LLNL-TH-734604

High Accuracy Measurement of theNuclear Decay of U-235m and Search forthe Nuclear Decay of Th-229m

F. Ponce

July 12, 2017

Disclaimer

This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

A High Accuracy Measurement of the Nuclear Decay of 235mU and Search for the NuclearDecay of 229mTh

By

Francisco PonceB.A. (University of California, Berkeley) 2007M.S. (San Francisco State University) 2012

DISSERTATION

Submitted in partial satisfaction of the requirements for the degree of

DOCTOR OF PHILOSOPHY

in

Physics

in the

OFFICE OF GRADUATE STUDIES

of the

UNIVERSITY OF CALIFORNIA

DAVIS

Approved:

Rena Zieve, Chair

Stephan Friedrich

Daniel Cebra

Committee in Charge

2017

i

A High Accuracy Measurement of the Nuclear Decay of 235mU and Search for the NuclearDecay of 229mTh

Francisco PonceDavis, California

2017

Among all nuclear decays, there exist two isomeric states with very low energy that belong

to 229Th (7.8 ± 0.5 eV) and 235U (76.8 ± 0.5 eV). Of particular interest is 229Th, because

the decay energy is in the ultraviolet and therefore in the range of modern tunable lasers.

The isomer can potentially be used as the basis for a nuclear clock that is expected to be

two orders of magnitude more precise than atomic clocks. However, the 229mTh nuclear

decay energy is not sufficiently well known to design the necessary laser system for a nuclear

clock. This work describes the development of a new technique using superconducting tunnel

junction (STJ) detectors to directly measure the nuclear decay of low energy isomers with

a high level of accuracy. The strength of the technique is demonstrated by measuring the

decay energy of the 235U isomer at 76.737 ± 0.018 eV, over an order of magnitude more

accurately than the current literature value. The technique is then applied to search for the

transition in 229mTh and measure its energy with comparable accuracy. These experiments

are unsuccessful and are discussed in light of the recent measurement of the 229mTh half-life

of 7 ± 1 µs. Modifications to our experimental technique to enable an accurate measurement

of 229mTh are suggested as the next step towards the ultimate goal of building a nuclear clock.

ii

ACKNOWLEDGMENTS

This work was in large part possible through the collective efforts of many individuals

that guided me along throughout the years. This will be an attempt to thank everyone but

I am likely to miss someone.

On the LLNL side I would like to thank my research advisor Stephan Friedrich, he taught

me everything he knew about cryogenic detectors. His positive attitude always helped when

morale was low and nothing was working, particularly at the beginning of this project.

On the UC Davis side I would like to thank Rena Zieve my faculty advisor, for being

willing to work with me. She always provided me with timely feedback to my questions.

I would like to thank all of the supporting researchers, and staff at LLNL. Particularly,

Jason Burke and Erik Swanberg for their advice in everything nuclear related, help getting

me started working with ROOT, setting up the pulsed laser, and wonderful conversations

throughout the years. To Roger Henderson and Sherry Faye for making our sources without

which this project would have been impossible. Marianne Ammendolia for helping to wire

bond our detectors on short notice. To Jonathan Dreyer, Owen Drury, and Camaron Bates

for teaching me the inner working of the ADR and laboratory. To Matthew Carpenter and

Simon George for their help and training at the ALS. To Jan Batteux, for all the help in

machining parts that we needed for the experiment and early morning conversations.

Finally, I would like to thank my family and friends for getting me out of the lab and

have a social life. Finally last but never least, my partner in crime Nicole, who puts up with

me on a daily basis and loves me with or without data.

iii

DISCLAIMER

This document was prepared as an account of work sponsored by an agency of the United

States government. Neither the United States government nor Lawrence Livermore National

Security, LLC, nor any of their employees makes any warranty, expressed or implied, or as-

sumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any

information, apparatus, product, or process disclosed, or represents that its use would not

infringe privately owned rights. Reference herein to any specific commercial product, pro-

cess, or service by trade name, trademark, manufacturer, or otherwise does not necessarily

constitute or imply its endorsement, recommendation, or favoring by the United States gov-

ernment or Lawrence Livermore National Security, LLC. The views and opinions of authors

expressed herein do not necessarily state or reflect those of the United States government or

Lawrence Livermore National Security, LLC, and shall not be used for advertising or product

endorsement purposes.

This work was performed under the auspices of the U. S. Department of Energy by

Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. It was

funded by the LLNL LDRD Grant 14-LW-073 and by U.S. DOE Grants DE-SC0004359 and

DE-SC0006214.

LLNL-TH-734604

iv

TABLE OF CONTENTS

1 Introduction 11.1 Nuclear Clocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 The Low-Energy Nuclear State in 229Th . . . . . . . . . . . . . . . . . . . . 31.3 Direct Measurements of the Decay of 229mTh . . . . . . . . . . . . . . . . . . 6

2 Superconducting Tunnel Junction Single-Photon and Particle Detectors 92.1 Superconducting Tunnel Junctions . . . . . . . . . . . . . . . . . . . . . . . 92.2 High Resolution Photon Detectors . . . . . . . . . . . . . . . . . . . . . . . . 112.3 STJ Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.4 STJ Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.5 Characterization with Soft X-Ray . . . . . . . . . . . . . . . . . . . . . . . . 202.6 Characterization with UV Light . . . . . . . . . . . . . . . . . . . . . . . . . 22

3 Experiment 303.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.2 Radioactive Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333.3 Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

4 235mU Measurement 434.1 Drift Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434.2 Offset Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444.3 Measurements of 235mU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514.4 Laser Calibration Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . 564.5 Detector Calibration Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . 564.6 Partition Width Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . 584.7 Bin Width Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.8 Fit Range and Background Uncertainty . . . . . . . . . . . . . . . . . . . . . 604.9 Uncertainty Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

5 229mTh Measurements 665.1 Alpha-Recoil Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665.2 Reduction of 233U Background . . . . . . . . . . . . . . . . . . . . . . . . . . 735.3 Search for 229mTh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815.4 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

6 Summary 91

v

LIST OF FIGURES

1.1 (From [1]) Energy levels and gamma transitions for 229Th . . . . . . . . . . . 31.2 (From [2]) Energy levels and gamma transitions for 229Th . . . . . . . . . . . 31.3 229Th gamma spectrum from Beck et al. . . . . . . . . . . . . . . . . . . . . 51.4 (From [3]) Half-life of neutral 229mTh . . . . . . . . . . . . . . . . . . . . . . 7

2.1 Excitation energy of STJ vs wave number . . . . . . . . . . . . . . . . . . . 102.2 DOS of superconductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.3 Outline of a basic STJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.4 I(V) for an ideal STJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.5 DOS of basic STJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.6 Outline of an STJ w traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.7 DOS of STJ w traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.8 STJ Amplifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.9 SEM image of STJ detector chip . . . . . . . . . . . . . . . . . . . . . . . . . 152.10 Elemental composition of newly fabricated STJ surface . . . . . . . . . . . . 172.11 Elemental composition of old STJ surface . . . . . . . . . . . . . . . . . . . . 172.12 ADR schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.13 STJ I(V) curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.14 STJ responsivity vs voltage bias . . . . . . . . . . . . . . . . . . . . . . . . . 212.15 STJ operation at the ALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.16 STJ ALS characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.17 STJ ALS FWHM vs energy . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.18 STJ resolution vs count rate . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.19 Characterization of STJ with pulsed laser . . . . . . . . . . . . . . . . . . . . 272.20 Long range calibration of STJ . . . . . . . . . . . . . . . . . . . . . . . . . . 282.21 STJ response to laser over hours . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.1 Outline of isomer implantation . . . . . . . . . . . . . . . . . . . . . . . . . . 313.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.3 Micro-machined Si collimator . . . . . . . . . . . . . . . . . . . . . . . . . . 323.4 Setup for depositing microdroplets . . . . . . . . . . . . . . . . . . . . . . . 343.5 Induced flux trapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.6 Large source holder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.7 Alpha spectrum of Pu source . . . . . . . . . . . . . . . . . . . . . . . . . . 373.8 Alpha spectrum of U source . . . . . . . . . . . . . . . . . . . . . . . . . . . 383.9 233U decay branch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.10 232U decay branch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

vi

3.11 XIA electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.12 Coincidence laser separation . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.1 STJ response to laser with 239Pu source (Raw) . . . . . . . . . . . . . . . . . 454.2 Fit to 5 minute partition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.3 Drift corrected and calibrated STJ response to laser . . . . . . . . . . . . . . 464.4 Centroid uncertainties to fully calibrated laser . . . . . . . . . . . . . . . . . 464.5 Variable laser illumination of (208 µm)2 STJ detector . . . . . . . . . . . . . 474.6 Centroid shift of three photon peak with laser intensity . . . . . . . . . . . . 474.7 Laser calibration versus average number of photons . . . . . . . . . . . . . . 484.8 Responsivity for (208 µm)2 STJ vs energy for variable laser intensities . . . 484.9 Responsivity for (138 µm)2 STJ vs energy for variable laser intensities . . . . 494.10 Centroid shift for (138µm)2 STJ vs laser intensity (Right) . . . . . . . . . . 494.11 Laser response of (138 µm)2 STJ detector to indirect illumination . . . . . . 504.12 Area of indirect illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . 504.13 Daily 235mU decay measurements . . . . . . . . . . . . . . . . . . . . . . . . 524.14 235mU isomer signal and fitted spectra . . . . . . . . . . . . . . . . . . . . . . 544.15 Residuals for 235mU fitted spectra . . . . . . . . . . . . . . . . . . . . . . . . 554.16 Calibration of pulsed 355nm laser . . . . . . . . . . . . . . . . . . . . . . . . 574.17 Laser dependence on temperature . . . . . . . . . . . . . . . . . . . . . . . . 574.18 Detector Calibration Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . 584.19 O and C fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584.20 235mU partition width uncertainty . . . . . . . . . . . . . . . . . . . . . . . . 594.21 235mU energy bin width uncertainty . . . . . . . . . . . . . . . . . . . . . . . 594.22 Background fits to 235mU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 624.23 235mU uncertainty due to background and fit range . . . . . . . . . . . . . . 634.24 Cumlative fits for 235mU decay measurements . . . . . . . . . . . . . . . . . . 65

5.1 233U Low energy background . . . . . . . . . . . . . . . . . . . . . . . . . . . 675.2 Alpha-Substrate interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . 685.3 ADC trace of alpha event on multiple detectors . . . . . . . . . . . . . . . . 695.4 ADC trace of recoil event on multiple detectors . . . . . . . . . . . . . . . . 695.5 Filtered trace of recoil event with crosstalk . . . . . . . . . . . . . . . . . . . 705.6 TOA of high energy events (with recoils) . . . . . . . . . . . . . . . . . . . . 725.7 Alpha particle only setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 735.8 TOA of high energy events (without recoils) . . . . . . . . . . . . . . . . . . 745.9 High energy spectra of time cut showing recoils . . . . . . . . . . . . . . . . 755.10 STJ detector laser calibration at low energies . . . . . . . . . . . . . . . . . . 765.11 233U signal with different data cuts . . . . . . . . . . . . . . . . . . . . . . . 775.12 Detector response after high energy alpha impact, long time scale . . . . . . 785.13 Detector response after recoil impact . . . . . . . . . . . . . . . . . . . . . . 785.14 Detector response after recoil impact in another detector . . . . . . . . . . . 795.15 Detector response after high energy impact, short time scale . . . . . . . . . 795.16 233U signal at 20 to 50 ms before and after recoil impact . . . . . . . . . . . 825.17 Reduced isomer signal on identical background . . . . . . . . . . . . . . . . . 83

vii

5.18 Projected isomer signal for collection time . . . . . . . . . . . . . . . . . . . 845.19 STJ spectra before and after recoil impact . . . . . . . . . . . . . . . . . . . 855.20 Difference Method relative to recoil impact . . . . . . . . . . . . . . . . . . . 865.21 Projected isomer response . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875.22 Projected acquisition time for a 10 ms half-life . . . . . . . . . . . . . . . . . 885.23 36-Pixel array of (208µm)2 STJ detector . . . . . . . . . . . . . . . . . . . . 88

viii

LIST OF TABLES

4.1 Experimental parameters for 235mU measurement . . . . . . . . . . . . . . . 524.2 Uncertainty Budget for 235mU . . . . . . . . . . . . . . . . . . . . . . . . . . 61

ix

Chapter 1

Introduction

This chapter motivates the current interest in the first excited state in 229Th because of

its potential use in nuclear clocks with unprecedented accuracy. It describes attempts to

accurately measure the energy of this state and motivates our experiment to improve them.

1.1 Nuclear Clocks

Among the thousands of nuclear energy levels in hundreds of different isotopes, the first

excited state in 229Th stands out because it is at an exceptionally low energy of only 7.8

eV above the ground state. This very low energy falls within the range of tunable UV

laser sources, and therefore could potentially enable the development of nuclear clocks with

unprecedented accuracy up to 10−19.

Currently, the international standard of time is set by atomic clocks based on the transi-

tion between two very narrow states in 133Cs atoms. The Cs atom acts as the clock resonator,

and a finely tuned laser is used to drive the electrons in the Cs atom between the two states.

The laser frequency is tuned to maximize the number of excited atoms, and this frequency

can be measured with very high accuracy to set the unit of time. Because the natural

linewidth of the transition in Cs is small, state of the art atomic clocks can achieve accura-

cies on the order of 10−16 or better [4, 5]. To achieve this level of accuracy a great deal of

1

effort is needed to shield the Cs atoms from external electric and magnetic fields, to cool the

atoms, and to isolate the optical components from vibrations.

The remarkably high accuracy of atomic clocks has led to their widespread use in scientific

and technological applications, most notably in GPS and network communications such as

the Internet and broadcasting. Still, some experimental needs go even beyond the accuracy

that atomic clocks can currently provide. For example, time synchronization for electronics in

high energy accelerators, measuring gravity at the Earth’s surface, and tests of fundamental

physics would benefit from clocks with increased accuracy [6, 7, 8, 9, 10, 11, 12, 13].

Nuclear clocks can potentially overcome the limitations of atomic clocks [14, 15]. Most

notably, the effect of external magnetic and electric fields on the nuclear states is greatly

reduced due to the size of the nuclear moments compared to that of the atom (≈ 3 orders of

magnitude). Additionally, nuclear states can have significantly narrower natural linewidths

than atomic transitions, especially the long-lived metastable states, based on the Heisenberg

uncertainty principle. The narrower linewidth of nuclear transitions would provide a higher

accuracy resonator compared to the current electronic transition in the Cs standard used in

modern atomic clocks. These advantages could give a nuclear clock an accuracy of up to

10−19, three orders of magnitude improvement over the atomic clock standard.

Building a nuclear clock requires a nuclear state with a small transition energy that can

be excited using modern laser technologies. Currently the only known candidate is the first

excited state in 229Th, whose energy of 7.8 ± 0.5 eV falls in the UV range. This makes 229Th

the only choice for the resonator in the development of a nuclear clock. Unfortunately, the

uncertainty in the energy ±0.5 eV is far too large and the linewidth unknown to justify a

laser search for the exact transition, because the resonance cross-section for laser absorption

is expected to be very small at 10−25 cm2 [16]. For example, assuming a 2 eV (or 2σ) range,

and a step size of 10−6 eV per 100 seconds a scan would take ≈6.5 years to complete. If the

natural linewidth is smaller than 10−6 eV no peak would be observed. Thus a more precise

measurement of the energy is needed to proceed.

2

Figure 1.1: (From [1]) Energy levels andgamma transitions in 229Th used in the ini-tial calculation of the energy difference be-tween the ground state and first excited nu-clear state.

Figure 1.2: (From [2]) Energy levels andgamma transitions in 229Th used to calcu-late the currently accepted energy differencebetween the ground state and the first ex-cited nuclear state.

1.2 The Low-Energy Nuclear State in 229Th

Evidence for the existence of a low lying nuclear state no more than 100 eV in 229Th was first

observed in 1976 by Kroger and Reich [17]. In studying the gamma emission of 229Th from

the alpha decay of 233U with Ge(Li) and Si(Li) detectors they noted several discrepancies.

Particularly the energy and parity of several emission lines could not be attributed to any of

the known rotational band heads of the 229Th atom. Since the rotational band heads mark

the lowest energy in the series of nuclear gamma transitions for a given angular momentum

j and parity π the symmetry of the rotation, they deduced that there existed an unobserved

rotational band head. Based on the energies of the observed emission lines they concluded

that the band head was jπ = 32

+with an energy below 100 eV above the ground state.

Reich and Helmer improved upon the initial measurements of the gamma ray emissions

using germanium detectors with higher energy resolution [1]. They deduced the energy of

the excited state from the difference of higher energy gamma lines (Fig. 1.1), assuming that

the marked lines were singlets and described only the transition as marked. They inferred

3

an energy of -1 ± 4 eV averaged from the three differences,

∆1 = E(97.1)− E(71.8)− E(25.3) (1.1)

∆2 = E(97.1)− E(67.9)− E(29.1) (1.2)

∆3 = [E(148.1)− E(146.3)] + [E(118.9)− E(117.1)] (1.3)

where E(#) is the energy of the # gamma emission. This was the first indirect measurement

of a low lying excited nuclear state in 229Th, and meant that the first excited state was

between 0 and 7 eV above the ground state with 2σ significance. Supporting evidence

for the existence of this state came by the fall of 1990 [18]. Burke et al. used a 17 MeV

deuteron beam incident on a 230Th target to generate 229Th and a tritium atom byproduct in

the 230Th(d,t)229Th reaction and compared the response to the well known 232Th(d,t)231Th

reaction. From the comparison they argued that the band assignments in [17] were correct.

Helmer and Reich followed up their earlier work in 1994 with a new measurement us-

ing new HPGe and Si(Li) detectors with improved resolution to remeasure the gamma ray

energies [19]. Using the same difference scheme and including additional lines an improved

measurement of 3.5 ± 1 eV placed the decay energy in the realm of UV radiation.

The low energy of the state spurred several attempts to directly measure the UV photon

of the decay, with several early claims of a successful observation [20, 21]. However, there

were discrepancies between the observations and neither could be reproduced elsewhere.

Others concluded that the observations could be attributed to nitrogen fluorescence induced

by α-particles traveling through air [22, 23].

In 2005, Guimaraes-Filho and Helene re-examined the data from [19] using improved

statistics and matrix formalisms. They took the same difference schemes, accounted for

branching ratios and deduced an energy of 5.5 ± 1 eV [24].

The latest and widely accepted value for the energy of the first excited state in 229Th

came in 2007 from Beck et al. using micro-calorimeters cooled to 90 mK with an energy

4

Figure 1.3: (From [2]) Low level gamma spectrum from 229Th used todetermine the nuclear decay energy.

resolution of 26 eV FWHM [2]. The improved measurements focused on four particular lines

close to the ground state (Fig. 1.2). These lines were unresolvable in earlier experiments

due to their close proximity and lower energy resolution of Ge detectors, but were cleanly

resolved with the microcalorimeter (Fig. 1.3). They extracted the energy of the first excited

state from the differences of the gamma-ray energies and initially obtained a value of 7.6

± 0.5 eV [2]. They later updated it with additional statistical analysis to the currently

accepted value of 7.8 ± 0.5 eV [25].

The main limitation of this measurement is the inability to resolve the two different decays

from the state at 29.19 keV one into the excited state and a weaker one into the ground state.

This makes the gamma line at 29.19 keV a doublet and introduces a correction term to the

energy of the first excited state due to the uncertainty of the associated branching ratios.

Proposals have been made to use metallic magnetic microcalorimeter (MMC) detectors to

make a similar measurement with even higher energy resolution that could separate the two

gamma emissions from the state at 29.19 keV [26]. Advancements in these detectors show

resolutions of 1.7 eV at 6 keV [27] and 46 eV at 60 keV [28]. Additional detector development

is therefore needed to achieve the required resolution to resolve the two gamma emissions

and measure the first excited state in 229Th more accurately.

5

1.3 Direct Measurements of the Decay of 229mTh

An extensive study to measure the energy and half-life of the excited nuclear state of 229Th

was carried out at Lawrence Livermore National Laboratory (LLNL) [29]. The experiment

used an electroplated 233U source on aluminized Mylar that alpha decayed into 229Th. The

source thickness of ≈10 nm was optimized for the escape of the 229Th recoils. The Mylar

was 1.4 µm so that α particles could escape from both sides, while 229Th recoils could only

escape from the side with the source. The source was positioned between a Si α-detector and

a microchannel plate (MCP) detector. An α detection in the Si detector initiated a search

for an associated 229Th recoil ion in the MCP and its potential subsequent decay to the

ground state if the recoil had been present in the first excited nuclear state. Measurements

were conducted in vacuum to mitigate possible effects from alpha induced fluorescence. A

239Pu source, which alpha decays into 235mU with a half-life of 26 minutes was used to verify

proper operation of equipment. This technique was sensitive to a half-life ranging from 10−8

to 10−3 seconds. Unfortunately, the nuclear decay of excited 229Th was not observed [29].

A similar experiment was carried out by the nuClock consortium in Europe, with the

main difference being the handling of the 229Th recoils after escaping from the source. As

with the prior experiment, a thin 233U source was used to generate 229Th ions with energies

up to 90 keV. The kinetic energy of the 229Th recoil ions was reduced in a buffer-gas stopping

cell filled with ultra-pure He through collisions with the He atoms. The slow 229Th ions were

then sent through a quadrupole mass separator, which isolates a specific ionization state

and mass by applying an RF voltage with a DC offset across the diagonals of four parallel

conducting rods. The isolated 229Th ions of different charge states are then deposited onto

an MCP at a controlled and very low energy where they are neutralized, enabling decay

by internal conversion. Internal conversion involves the transfer of the energy of the excited

nucleus to a bound electron whose binding energy is less than the energy of the decay thereby

ejecting the electron and leaving the atom ionized1. Early experiments showed that the

1It should be emphasized that this is not a β-decay, as Z remains constant in the process. The kinetic

6

Figure 1.4: (From [3]) Half-life of neutral 229mTh as measured from theionized (a) 229mTh2+ and (b) 229mTh3+

impact of ionized 229Th was occasionally followed by a subsequent decay signal in the MCP,

while ionized 230Th, 233U, and 234U showed no similar decay [30]. This was the first direct

detection of the existence of a long-lived state at the first excited nuclear level in 229Th. The

precision for the energy of the nuclear excited state was not improved in this experiment

but constrained to be between 6.3 and 18.3 eV in agreement with earlier measurements.

Furthermore, by storing 229Th2+ in the He buffer gas and the mass separator the half-life of

the ionized first excited nuclear state was shown to be > 1 minute. This was true only if

the 229Th was at least doubly ionized such that decay by internal conversion was suppressed.

Follow up experiments were performed with the MCP to measure the number of isomeric

decays as a function of the time after the Th-ion impact (Fig. 1.4). A half-life of 7 ± 1 µs

for the neutral excited nuclear state of 229Th was extracted [3], which is within theoretical

expectations [31].

These experiments showed that the lifetime of ionized 229mTh can be > 1 minute and

therefore sufficiently long to use 229mTh in a nuclear clock. However, they did not improve

upon the accuracy of the decay energy for the first excited nuclear state. It is therefore

still not possible to perform a laser search for the accurate energy of the transition between

the two states. This thesis describes experiments that attempt to improve the accuracy

and precision of the energy of this decay by an order of magnitude to enable such a laser

energy of the electron will be equal to the full energy of the excited state minus the binding energy of theelectron. This process is influenced by the chemical environment of the decaying atom.

7

search. The experiments use superconducting tunnel junction photon detectors whose energy

resolution of a few eV in the UV range is significantly higher than that of conventional Si and

Ge detectors. The improved resolution would enable a much more precise direct measurement

of the 229mTh decay. As in [29], we initially use 235mU to demonstrate the high accuracy and

precision of this experiment, and then describe the results of a search for the decay of the

229mTh.

8

Chapter 2

Superconducting Tunnel Junction

Single-Photon and Particle Detectors

This chapter describes the theory of high-resolution superconducting tunnel junction (STJ)

photon detectors. It describes the design and operation of the STJ detectors used for this

work. The detectors are characterized at UV and X-ray energies to show that their perfor-

mance is sufficient to measure the nuclear decay of 235mU and 229mTh with high precision.

2.1 Superconducting Tunnel Junctions

The superconducting state is characterized by a drop in resistivity to zero and the expulsion of

magnetic fields. At the onset of superconductivity electrons near the Fermi energy experience

a slightly attractive potential due to phonon screening and form a bound state referred to

as a Cooper pair [32]. These Cooper pairs have a binding energy ∆ on the order of ≈ 1

meV that results in the formation of a superconducting energy gap within the density of

states (DOS) (Fig. 2.1).The DOS of a superconductor in the semiconductor representation

is shown in figure 2.2. Cooper pairs are placed at the Fermi energy in the center of the

energy gap (2∆), which separates excited electron states and hole states. When a Cooper

pair is broken, the resulting quasiparticles possess both electron-like and hole-like properties.

9

Figure 2.1: Excitation energy of a super-conductor as a function of wave number, k.The superconducting energy gap forms atthe Fermi level.

Figure 2.2: At non-zero temperatures thereis a population of thermal quasiparticlesoccupying both electron and hole-like states.

At temperatures above 0 K there is a small population of thermal quasiparticles.

A superconducting tunnel junction (STJ) is composed of two superconducting films sep-

arated by a thin oxide barrier (Fig. 2.3). The oxide barrier is sufficiently thin to allow

particles to quantum mechanically tunnel across and is referred to as a tunneling barrier.

Both Cooper pairs and single quasiparticle can contribute to the tunneling current. At V =

0, Cooper pairs can tunnel across the barrier up to a maximum current of ± IC that depends

on the barrier transmissivity and the phase difference between the two superconductors.

This current is referred to as the DC Josephson current and can be suppressed by applying a

small magnetic field in the plane of the tunnel barrier [33, 34]. At 0 <V Cooper pairs tunnel

across the barrier at a frequency of 2eV~ , an effect known as the AC Josephson effect. If

the wavelength of the AC Josephson current matches the dimensions of the tunnel junction,

resonances in the I(V) known as Fiske modes will form [35]. Additionally, quasiparticles will

tunnel across the barrier resulting in a net leakage current from the imbalance of available

and filled states between the two electrodes [36]. If V = 2∆e, Cooper pairs can be broken

10

Figure 2.3: The layers of an STJ are brokendown into 3 main features the counter elec-trode (or absorber), an insulator layer (ortunneling barrier) and a base electrode.

Figure 2.4: I(V) for the ideal STJ atnon-zero temperature.

and the quasiparticles can tunnel into the conduction band of the other electrode, thereby

sharply increasing the tunneling current. For higher voltages, the I(V) characteristic of the

junction will asymptotically approach the behavior of a normal metal where the current

depends linearly on the applied voltage with a slope set by the normal metal resistance RN .

2.2 High Resolution Photon Detectors

An STJ may be operated as a photon detector by applying a small bias voltage V0. When

a photon is absorbed in one of the electrodes, Cooper pairs are broken and generate excess

quasiparticles in proportion to the photon energy. The generated charge will preferentially

tunnel in the direction of the applied voltage and create a current pulse whose magnitude

can be measured with a room temperature amplifier.

In the process of converting the photon energy into excess charges, the absorption of a

photon initially generates a single photo-electron. At first, the photo-electron will interact

mostly with other electrons generating additional cooler electrons as the energy is distributed.

Once the energy of the electrons becomes comparable to the Debye energy, electrons will

11

Figure 2.5: Density of states as a function of position inside of an STJ.Electrons are represented by solid black circles, holes by solid empty cir-cles and Cooper pairs by paired electrons. A photon is absorbed in theAbsorber (counter electrode) which generates a excess quasiparticle popu-lation (purple arrow). Quasiparticles diffuse throughout and the absorberand scatter down to lower energies (orange arrow). The particles will thentunnel across the barrier (green arrow) followed by additional scatteringdown to lower energies. Quasiparticles may use their hole-like properties(gray arrows) to tunnel back across the barrier (green dashed arrows) andtunnel multiple times. The quasiparticles that tunnel across the barriermay tunnel back and not be counted. Eventually all quasiparticles recom-bine into Cooper pairs (blue arrows).

12

couple to the lattice more strongly and generate high energy phonons, which in turn break

Cooper pairs and generate additional lower energy quasiparticles. This process continues as

long as the phonons possess energies > 2∆ and electrons possess energy > 3∆. In the end,

60% of the photon energy is converted into quasiparticles with the remaining 40% converted

into low energy phonons. The statistical variation in the number of quasiparticles generated

in this process has been determined from Monte Carlo simulations to be,

δE2Statistical = εEF (2.1)

where ε ≈ 1.7∆ is the average energy needed to generate an excess quasiparticle and depends

on superconductor energy gap ∆, E is the energy of the incident photon, and F is the

Fano factor determined to be ≈ 0.2 [37]. The Fano factor quantifies the fluctuations in the

generation of excess quasiparticles due to correlations in the energy relaxation cascade[38].

Since superconductors have energy gaps on the order of ≈ 1 meV, the energy resolution of an

STJ detector can be orders of magnitude higher than that of a conventional semiconductor

detector. At the energy of 7.8 eV for the 229mTh transition, the statistical limit is ≈ 52 meV.

During and after the cascade process quasiparticles will diffuse across the volume of the

absorber. Quasiparticles will tunnel across the barrier within a time constant [39],

τTun = 4q2n0V RN

√(∆ + qV0)2 −∆2

∆+ qV0

(2.2)

where q is the electron charge, n0 = τrecτtun

is the normal state DOS at the Fermi energy, V

is the volume of the superconductor, RN is the normal state resistance of the junction, ∆

is the superconducting energy gap, and V0 is the applied bias voltage. Since quasiparticles

are in a mixed quantum state having both electron-like and hole-like properties, they can

tunnel multiple time by returning across the barrier using their hole-like properties. Each

time they tunnel as a hole, they transfer charge in the same direction and add to the signal

current. Multiple tunneling will continue until eventually the quasiparticles will recombine

13

Figure 2.6: STJs with a secondary super-conductor of lower bandgap (green layer) in-crease signal output by trapping quasiparti-cles close to the barrier to enhance tunneling.

Figure 2.7: Charged particles that scatterinto the lower bandgap superconductor aretrapped near the tunneling barrier until tun-neling or recombination occur.

after a characteristic recombination time τrec [40]. Statistical fluctuations with the multiple

tunneling process contribute a theoretical uncertainty to the energy resolution of

δE2Multi−Tunnel = εE(1 +

1

〈n〉) (2.3)

where 〈n〉 is the average number of times a quasiparticle tunnels [41] [42] (Fig. 2.5).

The need for high absorption efficiency and fast tunneling times places conflicting de-

mands on the absorber thickness. To deal with the different needs, a trapping layer is added

to either side of the barrier that consists of a thinner superconducting layer with a smaller

energy gap than the absorber, ∆trap <∆abs (Fig. 2.6) [43]. The trap is added to either side

to maintain symmetry and allow tunneling to occur as outlined before (Fig. 2.7). The trap

creates a small potential well for quasiparticles to scatter into, which confines them in close

proximity to the tunneling barrier. The reduced distance effectively decreases the tunneling

volume and increases the rate at which the quasiparticles are incident on the barrier, thus

increasing the likelihood of tunneling (see Eqn. 2.2).

The STJ is read out using a low noise current sensitive preamplifier as shown in figure

14

Figure 2.8: General current amplifer. TheSTJ is referenced here as a simple currentsource, Ix.

Figure 2.9: STJ detector chip fabricated bySTAR Cryoelectronics. There are five pairsof detectors with sizes from upper left tolower right (284 µm)2, (208 µm)2, (138 µm)2,(68 µm)2 and (25 µm)2 which share a com-mon ground.

2.8, where Ix represents the X-ray induced signal from the STJ, V0 is the STJ bias voltage,

and Rf is the feedback resistor. The amplified output voltage is then given as Rf Ix. There

is a current noise spectral density for the electronics of the form [44],

∆E2Electronic ∝ i2tot = i2Shot + i2Johnson +

e2Tot

R2eff

(2.4)

where e2Tot is the voltage noise of the FET amplifier and Reff is the dynamic resistance of

the STJ at the bias point. The shot noise of the STJ is given by 2qIdc where q is the electron

charge and Idc is the leakage current of the STJ. The Johnson noise is due to the feedback

resistor and is given by 4kbTRf

where kb is the Boltzmann constant and T is the temperature

of the resistor.

15

2.3 STJ Fabrication

The detectors used in this study were Ta-Al-AlO2-Al-Ta STJ detectors fabricated at STAR

Cryoelectronics [45] using standard photolithographic techniques.

Tantalum was selected as the absorber because of its high atomic number Z, which

increases the absorption efficiency at higher X-ray energies, and because of its small energy

gap (∆Ta = 0.7 meV), which enables high resolution. Bulk Ta has a body-center-cubic

crystal structure, α-Ta, and a TC = 4.47 K. In thin films, Ta will grow in a mixture of

α-Ta and a second tetragonal crystal structure, β-Ta. The β-Ta film is undesirable as it has

a lower TC ≈ 0.5 K and a much lower energy gap which would limit the options for trap

material. To grow α-Ta, a thin Nb layer is used as a seed [46].

Aluminum was chosen as the trapping layer because it has a lower energy gap, ∆Al =

0.34 meV, and readily forms a uniform Al2O3 oxide which generally does not form pinholes

that short the detector. The oxide thickness is readily controlled by the initial pressure and

time of exposure to oxygen.

The first layer deposited in fabrication is a thin (≈ 5 nm) Nb layer to grow the 265

nm α-Ta base electrode. This is followed by the deposition of 50 nm of Al, then exposure

to oxygen which creates the barrier, followed by an additional 50 nm of Al. The second

layer of Al serves as a seed for the remaining 300 nm of Ta. The entire deposition is done

without breaking vacuum. The area of the detectors is defined by ion milling, with additional

passivation of the side walls to reduce leakage current. A small section of SiO2 is used to

prevent shorts from the Nb wiring layer to read out the devices. The final devices have five

pairs of STJ detectors in a diamond configuration with side lengths of 25 µm, 68 µm, 138 µm,

208µm, and 284 µm. For our measurements we use the two pairs of (138µm)2 and (208 µm)2

STJ detectors from the chip (Fig. 2.9).

The surface composition of a newly fabricated detector and a five year old detector were

tested by Auger spectroscopy by Evans Analytical Group (Fig. 2.10 and 2.11). The first 5

nm of the newly fabricated detector are dominated by the formation of a stable insulating

16

Figure 2.10: The STJ surface compositionwas measure by surface sputtering at EvansAnalytics Group. The oxygen comes fromoxidation upon exposure to atmosphere.

Figure 2.11: The STJ surface compositionwas measure by surface sputtering at EvansAnalytics Group. The carbon is from hydro-carbons on the surface.

Ta2O5 oxide layer as expected. Beyond 8 nm the composition is dominated by Ta. The older

detector contains some carbon on the surface, likely hydrocarbons collected on the detector

over the years. As before there is an oxide layer but it appears to have diffused deeper into

the Ta layer, likely due to differences in inital processing. After 15 nm the composition

becomes dominated by Ta.

2.4 STJ Operation

The detectors must be cooled well below TC for operation. In practice the temperature is

much smaller than TC/10 to reduce the population of thermal quasiparticles and thus the

contribution of the shot noise. The lower temperature also serves to increase the dynamic

resistance Rdyn which decreases the FET noise contribution (Eqn. 2.4). The Johnson noise

remains unchanged because the preamp is at room temperature.

The operational temperature < 100 mK is reached using an adiabatic demagnetization

17

Figure 2.12: ADR Schematic detectors are oriented for exposure to x-raysthrough IR windows or UV photons through a fiber optic. The paramag-netic salt pills are suspended from their own stages by Kevlar strings (notdepicted). They and are enclosed along with a superconducting magnetinside a magnetic shield which sets the 5 gauss line at just outside theADR. A mechanical heat switch is used to make thermal contact with theLHe bath during an ADR cycle.

18

refrigerator (ADR) [47]. The ADR is comprised of five temperature stages (Fig. 2.12). The

first stage is at room temperature and supports the vacuum inside the ADR. The second

stage is a 77 K shield attached to a liquid nitrogen filled tank. The third stage is a 4 K

shield attached to a liquid helium filled tank. Both shields are thermally isolated from other

stages and equipped with thin infrared (IR) windows to transmit X-rays.

To reach temperatures below 4.2 K we use two paramagnetic materials, gadolinium gal-

lium garnet (GGG) and iron ammonium sulfate, known in the field by its antiquated name

ferric ammonium alum (FAA). A heat switch allows coupling of the GGG (1 K stage) and the

FAA (0.1 K stage) to the 4 K stage with the He bath 1. With the heat switch initially closed,

the paramagnets are subjected to an increasing magnetic field through a large superconduct-

ing solenoid. The magnetic field decreases the entropy of the paramagnets by aligning their

spins. The heat of magnetization is carried away through the heat switch to the He bath

and the stages are allowed to equilibrate at 4 K. The heat switch is then opened, thermally

isolating the stages from one another and the He bath, before the magnetic field is reduced

to 0 T. As the magnetic field is removed, the spins in the paramagnets begin to randomly

orient by absorbing phonons from their surroundings. This decreases the temperature of the

GGG to below 1 K and of the FAA to below 0.1 K. The ADR hold time is approximately

24 hours at temperatures below 200 mK.

The STJ detectors are enclosed by a Cryoperm shield at 4K (not shown) to protect the

detectors from the strong magnetic field of the superconducting magnet during the ADR

cycle, which can cause flux trapping and degrade detector performance. Inside the shield, a

Helmholtz coil (or bias magnet) provides a small magnetic field to suppress the AC Josephson

effect and Fiske mode resonance for stable detector operation. Detectors mounted facing the

IR windows can be illuminated with X-rays from an external source.

For this project the ADR was retrofitted with a 50 µm diameter single mode optical fiber

(CeramOptec UV50/55p) to allow STJ exposure to UV photons. The fiber optic is heat

1The 1 K stage serves as an intermediary heat sink between the 0.1 K and 4 K stages to reduce thethermal load on the 0.1 K stage from the wiring.

19

sunk at 77 K and at 4 K2 and is glued with GE varnish to an Al mounting bracket on

the Cryoperm shield facing the STJ detectors. No special alignment is needed since the

photons leave the fiber with an opening angle of ≈30 degrees 3. A hot mirror (Edmund

Optics #B64-453 0◦ AOI) and heat absorbing glass (Edmund Optics #B49-092 KG-5) are

mounted in front of the end of the fiber optic to reflect and absorb IR radiation, respectively

[48]. This setup enables UV illumination of the detectors, mounted facing the fiber optic.

A typical I(V) curve for a (138 µm)2 and (208 µm)2 shows that the leakage current scales

with detector area (Fig. 2.13). Fiske mode resonances are visible as peaks in the curve on

both detectors, and the resonance voltage is higher for the smaller STJ as expected. We

have determined the responsivity of the detector as a function of bias voltage (Fig. 2.14)

and find that it is maximum at a bias voltage of 100 µV. This bias point was used for all

STJ measurements. Typical leakage currents at this bias were between 5 and 15 nA for the

(138 µm)2 STJs and between 15 and 30 nA for the (208 µm)2 STJs. The characteristic rise

time of these detectors is ≈6 µs with a decay time of ≈100µs.

2.5 Characterization with Soft X-Ray

The STJ detectors were characterized with soft x-rays at beam line 6.3.1 of the Advanced

Light Source (ALS) synchrotron at Lawrence Berkeley National Laboratories (LBNL). The

energy of the photons can be adjusted between 200 and 2000 eV for detailed measurement

of detector resolution as a function of energy. The energy resolution of the beam is ∆E/E

= 10−3, i.e. below 0.5 eV for photon energies below 500 eV and thus significantly smaller

than the linewidth of typical fluorescence sources. The beam size is 300 µm x 500 µm at the

back of the endstation where the ADR was mounted (Fig. 2.15). To reduce the beam size,

a 100 µm Pt pinhole on an XYZ stage was inserted into the beam path at a distance of 6

inches from ADR. The pinhole was manually adjusted to center the beam exposure onto the

2There is additional slack on either side of the mount to allow for thermal expansion between shields.3The rough alignment reduces the intensity of the incident laser onto the detectors

20

Figure 2.13: Leakage current for a (208 µm)2

and (138 µm)2 STJ. The increase in currentat 0 µV is the DC Josephson current andthe peaks past ± 50µV are suppressed Fiskemodes.

Figure 2.14: The responsivity of four STJdetectors was measured at 10.5 eV using apulsed UV laser.

STJ. A (208 µm)2 STJ detector was directly illuminated with beam energies between 200

and 280 eV in 10 eV steps. The electronic noise was measured with a pulser signal added

to the preamplifier input along with the STJ signal. The output of the preamplifier is fed

into shaping amplifier (Canberra Spectroscopy Amplifier Model 2020) and read out using an

ORTEC Model ASPEC-927 programmable dual-input multichannel analyzer that produces

a simple histogram of observed pulsed heights. A linear calibration was performed using the

centroid fits to the spectra (Fig. 2.16, top). Based on the residuals the linear calibration is

valid to within statistical uncertainty.

The energy resolution of the detector as a function of energy is plotted in figure 2.17,

together with the theoretical resolution according to equation 2.1 and 2.3 assuming 〈n〉 = ∞

and adding the measured electronic noise of 1.97 ± 0.01 eV in quadrature. The measured

resolution is below the theoretical value because the short shaping time measures the signal

height during the initial and less noisy part of the pulse [49].

21

Figure 2.15: ADR was mounted to the back end of Beam Line 6.3.1 at theALS. This allowed direct illumination of the STJs. A mobile pinhole wasplaced in between to selectively illuminate different detectors.

The detector resolution was also measured as a function of photon count rate (Fig. 2.18)

by direct illumination with the beam at 500 eV. At low rate, the resolution of ≈ 4 eV FWHM

is determined by statistical and electronic noise. At rates above a few 100 Hz, pile-up starts

to further broaden the peak. For comparison, we include measurement of the width of the

oxygen K-edge fluorescence. The linewidth of O K X-rays is noticeably wider than the width

of the synchrotron beam because it is limited by an intrinsic width of 8 eV.

2.6 Characterization with UV Light

For characterization in the UC range, the STJ detectors were illuminated with 355 nm

photons from a pulsed laser (Spectra-Physics J40-8S-40). The laser uses a Nd:YVO4 crystal

with a fundamental wavelength of 1064 nm that is sent through a third harmonic generator

(tripler) to produce 355 nm UV photons. The crystal is internally temperature regulated

to minimize drift in the laser wavelength. The system can be operated at repetition rates

22

Figure 2.16: (Top) Calibrated response todirect illumination of STJ by monochro-matic synchrotron beam. The difference inpeak counts are caused by optical efficienciesat different energies. (Bottom) Residualto linear calibration with no statisticallysignificant deviations observable.

Figure 2.17: Resolution of a (208 µm)2 STJ(blue circles) along with the measured elec-tronic noise (gray diamonds) along with thestatistically limited resolution (solid blackcurve) and statistically limited with elec-tronic noise (dashed black curve). The dif-ference from theory may indicate an oversimplified uncertainty of the multi-tunnelingprocess.

23

Figure 2.18: The resolution of the STJ as a function of count rate for flu-orescence (blue circles) and direct illumination (green squares) is constantbelow 200 cps.

24

between 15 Hz and 300 kHz with an external trigger. The pulse width is < 12 ns. The laser

was heavily attenuated outside the ADR and a single pinhole 150 µm Pt-Ir collimator is used

to restrict the illumination to the STJ area.

The laser was computer controlled through the program TeraTerm and automated using

.JOB files on the MAESTRO software of the ORTEC system. For repetition rates below

5 kHz the laser was externally triggered using a four channel digital delay/pulse generator

(SRS Model DG535)4.

The detectors were characterized under direct illumination of the pulsed laser at a repe-

tition rate of 5 kHz (Fig. 2.19, top). The spectrum shows several peaks at integer multiples

of the single photon energy because the detector rise time is much greater than the laser

pulse width. The peaks are labeled with the number of simultaneously absorbed photons in

that peak. The laser intensity sets the average number of photons observed in each pulse,

which follows Poisson statistics. The response profile F(E) is therefore expected to follow

a Poisson distribution convolved with a Gaussian function with a width set by the energy

resolution of the STJ:

F (E) =∞∑n=0

µne−µ

n!· e−

(E−nE1)2

2σ2n . (2.5)

Here µ is the average number of photons, E1 is the energy of the photon, σn is the standard

deviation of the nth Gaussian peak and n refers to the peak index. Spectra are typically

fitted to a subset of observable peaks with a superposition of N-Gaussian functions, where

N is the number of peaks with count above 500. The residuals to the fit show that the fit

is consistent with the detector response to within statistical uncertainty,√counts, at high

repetition rates and short acquisition times.

The average of the Poisson distribution, which depends on laser intensity, can be con-

trolled by adjusting the pump diode current to calibrate the STJ detectors over a range of

energies (Fig. 2.20, top). Detector size affects the available energy range for a given laser

intensity with a lower range for a (138 µm)2 STJ and higher range for a (208 µm)2 STJ. The

4This setup used method D for external triggering as outlined in the Spectra-Physics user’s manual.

25

difference in resolution is expected as the resolution depends on detector size [45]. More

importantly the resolution changes with deposited energy as expected according to equation

2.1 and 2.3.

Extended exposures over many hours reveal that the laser intensity can change abruptly

at random times, and that the STJ responsivity can drift slowly over time scales of hours.

We illustrate the effects with a two dimensional histogram (Fig. 2.21). The STJ was set to

periodically collect spectra at 5 kHz over a period of 18 hours, and the relative number of

counts is represented by the differing colors. Each peak is represented by a bright row, with

the row at MCA channel 100 corresponding to the 5-photon peak. Sudden shifts in laser

intensity, observed here after 5 hours, distort the Poisson nature of the laser profile. Addi-

tionally, the detector starts to drift after ≈ 12 hours because of the unregulated temperature

of the ADR. These two effects require using a superposition of multiple Gaussian functions

with independent parameters to fit the laser profile properly.

The precision of the energy calibration, and thus the precision in the measurement of

a nuclear decay, is determined by the uncertainty δCentroid in the measurement of a peak

centroid. It is given by

δCentroid =σ√A

(2.6)

where σ is the standard deviation and A is the number of counts in the peak (or peak area).

A high precision measurement of the metastable nuclear states therefore requires both high

detector resolution (low σ) and a large number of counts.

The pulsed laser provides several well defined, evenly spaced, peaks of known energy

that can be determined with a precision of meV. This enables an accurate calibration of our

detectors to the same order [50]. The STJ resolution is limited by the electronic noise of ≈

2 eV below 100 eV. In combination with the laser, an STJ is ideal for both high accuracy

high precision measurements in the UV/EUV energy range.

26

Figure 2.19: (Top) The STJ response to the pulsed laser (green curve)is nominally a Poisson distribution of events with an integer number ofabsorbed photons, convolved with a Gaussian function due to the energyresolution of the STJ (black curve). The number above each peak rep-resents the number of photons for that peak. (Bottom) The residuals tothe fit (black circles) agree with the uncertainty expected from countingstatistics (green shaded region).

27

Figure 2.20: (Top) The laser intensity was varied from the lowest to the highest settingsat a repetition rate of 2.1 kHz to calibrate the detector across a wide range of energies.(Bottom) The resolution of the detectors scales as a function of energy as expected. Theobserved differences in range and resolution are due to detector size.

28

Figure 2.21: STJ response to pulsed laser at 5 kHz with period acquisi-tions. The laser intensity is not constant in time as seen by subtle changesin brightness of individual peaks. This effect is most pronounced in thesudden change at 5 hours. The decrease of the signal after 12 hours is dueto a change in detector response. The bright line near zero is the cut offnoise peak.

29

Chapter 3

Experiment

This chapter describes the technique used to measure the nuclear decay energy of metastable

229mTh and 235mU with STJ detectors. It also describes the radioactive sources and the data

acquisition system used in these measurements.

3.1 Experimental Setup

A schematic of the setup to measure the isomer decay energy is shown in figure 3.1. The

alpha decay of the radioactive source continuously provides isomers of interest as recoil ions,

which can leave the source because it is very thin. The recoils are embedded inside the

STJ detector, where the isomer decays to the nuclear ground state with its characteristic

lifetime. Since all decay products are fully captured by the detector the full energy of the

nuclear decay is observed regardless of the recoil chemical state. The STJ is calibrated with

the pulsed laser, which is transmitted through the radioactive source. A collimator restricts

both the laser and source exposure to the detector area.

A picture of the experimental setup without the source is shown in figure 3.2 (left). The

hole in the Al holder is the area of the radioactive source when installed. The Cu holder

shields the detector chip from illumination by the source and suspends a Si collimator above

the detector chip. A wiring pad extension is used to wire bond the detector.

30

Figure 3.1: The radioactive source embeds metastable recoil ions into theSTJ, whose subsequent nuclear decay can be measured with high precision.The detector is calibrated with a 355 nm pulse laser through the thinsource.

The right of figure 3.2 illustrates a cross-sectional cut of the assembled setup. The Al

holder with the source rests on the Cu holder to set the detector-source distance to ≈ 2.5

mm. A 239Pu source is used to generate 235mU, and a 233U source is used to generate 229mTh.

The source is electroplated onto the aluminized mylar to a thickness of ≈ 10 nm optimized

for the metastable recoils escaping based on an energy of 90 keV. The Si collimator is glued to

the underside of the Cu holder using GE varnish such that the collimator-detector distance

is ≈ 50µm. Each detector is matched with a pinhole in the collimator that inscribes the

detector area and is aligned under a microscope (Fig 3.3). The last two pinholes in each row

are larger than the (25 µm)2 STJ detector underneath so that they are visible through the

pinhole.

31

Figure 3.2: (Left) Top view of the experimental setup without the radioac-tive source. (Right) Cross-section rendition showing source (red) on theAl mylar thin enough for the laser to pass. The laser beam (blue arrows)is sufficiently dispersed to illuminate all detectors.

Figure 3.3: The Si collimator is micro-machined by STAR Cryoelectronics tomatch the dimensions of their STJ detectorchip. The holes are smaller than the detec-tors except for the smallest (25 µm)2 STJ de-tector, which is visible through the pinhole(inset).

32

3.2 Radioactive Sources

The 233U used in these experiments was made at the MIT nuclear reactor in the 1960’s by

irradiating a pure 232Th target with neutrons. This creates 233Th by neutron capture, which

subsequently beta decays with a half-life of 22.3 minutes to 233Pa, which in turn beta decays

with a half-life of 27 days into 233U. Since the reactor neutrons are modulated to low energy,

the 233Pa(n, 2n)232Pa reaction during irradiation is suppressed so that few 232U impurities are

created by the subsequent β-decay of 232Pa. This process therefore allows for the generation

of very pure 233U with trace amounts of 232U at 0.2 ppm, which has a relatively short half-life

of 68.9 years and typically dominates the radioactive background.

Prior to fabrication of new sources the 233U is purified to remove daughter impurities. For

this the 233U is initially dissolve in 8M nitric acid and passed through an anion exchange to

remove Th impurities. The solution is then dried, the material dissolved in 9M hydrochloric

acid and passed through an anion exchange to remove the U. Additional 9M hydrochloric

acid is then passed through anion exchange to remove the remaining impurities. The U

is then washed out with 1M hydrochloric acid, dried and finally dissolved in nitric acid

and added to alcohol. The chemical purification process does not remove any 232U isotope

impurities, which makes starting with an isotopically pure sample of paramount importance.

For highest detection efficiency, we initially deposited a thin layer of purified 233U directly

onto the two (208 µm)2 STJ detectors. The layer was deposited with a 100 µm capillary

attached to a micro-pipette on a micro-controller that was lowered to within tens of microns

of the detector under a microscope (Fig 3.4). The capillary did not make contact with the

detector to avoid damaging the fragile tunnel barrier underneath. A micro-droplet of the

liquid source was pushed out of the capillary to make contact with the detector surface.

The capillary was then retracted, allowing surface tension to keep a thin drop of the source

solution on the detector that was allowed to air dry.

The attempt to measure the decay of the 229mTh from this source failed because the

STJ detector performance degraded rapidly during each ADR cycle. To explain this effect

33

Figure 3.4: A micro-droplet of radioactive material was directly depositedonto the STJ detectors under a microscope using a 100 µm capillary con-nected to a micro-pipette. The capillary was precision controlled by aseries of micrometers along the x-y-z axes. STJ test chip with a drop of233U on the surface of both (208 µm)2 STJ detectors.

34

six detectors were monitored for 3.5 hours by periodically measuring their leakage current

at a bias voltage of 100 µV (Fig 3.5). The four detectors without the source maintain a

constant leakage current throughout the measurement. However, the two detectors with the

radioactive source show a rapidly increasing leakage current during regular operation with

the bias magnet on, and a reduction in leakage while the bias magnet is off. This suggests

that both the radioactive source and the magnetic field are responsible for the degraded

performance. We attribute the increase in leakage current to magnetic flux trapping caused

when alpha particles traverse the STJs at shallow angles and drive a section of the STJ

normal. To limit the induced flux trapping, the source must be spaced from the detectors

by several mm so that alpha particles pass through the STJ detectors at normal incidence.

Alpha particles will deposit only a fraction of their total energy, while recoils deposit their

full kinetic energy into the detectors.

For the experiments in this thesis a 233U and a 239Pu source were electroplated onto

a 1.4 µm thick aluminized mylar to a thickness of ≈ 10 nm and an activity of ≈ 0.2 µCi.

The source thickness is limited to 10 nm because recoils cannot escape from deeper with a

maximum energy of 90 keV. The mylar was supported by a thin 132

inch aluminum plate with

a centered half inch diameter hole (Fig 3.6). Sources were characterized using an ORTEC

Soloist Alpha Spectrometer equipped with a Si detector (Model SOLOIST-U0900). The

alpha spectrum was collected for 2 hours as a distance of 58inches from the Si detector under

vacuum.

Based on the alpha spectrum of the Pu source, the composition is mostly 239Pu with

trace quantities of 238Pu that cannot be removed by chemical separation (Fig 3.7). The

straggle of the main 239Pu peak masks all lower energy decays from all isotopes. No lines

from the daughter chains are observed due to the long half-life of 235U (7.04·108 years) and

234U (2.46·105 years). Since the 239Pu decays to 235mU with a branching ratio of 100% and

the concentration of 238Pu is two orders lower than 239Pu, effectively all recoils are 235mU.

Based on the alpha spectrum of the U source, the composition is mostly 233U with trace

35

Figure 3.5: The leakage current of the(208 µm)2 STJ detectors with a source in-creases rapidly under normal operationswith the bias magnet (white shade). Whenthe magnet is turned off (gray shade) theleakage current recovers slightly. The STJdetectors without the source remain con-stant.

Figure 3.6: This holder serves as the tem-plate of the large area sources and the AlMylar is thin enough to see through.

36

Figure 3.7: Alpha spectrum of the 239Pu source used to produce 235mUisomers in the work. Trace amounts of 238Pu are present in the source.No decays of the 235U or 234U daughters are observed due to their long lifetimes.

37

Figure 3.8: Alpha spectrum of the 233U source used to produce 229mThisomers in the work. The daughter atoms have grown in and there aretrace amounts of the 232U decay chain.

quantities of 232U along with corresponding daughter nuclei (Fig 3.8). The observed lines

correspond to alphas from the main decay chain for 233U (Fig 3.9), and 232U (Fig 3.10). This

233U source was fabricated in 2010, so the daughter products were expected to have grown

in. Since the 233U decays to 229mTh with a branching ratio of 2% and the concentrations of

contaminants is low, the isomer nuclear decay signal is expected to be 2% of the incident

recoil signal.

3.3 Data Acquisition

A commercial data acquisition system built by XIA LLC specifically to operate and readout

STJ detectors [51] is used in this work. The XIA system consists of four components: an

STJ-32AD (preamplifier), an MPX-32D (digitizer), an STJ-ACRATE-4 (PIXI crate), and

38

Figure 3.9: Decay branch for 233U. At least one member of each of theprimary alpha decays is observed in the alpha spectrum of the U source.

39

Figure 3.10: Decay branch for 232U. At least one member of each of theprimary alpha decays is observed in the alpha spectrum of the U source.

40

an STJ-APWR (power supply). The STJ-32AD unit is a compact set of 32 analog current

sensitive preamplifiers modeled from a custom-built single channel low noise preamplifier

[52]. The MPX-32D unit is the corresponding 32 channel digitizer with two FPGA processors

that perform the calculation on the waveforms from the STJ-32AD. The STJ-ACRATE-4 is

a standard PIXI crate and the STJ-APWR is a custom-built low noise power supply for the

STJ-32AD. A PXI-PDM (PXI Preamplifier Power and Trigger Breakout Module) was used

as a clock distributor to enable coincidence and anti-coincidence measurements (Fig 3.11).

The FPGA uses a simple trapezoidal filter with a user defined rise time and decay time.

The energy of each pulse is based on the height of the filtered signal above the baseline. A

digital gain enables measurements at either low or high energies.

For this project several upgrades to the accompanying ProSpect software was made to

enable histogram, list mode and full pulse acquisition modes to below 7 eV. The firmware

of the FPGA processors was modified to work with the PXI-PDM to check the arrival time

of events against the on/off state of an input TTL. Events that occured when the TTL was

on were marked in coincidence and otherwise they were marked in anti-coincidence. This

enables separation of externally triggered pulsed laser events from the radioactive source

events in post-processing (Fig 3.12). The signal from a single STJ could not be split to

make a measurement at two distinct gains, so the software was modified to keep the time

stamp of overfill (high energy) events and place these events in the largest bin, 8192. The

arrival times of high energy events can then be compared to low energy events to look for

correlations.

To reduce electromagnetic pick up, the STJ-32AD preamplifiers were mounted directly

onto the ADR via the 78 pin D-connector. The cooling fan of the STJ-32AD turned out to

induce excess noise, and since it is not essential for operating a single preamplifier card, it

was disabled [53].

41

Figure 3.11: The XIA system is designed to work in unison and additionalfiltering and amplifiers are not possible. The system allows for multipledetectors to be operated together in histogram, list, or full trace mode.

Figure 3.12: Laser events are tagged duringacquisition and separated from sourceevents in post-processing.

42

Chapter 4

235mU Measurement

This chapter summarizes the measurements of the 235mU decay energy with STJ detectors.

It discusses statistical and systematic uncertainties of the measurements and suggests an

improved value for the energy of 235mU.

4.1 Drift Corrections

All experiments were carried out using several STJ detectors on a single chip, the pulsed

355 nm laser and the XIA data acquisition system. The detector chip was wired to operate

two (138 µm)2 and two (208 µm)2 STJ detectors. The laser was operated with an external

trigger at a rate of 100 Hz, and its intensity was centered around the energy of the 235mU

decay. The XIA data acquisition system was operated in list mode using the TTL signal of

the external trigger as a timing signal for the laser. Typical acquisitions lasted 16 - 24 hours

per ADR cycle, and data were processed using custom analysis routines written in ROOT

[54]. Laser events were separated from signal events in post-processing using the coincidence

tagging of the XIA system with a gate window of 150 µs.

Throughout all measurements, we observe that the STJ detector responsivity changes as

a function of time. This is due to flux trapping induced by the radioactive source when it

drives certain regions of the STJ normal and allows some of the applied magnetic field to be

43

trapped. To correct for this drift, detectors need to be calibrated continuously with the pulsed

355 nm laser throughout the acquisitions. In addition, the intensity of our calibration laser

fluctuates over time, sometimes with significant changes over short times scales. Figure 4.1

shows a typical STJ response to the pulsed laser over 22 hours. The bright lines are the peaks

of the laser, with the line at MCA Ch 630 at the start of the measurement corresponding to

77 eV, or 22 photons. Two things are noticeable: a slow drift in the response over time scales

of hours, and sudden changes in the responsivity at certain times (seen here at 2.5 hours).

The slow drift in the detector responsivity is a result of the changing decay time of the

STJ detectors, which causes a mismatch with the decay time of the trapezoidal filter of the

acquisition system. We attribute the sudden changes in the responsivity to sudden changes in

flux trapping in the STJ detector. To compensate for these changes, all data acquisitions are

partitioned into 5 minute intervals over which the detector responsivity and laser intensity

are considered constant. Each partition is fitted independently to a superposition of multiple

Gaussian functions to peaks with more than 500 counts. The residuals between the fit and

the data fall within the expected error bars based on counting statistics (Fig. 4.2). The

detector drift is calibrated out with a linear fit to the five peaks corresponding to 70 through

84 eV (Fig. 4.3). After drift correction, the changing laser intensity is still apparent. When

all 5 minute spectra from the 22 hour run are summed into a single spectrum (Fig. 4.4), peak

centroids can be determined with a precision below 1 meV in the range of interest around

77 eV and with a precision below 15 meV over the entire 125 eV range. Drift corrections

therefore do not cause a problem in the calibration of our detectors.

4.2 Offset Corrections

During drift correction of the data, we noticed that the calibration offset varied with laser

intensity. To quantify this offset, a (208 µm)2 STJ detector was monitored at 8 different laser

intensities at a repetition rate of 5 kHz. The laser intensity was constant for each acquisition

44

Figure 4.1: Unprocessed STJ response tothe pulsed laser and the 239Pu source over a22 hour period. Bright lines correspond tothe laser peaks, with the line at MCA Ch630 at time zero corresponding to 77 eV.

Figure 4.2: (Top) Five minute partitionof laser events (green) fitted with a su-perposition of multiple Gaussian functions(black). (Bottom) Residuals (black) of thefit lie within errors from counting statistics(green).

45

Figure 4.3: Drift corrected and calibratedSTJ response to the pulsed 355 nm laserover a 22 hour period. Note the fluctuationsin laser intensity after 4, 9, 13, and 21 hours.

Figure 4.4: Sum of drift corrected and cali-brated STJ response to laser over 22 hours.The uncertainty of the centroid is lowest forpeaks with a large number of counts, below1 meV for the peaks near 77 eV.

time of 5 minutes and the radioactive source was not present. In addition, two (138 µm)2

STJ detectors were monitored with the laser at 2 kHz and 8 intensities for a total time of

6.5 minutes in the presence of a radioactive source.

The uncalibrated response of the (208 µm)2 STJ detector is plotted in figure 4.5 to show

the spectra for increasing laser intensity. The spectra are normalized to the same amplitude

for the strongest peak. A shift in peak centroid is visible with laser intensity, especially at

the 3-photon peak around channel 110 that appears in all of the spectra. Centering on this

3-photon peak and normalizing the amplitudes, the centroid shift is readily apparent (Fig.

4.6).

For a linear calibration centered around 21 eV, the calibration coefficients are plotted

as a function of the average number of absorbed photons, which scales directly with the

laser intensity (Fig. 4.7). The calibration offset increases by a factor of 2 for these given

intensities, and a linear fit shows that the offset goes to zero as the laser intensity decreases.

46

Figure 4.5: Response of the (208 µm)2 STJdetector to different laser intensities. Theshaded region highlights the three photonpeak shown in figure 4.6.

Figure 4.6: Laser spectra normalized to theamplitude of the three photon peak showthe laser-induced offset.

By contrast the calibration slope, or laser responsivity, does not change significantly with

increasing laser intensity for the given intensities. Thus the calibration offset is correlated

with the laser intensity, and not with a change in the responsivity of the detector. For

accurate energy calibration, we therefore calculate the responsivity of the (208 µm)2 STJ

detector in the energy range between 7 and 60 eV from the distance between neighboring

peaks (Fig. 4.8). The responsivity values from different spectra match in the region of

overlap and are constant with energy within the accuracy of the measurement.

For the (138 µm)2 STJ detectors with the radioactive source the laser intensity was varied

over a larger range. We again observe an offset that scales with laser intensity, and therefore

calculate the responsivity of the detector between 10 and 230 eV from the difference between

neighboring peaks (Fig. 4.9). The responsivity of the detector matches in regions where the

spectra overlap for all intensities and is again constant over the entire range (Fig. 4.10).

Since we cannot take a spectrum at zero intensity, the centroid shift is measured relative to

47

Figure 4.7: Laser calibration for the fourhigher laser intensities of figure 4.5. Thecalibration slope, or detector responsivity, isconstant with increasing average number ofphotons, or laser intensity. The calibrationoffset changes linearly with increasing aver-age number of photons, decreasing to zeroand the intensity decreases to zero.

Figure 4.8: Responsivity of the (208 µm)2

STJ detector for variable laser intensities(marker color and shape). The responsivitymatches in regions of overlapping energyand does not depend on laser intensity. Nodependence on energy is observable withinthe accuracy of the measurement.

48

Figure 4.9: Responsivity of the (138 µm)2

STJ detectors as a function of energy fordifferent laser intensities (marker color andshape). The responsivity matches in regionsof overlapping energy with no observable de-pendence on laser intensity.

Figure 4.10: Centroid shift of the (138 µm)2

STJ detector for different laser intensities.The centroids shift as a function of laserintensity, but are constant with energy forall fixed laser intensity.

49

Figure 4.11: Laser response of (138 µm)2

STJ detector to indirect laser illumination atthree different intensities. The single photonpeak is visible at the lowest intensity and thetwo photon peak is visible at the medium in-tensity.

Figure 4.12: STJ detector chip used with asingle pixel collimator. A minimum indirectlaser illumination area (red circle) is inferredfrom measurements with the (138 µm)2 STJdetector ≈ 400µm away from the area di-rectly below the collimator (white dot).

the spectrum with the lowest intensity. As was the case for the calibration of the larger STJ

detector, the centroid shifts depend only on the laser intensity and are constant for all peaks

at the same intensity. At these laser intensities, the offset shifts between 0.25 and 1.4 eV.

Such a shift in centroid is possible when the laser illuminates the substrate around the

STJ detector [55]. Absorbed photons create an excess population of high energy phonons

that propagate to the STJ after laser exposure, and only then. These phonons have sufficient

energy to break Cooper pairs and contribute to the signal in the STJ detector. The additional

energy will vary from pulse to pulse, but will statistically average out to be proportional to

the laser intensity. At a given laser intensity, all peaks are offset by the same average energy

due to substrate phonons.

To confirm the illumination of the chip around the STJ detector, a 150 µm Pt-Ir pinhole

was used to restrict laser illumination to the (208 µm)2 STJ detector. The neighboring

(138 µm)2 STJ detector (covered by the collimator) was then monitored with the laser at 3

50

different intensities (Fig. 4.11). Although this STJ is≈ 400µm away from the pinhole a single

photon peak is observed above background at the lowest intensities and the two photon peak

becomes visible at the medium intensity. We conclude that while the collimator prevents any

direct illumination of the substrate, reflections from surfaces indirectly illuminate a region

several times larger than the (208 µm)2 STJ area (Fig. 4.12).

These measurements show that the STJ detector responsivity is constant, but that spectra

are offset in energy due to substrate phonons generated by the laser. This offset does not

affect the isomer decay, which is measured in anti-coincidence with the laser. We therefore

calibrate the signal using only the linear term of the calibration and set the offset term to

zero.

4.3 Measurements of 235mU

To measure the decay of the 235mU, we use a thin electroplated 239Pu source to provide a

continuous supply of 235mU recoil ions to the four STJ detectors. Of the four available detec-

tors, only the (138 µm)2 STJ detectors consistently possess sufficient resolution for accurate

energy calibration. The spectra from the (208 µm)2 STJ detectors show a rapid degradation

in detector resolution and are therefore excluded from the measurement of the nuclear decay

of 235mU. As before, the laser was operated with an external trigger throughout the acqui-

sition to perform a continuous calibration. The XIA data acquisition system was operated

in list mode with the TTL signal from the external trigger of the laser to mark events that

arrive at the STJ detectors in coincidence with the laser pulse. The laser was separated from

the radioactive signal in post-processing and processed in ROOT. All detectors were used to

perform coincidence vetoing to reduce the low energy background. Events are considered in

coincidence if they occur within 10 µs of each other on multiple detectors.

We conducted two experiments with new unradiated detectors on two different chips

from the same wafer, referred to as chip D and chip H (Table 4.1). In the first experiment

51

Experiment: Dec. 2015 Oct. 2016Detector Chip D HSTJ Detector Area (138 µm)2 (138µm)2

Laser Intensity Center (eV) Variable 77Acquisition Time (days) 21 7

Table 4.1: Experimental parameters for measuring the nuclear decay energy of 235mU.

Figure 4.13: Values of the 235mU decay energy from individual 1-day runsof the four different (138 µm)2 STJ detectors. The gray line and shademark the literature value of 76.8 ± 0.5 eV [56].

52

(December 2015), the laser intensity was varied to capture a full spectrum of peaks from

20 to 100 eV. In the second experiment (October 2016) the laser intensity was centered at

77 eV near the energy of the 235mU decay. For both experiments, the data are separated

into laser events and signal events using the coincidence tagging of the acquisition system.

The time stamps of the events are then used to partition the data into 5 minute intervals.

The laser partitions are used to generate a linear calibration for each 5 minute interval of

the acquisition. The signal partitions are then calibrated with the offset term set to zero to

avoid adding the contribution of the substrate phonons. After calibration, the signal data

are rebinned into a histogram with a bin width of 0.2 eV. The isomer signal is fitted to a

Gaussian function on a background of a sum of two exponential decays over a fit range of

50 eV. The individual 1-day measurements of the isomer energy for each detector are shown

in figure 4.13. All measurements from the four detectors fall within the accepted literature

value of 76.8 ± 0.5 eV. Measurements with poor statistics are omitted, and the variable size

of the uncertainties reflects the different number of counts in the isomer peak and changes

in detector responsivity from day to day. Based on the long acquisition December 2015

there appears to be no systematic drift due to radiation damage caused by exposure of the

STJ detectors to energetic recoil and alpha particles. Measurements from detectors operated

simultaneously are self-consistent with the same general spread in observed isomer energy.

There is no observable systematic difference in the individual measurements between the

two experiments. The individual measurements fall within 1 - 2 standard deviations of one

another regardless of detector and time of measurement.

The data are summed to obtain a final spectrum for each detector and refitted with the

same background and fit range (Fig. 4.14). The residuals to the fits for each detector are in

excellent agreement with the expected uncertainty due to counting statistics over the entire

fit range (Fig. 4.15). The four individual measurements of the nuclear decay energy of 235mU

are 76.739 ± 0.022stat, 76.741 ± 0.028stat, 76.758 ± 0.024stat, and 76.694 ± 0.025stat eV for

STJ 1 through 4, respectively. The energy resolution for STJ 1 through 4 is 1.94 ± 0.02,

53

Figure 4.14: Summed spectra for the four (138 µm)2 STJ detectors used inthis work. The black fits to the spectra assume the sum of two exponentialdecays as a background and a fit range of 50 eV.

54

Figure 4.15: Residuals of the fits to the isomer signals in figure 4.14. Theresiduals lie within the expected uncertainty due to counting statistics(shaded region) for each detector.

55

2.07 ± 0.03, 1.80 ± 0.02, and 1.61 ± 0.02 eV FWHM. These values are in agreement with

the energy resolution of the laser at 77 eV, typically below 2 eV.

4.4 Laser Calibration Uncertainty

The energy we extract for the 235mU decay depends directly on the single-photon energy of our

laser. The laser energy is measured relative to the well known reference lines from a mercury

vapor lamp at 253.6521 ± 0.0001 nm and 365.0158 ± 0.0001 nm [57]. The wavelength of

the pulsed 355 nm laser and two reference lines from the Hg vapor lamp were measured

using a UV grating spectrometer (THORLABS CCS150), (Fig. 4.16). The wavelength for

our laser is measured as 354.377 ± 0.016 nm, which is slightly lower than the 354.71 ± 0.01

nm found in the literature [58]. This difference can be attributed to the aging of the pump

diode of the laser [59]. We have then measured the wavelength of the laser as a function of

pump diode current to look for possible changes in laser wavelength due to heating of the

crystal [58] (Fig. 4.17). No systematic drift in wavelength was observed, indicating that the

temperature of the laser crystal did not vary significantly with the pump diode current.

The measured wavelength for our laser corresponds to a single photon energy of 3.49865

± 0.00015 eV. This adds a systematic uncertainty 1 meV at 7 eV and 4 meV at 77 eV.

4.5 Detector Calibration Uncertainty

We use a linear fit to the five energy peaks between 70 and 84 eV to calibrate the laser

spectrum with high accuracy. For a single 5 minute interval, the calibration peaks jointly

contain ≈40% of the total counts and have uncertainties below 20 meV. For a full day, each

peak contains over 150000 counts and has an uncertainty below 3 meV.

In addition, we consider the systematic uncertainty of the energy calibration due to the

offset of the laser spectra relative to the source spectra. The offset scales with laser intensity,

and moves the calibration peaks to higher energy compared to the 235mU signal, which is

56

Figure 4.16: The laser wavelength (greencurve) was calibrated using the 253 and 365nm lines of a Hg vapor lamp (gray curve).

Figure 4.17: The calibrated laser wavelengthshows no dependence on the pump diodecurrent for the Nd:YVO4 crystal. A changein wavelength would be expected if the tem-perature of the crystal changed with thepump diode current.

acquired when the laser is off. Figure 4.18 shows the change in laser peak positions as

a function of offset. For zero offset, the position reduces to the actual energy value. The

accuracy with which we can determine this offset for finite laser intensity adds an uncertainty

of ±10 meV to the detector calibration, which dominates the calibration accuracy.

We also considered a potential non-linearity of the detector as a source of calibration

error. Since all calibrations use the same five peaks, any non-linearity would contribute

equally to all peaks and cause a systematic shift in residuals of the peak at zero offset.

The average residual of the five peaks is -4.6 meV for the values in figure 4.18. Since the

uncertainty of the detector calibration is 10 meV, the observed offset cannot be distinguished

between random statistical fluctuations and a detector non-linearity.

We have confirmed that the detector non-linearity is negligible by extending the measured

energy range and searching for alpha-induced carbon and oxygen fluorescence (Fig. 4.19). It

turns out that weak C K and O K fluorescence signals are visible above the background in all

57

Figure 4.18: Peak energies of the shiftedlaser between 20 and 24 photons as a func-tion of the calibration offset. The fit to eachdistribution enables an estimate of the signalcalibration uncertainty.

Figure 4.19: Alpha induced fluorescence ofoxygen and carbon from the surfaces insidethe ADR and source. The measured energiesare within the accepted values of 277 ± 2 eVfor C and 524.9 ± 0.7 eV for O.

four detectors for acquisitions lasting more than a few days. The lines are very weak because

they likely originate from the mylar behind the 239Pu source, and the thin Al film on the

mylar absorbs most of the fluorescence. Given that the STJ detectors are calibrated around

77 eV only, it is striking that these weak lines are visible at all. For our STJ calibration at

70 to 84 eV, the measured energy of carbon K is 278.4 ± 0.8 eV and for oxygen K is 524.0 ±

0.6 eV. These values are consistent with the literature values of 277 ± 2 for carbon and 524.9

± 0.7 eV for oxygen [60]. We conclude that the non-linearity of the detector is negligible in

our measurements.

4.6 Partition Width Uncertainty

Drifts in the STJ response and fluctuations in laser intensity require piecewise calibration

of the spectra, and the choice of the interval length may introduce systematic errors into

the calibration. Short partitions would be more robust against fast changes, but suffer from

58

Figure 4.20: A subset of the data was re-processed using different partitions widths.There is a 2 meV uncertainty for the aver-age centroid value due to the variation inpartition width.

Figure 4.21: A subset of the data was repro-cessed using different histogram bin widths.There is a 4 meV uncertainty for the aver-age centroid value due to the variation in binwidth.

increased uncertainty in the calibration. Long partitions will be more susceptible to changes

in laser intensity. The use of a 5 minute partition was chosen as a compromise between

these conflicting requirements. To quantify the systematic uncertainty due to this choice,

a subset of the data with large statistics was reprocessed using partition widths between 2

and 8 minutes. The position of the 235mU centroid for different partitions widths was de-

termined experimentally (Fig. 4.20). The statistical uncertainty of the centroids is roughly

20 meV, and the observed variation in the centroids for different partition widths is signif-

icantly smaller at 2 meV. The exact choice of partition width is therefore not particularly

consequential.

4.7 Bin Width Uncertainty

To add histograms, their bin sizes must be equal and their bin edges must be aligned. After

calibration, neither of those statements are generally true, and the resulting spectra need to

59

be transformed into spectra with common binning. The choice for the bin width in these

experiments is 0.2 eV, because it is ≈10x smaller than the energy resolution of the STJ and

allows an accurate measurement of centroid values. To quantify the systematic uncertainty

due to this choice, we reprocess the 5 minute partition data from before using histogram

bin widths between 0.1 and 0.8 eV (Fig 4.21). The statistical uncertainty of the centroid

values in the individual fits is roughly 20 meV, and the observed variation for different bin

width is significantly smaller at 4 meV. Therefore rebinning of the data does not contribute

significantly to the overall uncertainty.

4.8 Fit Range and Background Uncertainty

The spectral background observed at the energy of the 235mU decay is due to a mixture

of low energy electrons, high energy beta particles that go through the collimator, and

secondary excitations caused by the alpha particles. Some effects can be removed using

coincidence vetoing, but the majority are not fully understood. We therefore do not know

the functional form of the background under the 235mU signal. To estimate the uncertainty

of the 235mU centroid due to the choice of background we approximate the background by

different functional forms and determine the 235mU centroids for fits over different ranges.

We use exponential decays, polynomials and inverse functions,

p0√2πp22

e− (E−p1)

2

2πp22 +Background =

p3p4e− E

p4 (SingleExp.)

p3p4e− E

p4 + p5p6e− E

p6 (DoubleExp.)

p3 + p4E (Linear)

p3 + p4E + p5E2 (Quadratic)

p3p4+E

(Inverse)

(4.1)

60

Source Uncertainty (meV) Method of Determination

Statistics 13 Statistical uncertainty of sum spectrumDetector Calibration 10 Uncertainty of offset correctionBackground-Range 6 Variation of fit function and rangeLaser Calibration 4 Calibration using Hg lampEnergy Bin 4 Choice of histogram bin widthPartition Width 2 Choice of time interval

Total: ± 13Stat ± 13Sys Added in quadratureFinal: 76.737 ± 0.018 eV

Table 4.2: Uncertainty budget for the 235mU nuclear decay energy

where p3 to p6 are the fit parameters we use in addition to p0 to p2 for the Gaussian

function to fit the isomer decay. As before, a subset of the data with high statistics is

processed assuming these five background functions and fit ranges between 20 and 100 eV

(Fig. 4.22). At a fit range of 100 eV there are clear deviations from the measured background

for all fit functions except the background given by the sum of two exponential functions.

However, for fit ranges below 55 eV, the different fits begin to match the data and become

indistinguishable from one another within the statistical accuracy of the measurement (Fig

4.23). The uncertainty of the centroid due to the choice of background function is calculated

from the observed variations as 6 meV. In this work, we use a sum of two exponential decays

as a background with a fit range of 50 eV.

4.9 Uncertainty Budget

The total uncertainties are summarized in Table 4.2. The statistical uncertainty of the four

individual detector measurements has an average value of 25 meV. The fit values for the

four detectors are shown in figure 4.24. The four measurements are in very good agreement,

within 1 - 2 standard deviations of each other. The nuclear decay energy of the 235mU state is

extracted from the fit to the sum of all data as 76.737 ± 0.013stat eV. This is an improvement

by a factor of 25 in precision and accuracy over the prior measurement of the 235mU nuclear

decay energy.

61

Figure 4.22: Example of fits to the 235mU signal for different functionalforms of the background over a range of 100 eV. For this large range, thereare significant deviations from the background for all functional formsexcept the sum of two exponential decays (red curve). For shorter fitranges, functional forms converge and are statistically indistinguishablewithin the accuracy of the measurement.

62

Figure 4.23: Inferred 235mU energies for dif-ferent functional forms of the background anddifferent fit ranges. Best fits were for rangesbelow 50 eV for all functions except the inverserelation.

The largest contribution to the systematic uncertainty is from the detector calibration.

Based on statistics alone we would expect this contribution to be much smaller considering

that the laser peaks can be determined with 1 meV precision. However, the offset due to

substrate phonons can only be determined to an accuracy of ±10 meV, which dominates the

uncertainty of the detector calibration.

The second largest contribution to the systematic uncertainty is from the background

and fit range. The effect of the choice in background on the centroid value was estimated

using five different functional backgrounds over different fit ranges. The background is best

approximated by the sum of two exponential decays for a large fit range. For short fit ranges

the fits to the background are statistically indistinguishable from each other.

The calibration of the laser energy was paramount to keeping the overall uncertainty low,

because the uncertainty scales with the number of observed photons (22 at 77 eV). The high

precision here is a result of using a UV grating spectrometer with a well known calibration

63

source from a Hg vapor lamp.

The systematic uncertainty from rebinning in order to sum spectra was small, and so

was the uncertainty due to partitioning the data into different time intervals to correct for

drift in the detector responsivity. The fact that the binning uncertainty is larger than the

partition uncertainty is due to binning effects when the bin size exceeds ≈FWHM/5.

The energy measured here is in very good agreement with the current literature value

of 76.8 ± 0.5 eV, which was determined using photo-electron spectroscopy [56]. In the

earlier experiment, a 239Pu source was used to embed 235mU recoil ions in a UF4 foil with

the expectation that the recoils would displace other U atoms and form 235mUF4 in the

foil. The conversion electrons from the decaying 235mU were then compared to the energies

of photo-electrons excited by Alkα X-rays in the same film. If the 235mU atom had not

been present as 235mUF4, the measurement may have been affected by chemical shifts. Our

measurements match the earlier value of the 235mU decay very well, and we may conclude

that the chemical state of the isomer was in fact UF4 as they expected for their measurement.

We recommend a new value of 76.737 ± 0.018 eV for the energy of the first excited state in

235U.

64

Figure 4.24: Summary of all experiments using a total of four detectors.The gray line and shade is the fitted centroid and uncertainty for the sumof all data.

65

Chapter 5

229mTh Measurements

This chapter summarizes the search for the nuclear decay of 229mTh. It covers distinguishing

recoils from alpha particles and methods used to reduce the low energy background. It also

discusses why no signal from the decay of the 229mTh is observed, and how to improve the

experimental setup to enable such an observation.

5.1 Alpha-Recoil Identification

Since the alpha decay of 233U populates the 229mTh state with a branching ratio of only

2%, the expected 229mTh signal will be 50 times weaker than the observed 235mU signal.

Additionally, the decay energy of 7.8 ± 0.5 eV for 229mTh is an order of magnitude lower

than for 235mU, and the background below 10 eV is extremely high (Fig. 5.1). The low

energy background from 233U would overwhelm any signal from 229mTh. In order to observe

the decay of 229mTh, the low energy background would need to be reduced by 4 - 5 orders

of magnitude. To accomplish this, we exploit the fact that a 229mTh signal can only occur

soon after the impact of a recoil ion, and restrict the search to these conditions.

Impacts from recoil ions can be distinguished from alpha particles based on their interac-

tion with the STJ detectors. Alpha particles will go through the detector and be absorbed

by the Si substrate. They will deposit over 95% of their energy in the Si, and generate

66

Figure 5.1: The signal from the 233U source (blue) has a very large lowenergy background. To observe the 229mTh nuclear decay the backgroundneeds to be reduced by 4 - 5 orders of magnitude.

67

Figure 5.2: Alpha particles pass through theSTJ and are absorbed in the Si substratecreating very energetic phonons that areobservable by all detectors. Recoils stop inthe first 10 nm of the STJ surface, and anyphonons that escape from the STJ into the Sisubstrate have energies below the thresholdto break Cooper pairs in other detectors.

phonons that propagate uniformly into a 4π solid angle (Fig 5.2). When these energetic

phonons propagate through the Si to the neighboring STJ detectors, they can be absorbed

and deposit sufficient energy to be observable as large pulses. On the other hand, recoil ions

are stopped in the first 10 nm of the Ta absorber of the STJ based on SRIM simulations

[61]. They carry up to 90 keV of kinetic energy, but since the entire energy is converted to

low-energy excitations inside the STJ, none of the phonons that escape into the Si substrate

have enough energy to produce a signal in any of the neighboring detectors. Recoils are thus

observed as large pulses in a single detector, with a pulse height similar to alpha particles.

To capture different types of signals, the data acquisition system is operated in full trace

acquisition mode at the lowest digital gain, using the four STJ detectors on a single chip.

Alpha particle events are observed regularly since they can pass through multiple pinholes in

the Si collimator and produce a large simultaneous signal in all four detectors (Fig. 5.3). The

magnitude of the observed alpha signal is on the same order across all detectors, because the

detector response saturates for very large deposited energy. The alpha particle arrival times

vary by less than 1 µs between the detectors due to the short distances between detectors and

68

Figure 5.3: ADC trace of an alpha eventon multiple detectors. There is some initialcrosstalk at t = 0 and a short arrival timedelay <1 µs for the other detectors observingthe impact (Inset).

Figure 5.4: ADC trace of a recoil eventon multiple detectors. The crosstalk isreadily visible at t = 0 as a negative pulsewhich returns to the baseline in 10 µs (Inset).

the speed of sound in Si. A small electronic crosstalk is observed as a dip in the waveform

at t = 0. Alpha particles may thus be identified based on the observation of large pulses in

multiple detectors with arrival time differences < 1 µs. Recoil ions are observed at a lower

count rate (Fig. 5.4). The detector in which the recoil is embedded observes a pulse on

par with an alpha impact, but the remaining detectors observe no signal beyond the small

electronic crosstalk.

The most straightforward way to separate recoil events from alphas would therefore be

to look for a single large signal in only one of the four STJ detectors. At the high digital gain

needed to measure signals of a few eV, an alpha should produce four off-scale signals, while

a recoil should produce only one. Unfortunately, when using this criterion at high gains, any

radioactive source background signal that is off-scale will appear as a false positive recoil.

Fortunately and serendipitously, the crosstalk from the recoil impact in the other detectors

is transformed into a bipolar signal by the trapezoidal filter of the acquisition systems (Fig.

69

Figure 5.5: Traces of a recoil event on multiple detectors after trapezoidalfiltering. The filtered crosstalk generates a small bipolar response in theother detectors with a delay of 2 - 5 µs in the arrival time between detec-tors. The filtered signal of the recoil impact is over one hundred timesbigger than the bipolar signals (inset).

70

5.5). This crosstalk will be recorded as a ’low energy’ event in coincidence with the other

detectors. Only recoil impacts have enough energy to produce a measurable crosstalk signal

in other detectors. Recoils may thus be identified based on the coincident observation of a

large pulse in one detector and significantly smaller pulses in other detectors with an arrival

time difference between 2 and 5µs. We may then use the variation in the time of arrival

between the coincidence observation to distinguish recoil ions from alpha particles.

For these experiments, the data acquisition system is operated in list mode on the lowest

digital gain with two (208 µm)2 and two (138 µm)2 STJ detectors. Only coincident events

<10µs apart in which at least one detector observes an energy well above the low energy

background are considered. We look at the difference in arrival times between signals in the

two larger detectors tSTJ 6 - tSTJ 5 relative to the arrival time differences in the two smaller

detectors tSTJ 8 - tSTJ 7 (Fig. 5.6). The alpha particles are very prominent at the center in

the shape of a lopsided hourglass with arrival time differences below ± 1 µs on both axes. The

recoils are observed on the axes at ± 3 µs, and the signals in different quadrants correspond

to the different detectors in which the recoil was embedded. The remaining background is

likely from high energy beta particles that pass through the Si collimator and uniformly hit

the substrate.

To confirm that these events do correspond to recoil impacts, a control experiment was

conducted with the 233U source facing away from the detectors. This removes the recoil

ions, which cannot pass through the Al mylar foil that the source is electroplated onto (Fig.

5.7). As before, we look at the difference in arrival time between the two larger detectors

relative to the two smaller detectors (Fig. 5.8). The alpha particles are still very prominent

at the center in the shape of a lopsided hourglass. However, the recoils on the axes have

disappeared, while the beta background remains.

We now restrict the analysis to events within the circled region in figures 5.6 and 5.8 that

are dominated by recoil ions. A histogram of these events is shown in figure 5.9. For the

spectra with the 233U source facing the detectors, a large peak with a wide energy distribution

71

Figure 5.6: Time of arrival between coincident events with the 233U sourcefacing the STJ detectors. The prominent feature at the center in the shapeof an hour glass are caused by alpha particles. The groups of events oneither side of each axis with large time delay is indicative of recoils.

72

Figure 5.7: Setup for observing alpha particleswithout recoils. The side with the 233U sourceis turned to face away from the detectors sothat recoil ions are absorbed by the mylar un-derneath.

is observed. The peak is attributed to the recoil ions, and it is wide because the recoils can

escape from different depths within the source resulting in kinetic energies between 0 and

90 keV. When the source is facing away from the detectors, the spectra have a non-zero

background due to beta particles, but no recoil peak is present. This is expected because

recoils cannot pass through the Al mylar. The number of counts in the recoil peak for the

(208 µm)2 devices is roughly twice the counts in the (138 µm)2 devices as expected based on

detector area. Furthermore, the corresponding coincident events for the recoils deposit low

energy in the other detectors, as expected for the recoil crosstalk. This confirms that the

events with large arrival time differences are due to recoil ions, and may thus be distinguished

from alpha particles using only timing information.

5.2 Reduction of 233U Background

As before, experiments were performed using the 233U source, the pulsed 355 nm laser, and

four STJ detectors on a single chip. The acquisition system was set to acquire in list mode

with high digital gain and the TTL signal from the trigger of the laser. The laser was

operated at 100 Hz at low intensities so that the calibration signal was centered at 7 eV. All

detectors had sufficient resolution for accurate energy calibration (Fig. 5.10), and were used

73

Figure 5.8: Time of arrival between coincident events with the 233U sourcefacing away from the STJ detectors. The lop-sided hour glass at thecenter are the alpha particles that passed through the pinholes of thecollimator. The background is due to beta particles that pass through theSi collimator. Compared to figure 5.6, no more recoil events are visible inthe circled regions.

74

Figure 5.9: Spectrum of the events in the circled region in Fig 5.6 and5.8. When the source faces the detectors (green), recoils with a widedistribution of energies are visible. When the source is reversed (blue),recoils disappear.

75

Figure 5.10: Calibrated laser response of two (138 µm)2 and two (208 µm)2

STJ devices.

to mark events as coincident when they were <10µs apart across detectors to distinguish

recoil ions and alpha particles. As with the 235mU data, the laser is used to perform a

linear calibration of the STJ detectors and the constant offset due to substrate phonons is

discarded.

To reduce the 233U low energy background we now apply different cuts to the data

(Fig. 5.11). All events in coincidence that are not distinguished as recoils are discarded first.

These events consist mostly of alpha particles that went through a pinhole and beta particles

with sufficient energy to penetrate the Si collimator. This reduces the background by over

two-thirds, which is significant considering that these events only represent the fraction of

76

Figure 5.11: STJ response to 233U (blue) source. Several cuts can beapplied to the data to reduce the background at low energies. The firstcut to the data is discarding all high energy coincidence events (green)and an additional 100 ms after impact (cyan). Looking at up to 100 ms(magenta) or up to 10 ms (red) after recoil impact greatly reduces the lowenergy background. The expected background per 10 ms (black) is thetime averaged background from 10 to 100 ms.

77

Figure 5.12: Calibrated laser response be-fore and after an alpha impact. A pertur-bation immediately after impact is observedthat affects the energy of the laser peaks andincreases the low energy noise peak abovethe trigger threshold.

Figure 5.13: Calibrated laser responsebefore and after a recoil impact in thedetector. A perturbation similar to analpha event is observed.

alphas that go through the pinholes in the collimator and not any secondary excitations.

We observe that the background is reduced by an additional factor of three by widening

the veto window to include 100 ms after an alpha particle impact. This was unexpected,

because these events turn out to be normal low energy events. However, their energy has

been shifted up due to the absorption of a high energy alpha or beta particle. It appears

that the STJ response is changed after a high-energy event for time scales much larger than

the ≈ 150µs that it takes the filtered signal to return to the baseline. Still, rejecting the

background caused by high energy particles can only reduce the total background by about

one order of magnitude, which is nowhere near the 4 - 5 orders of magnitude needed to

observe a 229mTh signal.

To develop an understanding of the STJ behavior immediately after a high energy alpha

impact, we look at the calibrated laser response from 35 ms before to 35 ms after any high

energy event (Fig. 5.12). The calibrated laser peaks are observed as bands of high point

78

Figure 5.14: Calibrated laser response inneighboring STJ detectors before and andafter a recoil impact in another detector.No perturbation is observed.

Figure 5.15: Calibrated laser response beforeand after a high energy impact for a few ms.The detector has a dead time of ≈ 0.25 msand lacks the resolution to resolve the laserpeaks until ≈ 1 ms after impact.

density, and the laser period is observable as ‘missing’ data at integer multiples of 10 ms.

Before impact at t = 0, the energy for each laser peak is constant because the laser has been

calibrated. Immediately after impact, the observed energy for each laser peak increases, and

only returns to the calibrated value after ≈12 ms. This appears to be similar to the phonon

generation in the substrate by the laser; however, the time scale of 12 ms is much larger than

what was observed with the laser. Additionally, there is a large increase of the noise peak,

which was below the trigger threshold before the high energy impact of an alpha particle. A

similar effect is observed in the laser response before and after a recoil impact in the detector

(Fig. 5.13). Since recoils are stopped in the first 10 nm of the STJ Ta absorber, we know that

this effect is unrelated to the Si substrate. This is confirmed in the response of neighboring

detectors when a recoil impact occurs in one of the STJ detectors, which do not show any

deviation from standard operation (Fig. 5.14). We believe that the STJ detector is simply in

a perturbed state after the violent impact of a high-energy particle. Most likely, low-energy

79

phonons remain in the STJ for time scales well beyond the decay of the current signal. These

phonons add energy to any subsequent event until they have completely escaped into the

Si substrate. The long time period that the STJ detector is operating in the perturbed

state after impact by a high energy event needs to be rejected in the search for the decay

of 229mTh. Alternatively, to include the data immediately after recoil impact and enable a

search for short decay times, we need a separate, time-dependent STJ calibration using only

the laser events between 0 and 12 ms. This secondary calibration would be complicated

by the short timescale (ms) compared to the drift corrections performed for the detector

calibration (min). At even shorter time scales, the detector has a dead time of ≈ 250 µs

before recording pulses again, albeit in the perturbed state (Fig. 5.15). This limits the

isomer half-life time scales that can be observed with this setup.

At the start of this project, the 229mTh half-life was unknown, and estimates ranged from

µs to hours with estimates around ≈1 ms considered most likely. We can therefore greatly

suppress the background by considering only time intervals at different lengths after a recoil

impact. The shorter the time interval, the better the background suppression at the risk of

missing the 229mTh decay if its half-life is longer.

If we consider only data within the first 100 ms after a recoil impact, the background

around 8 eV is reduced by almost 3 orders of magnitude, just shy of the 4 orders of magnitude

needed to detect a 229mTh signal for data acquisition times of 5 days (Fig. 5.11). If the data

are restricted to the first 10 ms instead, the background around 8 eV is reduced by an addi-

tional factor of ≈2. This is unexpected because the background should scale proportionally

to the collection time; however, the perturbed STJ responsivity during the first 10 ms after

recoil impact increases the observed energy of low noise events resulting in an artificially

high background. The true radioactive background with the detector in its equilibrium state

can be measured in the interval between 10 and 100 ms and scaled by a factor of 9 to get the

expected background per 10 ms. For this type of background we project that looking at 10

ms time interval would generate ≈ 300 background counts per 0.2 eV bin at 6 eV, and ≈100

80

background counts per 0.2 eV bin at 9 eV for 5 detector days and a single (138 µm)2 STJ

detector. This is at the predicted threshold for observing the nuclear decay in 229mTh (Fig.

5.1). It can be further increased by operating multiple detectors over larger time periods.

The signal-to-background ratio in the spectra of the isomer decay can be further improved

by subtracting any background that is known not to contain 229mTh signals. For this we

directly measure the background under identical conditions when no 229mTh decay is present

by considering only times just before the arrival of the recoil. The histograms in figure 5.16

represent the time cut from 20 to 50 ms and -50 to -20 ms relative to the recoil impact

for a total acquisition time of 5 days with a single (138 µm)2 STJ detector, excluding the

perturbed 10 ms time interval of the detector directly after a recoil impact. The two spectra

are nearly identical, indicating that the time before a recoil impact is a proper representation

of the source background with no isomer decay present. The difference spectrum of the two

curves will remove the background, leaving behind only the statistical variation and any

nuclear decays from an embedded 229mTh. This method reduces the low energy background

by varying amounts depending on the search interval, well below the needed levels provided

the half-life is long enough. This shows that we can reduce the background by 5 orders of

magnitude or more depending on the time interval we choose for the search of the nuclear

decay energy and a particular half-life of 229mTh.

5.3 Search for 229mTh

The 7 ± 1 µs half-life of the nuclear decay of 229mTh was not known at the start of this

work, and available theoretical values predicted widely differing values from 10−6 to 104

seconds. A particular problem of the available theoretical estimates was that they were

calculated assuming a nuclear decay energy for 229mTh of 3.5 ± 1 eV, the earlier accepted

value. Assuming this energy, decay by internal conversion would be prohibited, and only

radiative decay would be possible. Based on the currently accepted value of 7.8 ± 0.5 eV,

81

Figure 5.16: 233U spectra for search times between 20 and 50 ms aftera recoil impact when a 229mTh isomer would potentially decay (lime),compared to the spectrum -50 to -20 ms before a recoil event when no229mTh isomer decay is possible (brown). Their difference (violet) shows nopeak at 7.8 eV, which would be observable if the half-life were moderatelylong (inset).

82

Figure 5.17: Reduced background with isomer signal reduced

internal conversion is possible. This makes earlier estimates of the half-life unreliable, but

new values are expected to be skewed towards shorter times.

The current limitations of our experimental setup force us to discard 10 - 20 ms of data

after a recoil impact due to the perturbed state of the STJ detector. This limits the possible

isomer half-life times we would be able to observe based on the additional isomer signal

reduction, especially when the half-life is short (Fig. 5.17). At the current background levels

we would reasonably be able to resolve an isomer peak at 80% of the expected signal strength

within 5 detector days. This corresponds to an isomer half-life of over 100+ ms (Fig. 5.18).

83

Figure 5.18: The fraction of observed isomersignal as a function of the search interval.For short half-lives of the 229mTh isomer, alarge fraction of the signal will have decayedbefore the detector returns to its equilibriumresponse after 20 ms (black).

At shorter half-life times a larger fraction of the excited state will decay before the detector

has returned to standard operation, and the search time would have to be extended.

A total of 7.5 days of data was taken using the four STJ detectors. During this time, we

observed a total of 14244 recoils for all detectors: 5622 recoil events in STJ 5 (208 µm)2, 3327

in STJ 6 (208 µm)2, 2831 in STJ 7 (138 µm)2, and 2464 in STJ 8 (138 µm)2. The observed

number of counts in the (138 µm)2 STJ detectors agrees with expectations based on the

400 isomers per 24 hours we observed with the 235mU measurements. The difference in the

number of recoils between the larger detectors is due to a non-uniformity in the radioactive

source. Assuming a 2% branching ratio, we expect our data to contain ≈ 285 229mTh recoil

isomer decays distributed among the four detectors. We look at the time from 15 to 100 ms

before and after the recoil impact (Fig. 5.19). There was no discernible difference between

spectra taken before and after recoil impact. Subtracting the two spectra for each detector,

no peak is observed in any of the four detectors or the total spectra (Fig. 5.20). Neither did

84

Figure 5.19: STJ response to the 233U source for 7.5 days of data acquisi-tion, for search intervals from 15 to 100 ms before (green) and after (blue)a 229Th recoil impact.

85

Figure 5.20: STJ response to 233U source, difference between the beforeand after spectra in figure 5.19. The sum total (black) spectrum shows noisomer peak for the given time cut of 15 to 100 ms.

86

Figure 5.21: Expected 229mTh signal from a single (138 µm)2 STJ detectorassuming 285 counts, no background (magenta), 1 ms of background (red),10 ms of background (green) and 50 ms of background (blue).

87

Figure 5.22: Projected acquisition spectrafor a single (138 µm)2 STJ detector assum-ing a half-life of 10 ms and search intervalbetween 15 to 45 ms.

Figure 5.23: 36-Pixel array of (208 µm)2

STJ detector.

we observe a 229mTh signal for any other search interval we examined.

We may project what an isomer signal with 285 counts at 7.8 eV would look like for a

detector resolution of 1.6 eV FWHM and different background levels (Fig. 5.21). For this

we estimate the background level of the STJ with its equilibrium state for different search

intervals, and add the expected 285 isomer counts, assuming that all the isomer decays

are observed. For a search interval of 1 ms the expected isomer peak would be well above

background assuming the full signal was collected, for a search interval of 10 ms the peak is

barely observable, and for a search interval of 50 ms no peak would be discernible above the

background noise.

For a single (138 µm)2 STJ detector we extrapolate the days needed to observe an isomer

with a half-life of 10 ms and a search interval between 15 to 45 ms after recoil impact (Fig.

5.22). Under these conditions we would expect to observe ≈21% of the total isomer decays,

and the difference method would enable a small peak to be above the noise at ≈ 800 detector

days. A large peak with good statistics would take between 3200 and 4800 detector days.

88

This suggests that we would be able to observe a signal from 229mTh with one of our existing

32 pixel arrays of (208µm)2 STJ detectors in 12.5 days, and with good statistics in 50 to 75

days (Fig. 5.23). If in addition, we use a time-dependent energy calibration after an impact

of a recoil ion so that we can capture calibrated data after 1 ms of the impact, the signal

from the 229mTh could be increased from 21% to 89%, which would decrease the needed

acquisition time by a factor of four, or enable searches intervals for lower half-lives.

5.4 Future Work

The explanation for the lack of a signal from the isomeric decay of 229mTh came in mid-2016

when researchers from the European nuClock consortium published the first direct obser-

vation of that decay. The measurements showed that the half-life for 229mTh exceeded 60

seconds as long as the Th ion remained at least doubly ionized, but was significantly shorter

when the Th was embedded in a solid and neutralized [30]. Shortly afterwards, they pub-

lished measurements for the half-life of neutral 229mTh of only 7 ± 1 µs [3]. This half-life is

much shorter than the decay time of our STJ signals, so that our detector response will not

have decayed to zero after a recoil impact by the time the 229mTh isomer decays into the

ground state. We therefore cannot expect to see an isomer signal in the collected data with

our current setup.

However, the nuClock papers also showed a relatively simple way our setup can be im-

proved to measure the energy of this decay despite its short half-life. In fact, the short

lifetime of the nuclear decay of 229mTh could be made to work in favor of a measurement

with our experimental technique by combining it with the recoil thermalization and separa-

tion steps of the nuClock experiment. Using a buffered-gas stopping cell with a quadrupole

mass separator the 229Th ions can be thermalized and embedded in the detector with kinetic

energies of tens of eV [62]. Since the STJ detector rise time of ≈ 6 - 8 µs is comparable to

the half-life of 229mTh, the kinetic, ionization, and nuclear decay energy of the 229mTh ion

89

could then be measured as a single event. Because the alpha decay of 233U populates the Th

isomeric state with a branching ratio of only 2% we would naturally observe two peaks. The

first and larger peak corresponds to a 229Th ion in its ground state, and its energy is given

by the sum of the kinetic and ionization energy of the 229Th ion. The second peak with an

intensity of 2% of the first peak corresponds to a 229mTh ion in its first excited state, and its

energy is given by the same kinetic and ionization energy of the 229Th ion plus the energy

of the nuclear decay of the isomer.

For this improved experimental setup a differential pump can be used to accommodate

potential pressure differences between the quadrupole mass separator and our cryostat. The

IR blocking windows of our ADR need to be replaced with two pinholes of a few mm in

diameter to provide a direct line of sight to the STJ detectors for the ion beam from the

quadrupole mass separator. These pinholes may need to be cooled to reduce the heat load

to the ADR from thermal radiation. In addition, the acceleration voltage of the 229Th ion

beam needs to be well controlled to ensure that variations in kinetic energy do not degrade

the width of the observed peaks beyond the value set by the energy resolution of the STJ.

Since none of the nuClock experiments have measured the energy of the decay of 229mTh,

the need for this experiment is still as high as it was at the beginning of this work.

90

Chapter 6

Summary

This project set out to measure the energy of the nuclear decay of 229mTh with high accu-

racy. This excited state is currently of great interest to the scientific community because

it resides in the UV region at 7.8 ± 0.5 eV and therefore within the range of current laser

technology. This makes 229Th the only candidate for the development of a nuclear clock with

an unprecedented accuracy of 10−19. A nuclear clock would e.g. enable new fundamental

research such as the search for temporal variation in fundamental constants.

To measure this energy, we have developed a new technique for characterizing low-energy

metastable nuclear states. The technique relies on embedding a metastable nucleus inside a

superconducting high-resolution detector, and then measuring its decay into the ground state

with high accuracy. For our experiment we use superconducting tunnel junction detectors,

which have an energy resolution of ≈2 eV FWHM at UV energies and can be operated at high

count rates. The high resolution enables measuring the centroid with improved uncertainty,

while the high count rate enables the use of a pulsed 355 nm pulse laser to continuously

calibrate the detectors with high accuracy. Additionally, by embedding the isomer inside

the STJ we can avoid any effects on the measured decay energy due to the chemistry of the

decaying atom.

We have demonstrated the strength of this technique by measuring the second lowest

91

known nuclear decay from 235mU as 76.737 ± 0.018 eV. This measurement is a factor of

25 improvement in accuracy over the current literature value of 76.8 ± 0.5 eV [56]. The

uncertainty of our measurement has a statistical contribution of 13 meV and a systematic

contribution of 13 meV, to be added in quadrature. The systematic uncertainty was found

to be dominated by the detector calibration uncertainty at ± 10 meV due to an offset caused

by the laser illuminating the substrate.

A similar measurement of 229mTh with this setup proved unsuccessful. The inability to

observe the 229mTh peak can be attributed to the short half-life of the decay, which was not

known at the beginning of this project, but was recently measured to be only 7 ± 1 µs [3].

The half-life is too short for the STJ detector to return to its equilibrium state after the

impact of the Th recoil. However, our experimental setup could be modified in the future to

measure the energy of the 229mTh despite its short half-life. For this, the kinetic energy of

the 229Th recoil needs to be reduced, so that the ions impact the STJ with an energy of only

a few 10 eV. In this case, the decay of the 229mTh simply adds to the observed signal height,

and the STJ would detect two peaks, a large peak at the kinetic plus ionization energy of

the 229Th ion in its ground state, and a smaller one at a higher energy due to the added

energy of the nuclear decay of 229mTh.

While we did not succeed in our initial goal to measure the nuclear decay energy of 229mTh,

the experimental approach in general worked very well, and can be adapted for 229mTh in the

future relatively easily. This makes this experiment an important step towards the ultimate

goal of developing a nuclear clock.

92

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