1

Report on The Study of (α,n) Neutron Yield and

Energy Spectrum

Dongming Mei for the AARM collaboration

2

Motivation • (α,n) neutrons produced in the materials, which will be used to build

the detector components for low background experiments, are important backgrounds

• Both neutron yield and energy are important• Cross sections in particular resonances should be validated• The calculation done for alphas from 238U and 232Th are not sufficient

– 228Th and 226Ra needs to be taken into account• The yield and energy spectrum in different materials are only

calculated by a few groups• The validation of the calculations are needed

– Start with independent comparisons– Measurements and benchmarks are also on the way

3

Calculations by the SNO collaborators

• R. Heaton et al., Nucl. Geophys. V 4, 499 (1990).

• R. Heaton et al., Nucl. Instrum. Methods Phys. Res. A 276, 529 (1989).

4

USD Calculations

• D.M.Mei, C.Zhang, A.Hime, NIMA606, 651-660 (2009)

• Http://neutronyield.usd.edu

5

Calculations using SOURCES

• http://www.ornl.gov/info/reports/1992/3445603684010.pdf

• Radial Prot Dosimetry, 2005: 155 (1-4) 117-21

6

Comparison (USD vs SOURCES)Implemented By European Scientists

• Marco Selvi (one of several) The overall agreement in the neutron yield is

anyhow quite good ,I found everything within a factor of 2, which is not so bad I would say.

7

Comparison Accomplished by US Scientists

• K.Palladino (MIT), H.Qiu (SMU), S.Scorza (SMU)

8

Modus Operandi• TENDL vs SOURCES4 lib cross sections

• TENDL 2011 and 2012 have been considered as the USD website inputs. TENDL is a nuclear data library (validated) which provides the output of the TALYS nuclear model code system

• SOURCES4 cross section input libraries come from EMPIRE calculations and for some isotopes a combination of measurements and EMPIRE calculations

• USD website vs SOURCES4 calculations: compare the radiogenic neutron spectra coming from both codes and some simulation quick checks for Cu.

9

X section comparison ✔Good agreement in the (alpha,n) ROI (0-10MeV) for most of the isotope considered – details http://www.physics.smu.edu/cooley/aarm/webpage.html

✖C13, O17, … : SOURCES4 inputs match TENDL 2011 at low energy and then match TENDL 2012 at high energy

SOURCES4 input – TENDL 2011 –TENDL 2012

Cu65 -> ok! C13

10

X section input libraries For many isotopes SOURCES4 cross section input libraries consist in a combination of measurements and EMPIRE calculations -> we believe SOURCES4 cross section libraries the right choice.

- EMPIRE is the code recommended by IAEA.

- Neither EMPIRE nor TALYS can calculate properly resonance behavior which has been experimentally observed (if we trust the data)

11

Copper check - Inputs

• SOURCES4 calculation considers – 63Cu = 70%, 65Cu = 30%– 1ppb 232Th in Cu (100% 232Th)– 1ppb U in Cu (99.28% 238U + 0.72% 235U)

• USD website considers– Nat Cu– 1ppb 232Th in Cu (100% 232Th)– 1ppb U in Cu (100% 238U)

12

Copper Check- Neutron Spectrum Comparison

13

Spectra Integration (n/s/cm3)

SOURCES4 Th: 9.49 E-12 n/s/cm3

U: 2.90 E-12 n/s/cm3

USD website Th: 1.11 E-11 n/s/cm3

U: 3.46 E-12 n/s/cm3

SOURCES4/USD Discrepancies

Thorium ~15%Uranium ~13%

14

Simulation checkWe have performed some quick simulations: propagate both USD and SOURCES4 radiogenic neutron spectra in the same experimental geometry to check the background neutron rate

• 1 Million neutrons from U and Th decay chains each in a simple geometry: 1 Cu can (21760.6 cm3) around 100kg of germanium detector.

• The same Cu can has been contaminated with both USD and SOURCES spectra

• Cu contamination level: Th: 0.02mBq/kg U: 0.1 mBq/kg • Neutron rate from (alpha,n) reactions and has been reported

15

SOURCES4/USD ~20% discrepancy in the rate found in Ge detectors

USD rate > SOURCES4 rate

16

• Glass composition from Hamamatsu, more elements and higher neutron yields than just silicon, oxygen and boron due to inclusion of 4% each by mass fraction inclusion of sodium, aluminum and barium

Borosilicate glass check

17

Borosilicate Spectra Integration (n/s/cm3)

SOURCES4 Th: 1.27 E-10 n/s/cm3

U: 3.63 E-10 n/s/cm3

USD website Th: 6.98 E-11 n/s/cm3

U: 2.45 E-10 n/s/cm3

SOURCES4/USD Discrepancies

Thorium ~82%Uranium ~48%

18

Borosilicate Simulations• Simulations done within RAT, utilizing Geant4.9.5, and a cylindrical

45T liquid argon single phase detector surrounded by borosilicate glass mimicking PMTs as well as stainless steel and a water veto

• Simulation done with 1.65 times more thorium than uranium matching assayed values of glass (and old simulations)

# Simulated Events 12-25 keVee

& >65 cm from wall

& PSD cut MC single scatters in ROI

USD 5000000 11651 211 9 2Sources 5000000 12133 217 10 2

Borosilicate Conclusion: Shape differences are not a large effect, but at this assay value Sources would have 1.8x more neutrons produced than the USD code

19

Next steps• Check SOURCES calculations having Talys cross

section as inputs • Check USD calculations having Empire inputs (is

it possible?) • Cross check EMPIRE and TALYS cross sections

with other calculations• Benchmarking against the experimental nuclear data. (SOURCES has already provided some comparison

studies in the users guide but not for U/Th decay chains)

20

Conversion factor #->n/kg/yHere below the formula used for calculating the normalization factor needed to convert the number of neutron found in the IZip from simulation into a counting rate (n/kg/year).

21

Summary of the Comparisons

• Neutron yield agreement is within a factor of 2 for all materials compared so far

• The agreement in energy spectra from various materials is not good– Understand the cross sections including

resonances– The calculated kinetic energy of out-going

neutrons with respect to different excited states of the final nucleus

22

Neutron Screening Facility (LZ-veto)Gd-LS detector (Courtesy to the LZ collaboration))

23

LZ-Veto as a Neutron Screening The goals of this device are:• To further characterize the neutron and gamma ray environment adjacent to the LUX

liquid xenon, so as to better constrain possible background contributions to any apparent nuclear recoil signal from WIMPs.

• To screen a variety of components for neutron and gamma activity. Of particular interest are components containing fluorine like polytetrafluoroethylene (PTFE) used in noble-liquid dark matter detectors, including the LUX liquid xenon detector. Fluorine is particularly susceptible to emission of neutrons caused by impinging alpha particles, from radioactive contamination both on its surface and intrinsically. Such neutrons can potentially contribute to the background for a WIMP signal. In addition, this device can screen some materials more accurately for gamma-ray emission than standard germanium screening devices.

• To develop and establish safe and effective designs and procedures for deployment of Gd-LS deep underground.

• To conduct searches for rare processes involving bursts of neutrons, for example, the spontaneous fission of 232Th, or searches for super-heavy elements.

24

Future Work• Continue to work internationally as a collaboration• Monte Carlos Validation

– Energy spectra in different materials– Specifically focus on resonances in cross section– Include 228Th and 226Ra – Materials such as PTFE

• Measurements – USD neutron detector for measuring the (α,n) neutrons from

rock– Other measurements from various experiments– LZ-veto for measuring (α,n) the neutrons from various materials