Diego González-Díaz (Zaragoza University

and Tsinghua University), Stony Brook, 04-Oct-2012

1

TPCs for rare event searches

CAST (axion searches)

NEXT-100 (neutrino-less double beta decay)

T-REX (directional dark matter)ArgonDM (dark matter)……….

XENON (dark matter)

EXO-200 (neutrino-less double beta decay) 2

Microbulk Micromegas technology

x [μm]

y [μ

m]

50 μm

5 μm

115 μm50 μm

Main characteristics:

• Simple and robust all-in-one kapton-clad 2-copper sandwich structure.

• Very low outgassing and high radiopurity (<30 μBq/cm2 for 235U, 238U, 232Th chains).

• Multiplication takes place in ‘cells’. Geometrical UV-photon quenching seems to provide an improved stability at high pressures.

• Granularity demands are easily scalable.

copper

copper

kapton

Comsol simulation for a typical field configuration at high electron transparency: Edrift/Eamp~0.01

Very high quality of pattern!

Hector Mirallas

S. Cebrian et al, Radiopurity of Micromegas readout planes, Astropart. Phys. 34 (2011) 354-359

3

A sensible application for next-generation TPC experiments: ββ0-decay

Inverted mass ordering

Normal mass ordering

present ββ0-bounds constrained by ν-oscillations

cosm

olog

ical

con

stra

ints

A relevant figure of merit. Sensitivity to mββ: upper mass limit that can be claimed at 90%CL by a negative result in the next generation ββ0 experiments, as a function of their exposure.

end of inverted mass ordering landscape

Klapdor’s claim

4J. J. Cadenas et al., Sense and sensitivity of double beta decay experiments, JCAP(2011)

Why NEXT-100?

It covers a ‘technological gap’, providing simultaneously:

• Good topological information.

• Good energy resolution down to 0.5-1%FWHM@Qββ.

• Good prospects for scalability to 1Ton.

Canfranc

Underground Lab

2-blob ββ0 event at Qββ

background eventat Qββ

V. Alvarez et al., NEXT-100 Technical design report (TDR). Executive summary, 2012JINST 7 T06001 5

Why microbulk Micromegas?

• Flexibility for large area coverage.

• Possibly the only affordable technological concept that allows simultaneously for energy resolution and virtually unlimited tracking capabilities at high pressure and large areas (1-5m2).

Assets that will be surely useful for 1Ton experiments .

• Extremely radio-pure. .

• Not sensitive to the to signal .

• A priori compatible with electroluminescence .

Can improve resolution down to Fano factor.

Recover sensitivity to to.

work-line 1(this talk)

work-line 2(work in progress)

Why Xe-TMA?

1. They are known to form a Penning mixture, that is a desirable for energy resolution and maximum gain of gaseous detectors.

2. It increases drift velocity and reduces diffussion, recombination and attachment, enhancing the topological signatures.

• Not mature enough, specially for operation at high pressures.

work-line 1(this talk)

work-line 3(Dave Nygren et alat Berkeley)

1. Penning mixtures are known to reduce the Fano factor by 1/2-1/3.

2. If TMA fluoresces in the visible region one might expect to keep sensitivity to to and still being able to create electroluminescence (!).

J. Phys. Conf Ser. 309(2011)012006

6

General purpose chamber for R&D studies

Main characteristics:

• Fully stainless-steel vessel, h=10cm, ϕ=16cm.• Designed for standing pressures in the range 0-15bar.• Mini-TPC with microbulk Micromegas as anode.• Bake out system + turbo pump, allowing for vacuum down to 10-6mbar

after full TPC assembly.• Outgassing below 5x10-5 mbar l/s before gas filling.• Gas recirculation through SAES FaciliTorr + Messer Oxysorb

getters.• Characterization of system composition with a Pfeiffer OmniStar mass

spectrometer.• O2 and H2O impurities estimated (indirectly) to be below 30ppms in

running conditions. O2 and H2O –meters will be incorporated soon.• Acquisition with:

1) Canberra 2004/2022 amplifying chain + multichannel analyzer Amptek MCA 8000A.2) Oscilloscope.

3.5cm

10 cm

Micromegas(50μm gap, 50μm holes, 115 μm pitch)

Field cage: h= 1-6cm

10MΩ/resistors

radioactive sourcegoes here

7

Operation of Micromegas in Xe+TMA mixtures.General properties.

Good transparency even at 10bar. Only achievable through continuous gas purification.

Xe/TMA at ~98.5/1.5

Xe/TMA at ~98.5/1.5P = 8bar

Good description and good 1/√E scaling.

Diana C. Herrera

5

Used for this study.

S. Cebrian et al., Micromegas operation at high pressures in Xenon-Trimethylamine, arXiv

Find these results at:

8

too little too little

too little too little

too much too much

too much

too much

Penning at work-IDiana C. Herrera

9

Penning at work-II

Necessary increase of the field is much weaker than E/p mainly due to:• Scaling of α = αo p/po

• To a smaller extent to the dynamics of the Penning effect.

Drastic increase in gain at constant field with increasing concentration of TMA!

-> Presumably due to the activation of Penning-type energy-transfer mechanisms

Field for a gain=300 1.5-2.5% (optimal range is extremely narrow!)

Diana C. Herrera

up to x30-50 increaseat constant field

< x3

10

Strong exponential drop of the maximum achievable gain at high pressures.

-> Possibly due to the increased space charge at constant gain for high pressure. Is it possible to further improve?. Where is the limit?.

Xe/TMA at ~98.5/1.5

Xe/TMA at ~98.5/1.5

Energy resolution degrading at high pressure:

-> Operation at a much reduced E/p (down to 1/3-1/4) cools the electron swarm at high pressures.

22.1 keV

Pressure scan for Penning-optimized Xe/TMA mixtures (~97.5/2.5)

x400

Diana C. Herrera

11

pure Xe, C. Balan et al., 2011 JINST 6, P02P006

pure Xe, T. Dafni et al., J. Phys.: Conf. Ser. 309 (2011) 012009 (similar setup)

Xe/TMA, these measurements

𝜎𝑄𝑋𝑒𝛽𝛽 0

𝑄𝑋𝑒 𝛽𝛽0(2.48𝑀𝑒𝑉 )=0.9 % FWHM

Diana C. Herrera

within a factor x3 of the Fano factor limit for pure Xenon (0.27%FWHM)

Best energy resolutions for Penning-optimizedXe/TMA mixtures (~97.5/2.5)

12

pure Xenon

E [V/cm/bar]

v d [

cm/μ

s]Francisco IguazPreliminary modeling and scope

for large area TPCs

Preliminary comparison with TMA rather reasonable(despite TMA it is rated 3* in Magboltz at the moment)

241Amα

γ

Si-diode for triggering (to)

MM

drift region can be imaged

Region of maximum transparency in Xe/TMA mixtures

pushing the Magboltz-truth a step further…

x 4-5

E [V/cm/bar]

pure Xenon

DT

[μm

/cm

1/2 b

ar1

/2]

x 10!

13

Preliminary results with a medium size TPC

14

time-line

arrival pumping and bake-out system

field-cage

0.8cmx0.8cm pixelized microbulk Micro-Megas

T2K electronics (based on AFTER chip)

35cm30cm

15

First results for 1/4th of the readout plane (proof of principle)

57Co

241Am

Xe/TMA 96.3/3.7

Edrift = 170 V/cm,Eamp = 54 kV/cm,P = 1 bar

Unfortunately connectivity not yet perfect:~10/270 pixels are not properly connected.

We have recovered from a design problem by means of a tedious capacitive procedure that ensures a high (yet not perfect) connectivity. Work in progress.

Laura Segui

16

𝜎𝑄𝑋𝑒𝛽𝛽 0

𝑄𝑋𝑒 𝛽𝛽0(2.48𝑀𝑒𝑉 )=3 % FWHM

Next steps:

• Channel equalization

• Optimization of pedestal subtraction.

• Event filtering (for instance, sudden noise explosions).

• Improved fiducialization.

• Specially, development of an adequate analysis for this new stage.

Energy resolutionLaura Segui

17

some events at around 60keV Laura Segui

18

some events at around 90keV

x-ray candidate

Laura Segui

19

some events at around 120keV

x-ray candidate

x-ray candidate

Laura Segui

20

Conclusions

• Microbulk micromegas in Xe-TMA mixtures is an appealing technological option for rare event searches.

• The Penning transfer mechanisms seem to be optimally active within a mild 1-3% TMA concentration range, therein virtually un-affecting the experiment exposure (Xenon).

• Operation at a gain x400 and at 0.9%FWHM@Qββ,Xe at 10bar is possible.

• We would like to study the mixture more systematically with a new batch, specially the maximum gain, reproducibility and stability.

• Preliminary comparisons with Magboltz (TMA rated 3*) suggest that a dramatic factor x10 reduction of the transverse diffusion is within reach. Further studies resorting to event topology are required to validate this estimate.

• In order to make this option competitive, we are considering various approaches to determine the t o (an obvious one being TMA-fluorescence)

• Proof of principle demonstrated for a medium size ϕ=30cm, h=35cm TPC. Tracks can be clearly reconstructed, albeit a crude value for the energy resolution is about a factor x3 worse than for the case of un-segmented readout in small chambers.

• However, a large effort is still needed in order to achieve competitive results for the complete medium-size TPC at 10bar.

stay tuned!21

The team

• Igor Irastorza• Hector Gomez• Asuncion Rodriguez• Juan Castel• Hector Mirallas• Alicia Diago• Laura Segui• Theopisti Dafni• Diego Gonzalez-Diaz• Diana Carolina Herrera• Susana Cebrian• Gloria Luzon• Alfredo Tomas• Esther Ferrer-Ribas (CEA-Saclay)• Iannis Giomataris (CEA-

Saclay)

and

Rui Oliveira (CERN)Antonio Teixeira (CERN)

and the support of the CERN workshop

22

BACKUP

Status of NEXT-I(MM) on the last collaboration meeting (Nov11)

In a nut-shell

• Bake-out and pumping systems fully installed:

Gas tightness: <1mbar/day (T-corrected) at 11bar during 10daysVacuum: ~10-6 mbar after bake-out Out-gassing: <10-5 mbar l/s

• The HV for the 35cm-long drift region proved to be Paschen-tight up to 7kV@1bar and 26kV@8bar in pure Ar. Common wisdom suggests that for Xe it should be usually better.

• Mass-spectrometer working steadily. Calibration factors in Xe-TMA mixtures obtained.

• Re-circulation system installed. Not commissioned.

• T2K electronics connected. Not thoroughly tested. Analysis software still in early stage.

• Four pixelized μ-bulk MicroMegas (Φ=28cm) installed.

Status of NEXT-I(MM) todayFinal system pressure

Levels down to Pf=6x10-6 mbar achieved in the present system after ~72h heating time at 130deg.

However, x10-100 worse vacuum levels were used during most of measurements presented here.

Status of NEXT-I(MM) todayOut-gassing

Levels below Og=3x10-4 mbar l/s achieved in the present system.

From experience it will be much better after bake-out. Unfortunately it was not measured.

Gas tightness

After correcting for temperature variations: Previously: ΔP<1mbar/day at 11bar for 10 days (limited by pressure-meter).Presently: ΔP<3mbar/day at 1bar for 3 days (limited by pressure-meter).

No concern regarding gas leakage but more systematic measurements will come.

Jan-2010

Status of NEXT-I(MM) today

Electrical insulation

Xe-TMA mixtures much less Paschen-tight than pure Ar (Penning effect at work!), by~ x2.

The drift fields are still comfortable and transparency seems to be achievable.

Strong limitation for systematic study of attachment. An extra factor x3 will be handy

Status of NEXT-I(MM) today

If it is Paschen-tight for Xe-TMA, possibly ok for most practical mixtures.

From D. C. Herrera

~x2

really?? 0%TMA2%TMA

Chamber operation (I) Status of NEXT-I(MM) today

After grounding optimization and soft RC-filtering, signal is visible at Edrift= 160 V/cm/bar, and EMM = 58kV/cm (Xe-TMA ~ 98/2) with the following parameters:

• Mesh signal with single amplifying stage (CANBERRA-2004, tfall~50μs for δ-impulse excitation):

Eth~30keV, rmsEnoise~10keV (fairly stable)

• Mesh signal with double amplifying stage (CANBERRA-2004 + spectroscopic amplifier, gaussian response tshaping=8μs):

Eth~10keV, rmsEnoise~3keV (fairly stable)

• Pixel signals with T2K electronics (tshaping=2μs):

Eth~1.5keV, rmsEnoise~0.75keV (unstable)very seldom, even cosmic-ray triggered events can be directly seen from the mesh

external trigger signal

mesh signal

180 μs

System problems galore: -> cross-talk-> noise-> HV in 2 MMs unproperly applied-> dead pixels-> un-perfect cable-connector contact

Micro-bulk MicroMegas produces

nice signals in the scope.

electronics situation