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)

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pure Xenon

E [V/cm/bar]

v d [c

m/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 [μ

m/c

m1/

2 bar

1/2 ]

x 10!

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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

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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 to (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