Liquid Noble R&D for dark matter and double beta decay Tom - - PowerPoint PPT Presentation

• T. Shutt - Argonne, 1/11/2013

Liquid Noble R&D for dark matter and double beta decay

Tom Shutt Case Western Reserve University

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• T. Shutt - Argonne, 1/11/2013

Dark matter and ßß decay with liquid nobles

Double beta decay

EXO XMASS

Dark matter

LUX / LZ / XENON1T DarkSide XAX / MAX / Darwin XMASS DEAP / CLEAN

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Xe Ar Ne

• T. Shutt - Argonne, 1/11/2013

Challenges

• Dark matter: Two phase Xe TPC

— Improve S1/S2 discrimination:

• Cathode HV near 500 kV
• Highest light yield

— Kr removal — Xe purity for charge + light — External detector — Reduce backgrounds in PMTs plus other components

• Dark matter: Two phase LAr TPC

— Reduce huge 39Ar background

• PSD discrimination
• Removal
• Xe ßß decay

— Backgrounds, backgrounds, backgrounds — Light collection — Ba tagging

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Common goal: detector masses of tons to tens of tons

• T. Shutt - Argonne, 1/11/2013

Self shielding of gamma backgrounds

• Dark matter: single low-energy Compton scatter

must enter and escape

• 136Xe ßß Decay

— Q=2480 keV endpoint, above most lines, except:

• 208Tl: 2614 keV, 100% BR
• 214Bi: 2448 keV, 1.57% BR

— Simple self-shielding:

• Weaker than DM case
• Conclusion: ßß experiment requires much lower

backgrounds in components.

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scatter PMT

P(L) ∼ = L λ e− L

λ

P(L) = P

• e

− x λ

• T. Shutt - Argonne, 1/11/2013

S1/S2 Discrimination

• Sophisticated understanding of underlying processes

in Xe.

— Now: NEST simulation — Postdiction of Zeplin III result

• High field helps
• Light collection helps
• Calibration is challenge: tritiated methane source

(LUX)

• Situation in LAr is largely unknown.

5 5 Electron recoil band width

Recombination

C.E. Dahl, J. Kwong

ZEPLIN III: ~99.99% ~5 kV/cm

(

XENON100(level((530(V/cm)( ZEPLINQIII(levels((3.4(and( 3.9(kV/cm)( XENON10( XENON10(level((730(V/cm)(

• blem.(

(figure(from(M.(Szydagis)(

NEST

Energy [keVnr] 10 20 30 40 50 /S1)-ER mean

b

(S2

10

log

• 1.2
• 1.0
• 0.8
• 0.6
• 0.4
• 0.2

0.0 0.2 0.4 S1 [PE] 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38

XENON100 99.75%

• T. Shutt - Argonne, 1/11/2013

Pulse shape discrimination in LAr

• Remarkably good: roughly matched to

very large 39Ar background

• Very sensitive to amount of light

collection

• DEAP-1: believe threshold is ~60 keVr
• R&D: map to low energy, high

separation

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9

   



 = ¡fraction ¡of ¡“prompt”/total ¡light DEAP-1 data 120-240 pe 60 keVr thres. with 8 pe/keVee

~30 ~12 ~7

• T. Shutt - Argonne, 1/11/2013

Light collection

• LAr:

— Waveshifters

• Needed because of XUV photons: PMTs, reflectors
• Doesn’t poison LAr for charge drift

— Large phase space for optical design, especially single-

phase detector.

— R&D: geometry, quality of waveshifter film: reflectivity

• LXe

— PTFE has remarkably high reflectivity in LXe,

apparently consistent with 100%.

• Not the gas in gas Xe under same conditions.

— Why? — Can this be reproducibly controlled?

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S1 (p.e.)

1000 2000 3000 4000 5000 6000 7000

Rate (mBq/p.e.)

5 10 15 20

661.7keV

DarkSide10: 8.9 pe/keV LUX: 8 pe/keV

• T. Shutt - Argonne, 1/11/2013

LAr: 39Ar reduction

• Raw background ~ 1 Bq/kg
• Minimal goal for TPC running: reduce by 10-100.

— Factor of ~108 would allow lowest threshold

• Old Ar in underground gas, requires extensive separation system -

Princeton

— <6.5 mBq/kg — 143 of 150 kg collected, stable

production at 1/2 kg/day

— NSF funded expansion up to 50 kg/

day

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Energy/keV 200 400 600 800 1000 Rate/(Bq/keV)

• 6

10

• 5

10

• 4

10

• 3

10

• 2

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Underground Argon Measurements

AAr, @KURF UAr, @Surface UAr, @Surface, Muon Vetoed UAr, @KURF

• T. Shutt - Argonne, 1/11/2013

High voltage

• High voltage on cathode sets drift field

— Key driver of S2/S1 disc. in Xe, at least.

• Difficult in both LXe and LAr

— Not a distinguished history to date — Need > 100kV in Xe to improve discrimination in

LZ beyond ~99.5%

• Feedthrough design pursued by several

groups /experiments

— Core challenge: passage through gas phase

• Alternative - generate in liquid (e.g.,

Crockcroft Walton)

• Electroluminesence in detector also a

challenge

— Demonstrated in small detectors at needed field

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LUX W Ra fi No fl

100 kV achieved in 2 / 2 feedthroughs with no sign of breakdown or aging

Example: LUX HV feedthrough (Yale) Design appears scalable to ~300 kV

• T. Shutt - Argonne, 1/11/2013

PMTs

• Hamamatsu R11410mod, 3” with high QE is

new “default”

— Backgrounds ~ 20 times lower than

previous 2” tube (R8778)

— Beginning to become non-dominant

compared to current TPC mechanics

• Reliance on Hamamatsu!
• QUPIDs - no longer being

developed(?)

• PMTs unlikely to be replaced for DM
• > highest possible coverage
• SiPMTs being pursed by EXO

— Light yield -> resolution, but background

essential.

PMT

238U

[mBq/ PMT]

232Th

[mBq/ PMT]

40K

[mBq/ PMT]

60Co

[mBq/ PMT] R8778 9.5±0.6 2.7±0.3 66±2 2.6±0.1 R11410 MOD <0.4 <0.3 <8.3 2.0±0.2

R11410mod

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• T. Shutt - Argonne, 1/11/2013

External detector system: LZ, DarkSide

• Veto
• Near-hermetic background measurement: systematics
• LZ

— Neutron and gamma vetoing — Xe “skin” — Gd-loaded LAB scintillator — “Daya-Bay” architecture

• DarkSide

— Neutron veto — B-loaded scintillator -> n-alpha

• Doesn’t require high gamma efficiency

— SS vessel with high PMT count

• Relative importance decreases as mass increases: self shielding

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Active Xe Xe Skin Liquid Scint. γ n capture PTFE Ti LZ Dark Side

• T. Shutt - Argonne, 1/11/2013

Materials backgrounds

• Ti - are LUX Ti backgrounds reproducible? How low are they?
• How low can PMTs go?
• Rn control

— Rn in liquid, from all components: need few mBq in Xe TPC

• Requires SNO/Borexino “lite” emanation program

— Rn daughters on PTFE.

• alpha-n
• Pb recoil in single-phase - acute issue.

— Rn daughters in PTFE?

• Kr in Xe, 39Ar in LAr
• ßß decay: everything. PMTs far from being allowed, all materials

require extreme cleanliness

— Screening an enormous challenge. — Need whole-body screener with radically lower backgrounds

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• T. Shutt - Argonne, 1/11/2013

Xe purification

• Electron drift over meters is significant challenge in LXe

— apparently harder than LAr.

• Commercial heated getter removes impurities: challenge is

liquid circulation

• LUX, others: high efficiency dual-phase heat exchanger
• Analytical techniques: simple RGA + novel cryo-trap (UMD)
• Liquid purification

— Pump — Variation of spark purifiers

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0.5 ppt Kr

• pen leak valve

LZ scheme

• T. Shutt - Argonne, 1/11/2013

Kr separation from Xe: dark matter

• Equivalent pp-solar neutrino rate is 0.3 ppt.

— 0~.01 ppt for good pp-solar measurement

• Commercial Xe typically ~100 ppb.
• LUX - chromatography at ~< 2 ppt @ 8 kg/day
• XMASS, XENON100 - distillation - XMASS: < 2 ppt at 4.7 kg/

day

• Sub ppt:

— Key is sampling system — Emanation / leaks need serious attention for sub ppts.

• Pre-bake of all required plastic

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