LUX. Some thoughts on large detectors.Comments on Xe microphysics.

Tom ShuttCase Western Reserve University

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LUX

2

• 300 kg Xe, expected 100 kg fiducial

• At Sanford lab, Homestake SD.

• First water-shielded large dark matter detector

Surface Facility at Sanford Lab

• Old Warehouse• Occupancy starting Dec 09• LUX has been:

— Assembling detector— Helping create Sanford Lab

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Precision cleaned cryostat returns - July 19, 2010

Surface Operations

!

4

2009

Davis Campus @ SURF

5

1964

April2011

$8.1M outfitting contract - March 2011.

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Homestake DUSEL

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Deep Underground Science and

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Nov2011

LUX Status• Cryogenic run - May 2011• Full run now underway• Davis Cavern: Beneficial occupancy March 2012• Full running: Fall 2012

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During Lunch

Underwater

LUX Collaboration

7Oct 2011, Sanford Lab

BrownCaseMarylandLLNL

LIP-CoimbraRochesterSouth DakotaSDSMT

Texas A&MUC Berkeley/LBNLUC Davis

UC Santa BarbaraYale

Projected Reach of LUX

• No expected background in 100 kg x 300 days• “Beneficial occupancy” underground - March 2012• Full running - Fall 2012

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Simulated LUX data(100 kg fiducial, 100 days)

mX = 100 GeV/c2

σ = 5 10-45 cm2LUX - 100 kg x 300 days

LZS 1.5 ton

LZD 20 tonXENON100

(40 kg fiducial, 100 days) - ~800 ER event

LUX Upgrade• Replace old PMTs with new

— Requires only new array holders, detector reassembly

— BG well below measured limits: total background reduced by 40

• Much cleaner potential nuclear recoil signal

• Increase in fiducial mass• Target - 2013

0 50 100 150 200 250 300

10−6

10−5

10−4

10−3

10−2

10−1

Fiducial mass [kg]

Fidu

cial a

ctivi

ty [D

RUee

]

Background in Fiducial Volume vs. Fiducial Massfor R11410 MOD Bottom Array Upgrade

122x R8778Current ULs0.5 mBq U/Th/Co0 BG31+31 R11410Current ULs31+31 R114100.5 mBq U/Th/Co

T+Bot

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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

LUX

LUX upgrade

T. Shutt - March 11 2009

Some thoughts on scaling up this technology

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Self-shielding in liquid xenon

Effective for detectors large compared to ~10 cm gamma penetration distance: few 100 kg and up.

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300 kg LUX

fiducialcut

P (L) !=L

!e!

L!

PMTSingle, low-energy Compton scatter

Shielding• 4 m water shield + 4850 ft depth adequate up to at least 20 ton scale.• Liquid scintillator shield. Effective for

— Internal neutrons— Internal gammas— External, high-energy neutrons

• Titanium cryostat material— Significant new construction material for low background experiments— No measured contamination at limits of Oroville capability (< ~0.2 mBq/kg)— Enables active shield

0 1 2 3 4Shield Thickness (m)

Rock !

µ neutrons

Rock neutrons

175K scintillator shield

13!"#$%#&'('#)*+,-%-.#/+%-.#%+01

23'4"#5%##*06*)-

7.89:-0*;#

<-66-=

>*?@*)#A-090#

<-66-=

!

ISOHexane scintillator, housed immediately outside LXe, at LXe temperature. Goal: > 10-fold gamma veto + highly efficient neutron veto

UCSB/LBNL

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Kr Removal

Kr Xefeed

purg

e

reco

very

>> 1000 separation

• 85Kr - beta decay— Need Kr/Xe: 10 ppt, LZS 0.5 ppt, LZD 0.05 ppt— Commercial Xe/Kr ~ few ppb— Chromatographic system: < 2 ppt @ 2 kg/day

production

• Scaling up current system— 60 kg charcoal column— Vacuum phase “recovery” stage— High capacity Xe condenser

0 2000 4000 6000 8000 10000 12000

10−9

10−8

10−7

time(sec)

curren

t(Amp)

1Kg2kg1Kg300Torr4Kg1.5Kg

RGA

curre

nt (a

rb. u

nits)

time (seconds)

4 kg

1.5 kg

1 kg

1 kg (@300 torr)

2 kg

Kr-Xe cocktail: no noticeable saturationup to 4 kg Xe per cycle

Xe purification and analysis• Gas phase getter purification - standard• Very high efficiency two-phase heat exchanger

— Removes large thermal penalty to recondense

• In ~60 kg prototype, obtained purity in few days• Cold-trap enhanced mass

spectrometry: first sensitivity to required impurity levels

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Table 1. Primary components of the LUX 0.1 detector and their heat capacities.

Item Material Mass Heat(kg) Capacity

(J/K)

DisplacementBlocks aluminum 230 1.82!105

InnerFlange copper 162 6.32!104

IR Shield copper 112 4.37!104

Cryostat stainlessCan steel 100 5.10!104

Xenon xenon 55 8.69!103

Subtotal 659Everything

Else 115

Time (s)0 2000 4000 6000 8000 10000

Po

wer

(W)

-5

0

5

10

15

20

25

30

35

IR Shield

Cryostat Can

Xenon

Inner Flange

Displacement Blocks

Heater Power

Time (s)0 2000 4000 6000 8000 10000

Te

mp

era

ture

(K

)

176

177

178

179

180

181

182

183

184

Temperature Reaction to Xenon Flow Rate Change

IR Shield

Cryostat Can

XenonInner Flange

Displacement Blocks

0 slpm 7.2 slpm 42.0 slpm

Figure 5. Data taken during prototype operation showing in the top plot the effect of changing the xenonflow rate on the temperatures. The bottom plot shows the smoothed instantaneous power measured using thecomponents of the experimental setup.

Using these temperatures the heat load was determined for all of the major components of thedetector. These were then used to adjust the change in the power applied by the PID controlledheater, and the total heat load due to circulation was calculated. The result of these calculations is

– 10 –

Circulation off On: ~400 kg/day

Data from coldtrap/RGA arXiv: 1002:2742

open leak valve,open leak valve,

bypass gas purifierbypass gas purifierflow through flow through

gas purifiergas purifierbypassbypass

gas purifiergas purifier

Xe is constant due to cold trap

18 ppb N2

5 ppb O2

0.25 ppb CH4

close leakclose leak

valve to measurevalve to measure

backgroundsbackgrounds

~few ppm Ar

Sensitivity : 0.1 ppb O2

1.0 ppb N2

0.06 ppb CH4

Thermosyphon configuration testing

Thermosyphon Cryogenics• Uniquely suitable for very large scale.

— Extremely high capacity: equivalent to ~1 m Ø Cu bar.

— Remote deployment of multiple cold heads.— Tunable to low power for fine control.

• Intrinsically safe against power failure

• Cryogenics + Xe systems vetted by lab safety.

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LN Dewar.

Control Panel

Five thermosyphons in

this conduit

05/15 00:00 05/22 00:00 05/29 00:00 06/05 00:00180

200

220

240

260

280

300

Date

Avera

ge D

ete

cto

r T

em

pera

ture

[K

]

~0.3 K/hr

~1 K/hr

First LUX Cooldown

LUX

Light collection at the multi-ton scale• Rayleigh scattering: not yet a problem• PTFE walls: extraordinarily reflective at 175 nm (7eV)

— r ~ 98% or greater: “mirrored box”

• Purity should be achievable - comparable to requirements for charge drift

• With r ~98%, should get 2-3 x light of X10, X100: directly lowers threshold

17Xenon10 MEGA

LUX

LZ20

0.1 ppb: 1 km => 2% loss

LXe

Rayleigh scattering penalty

LZD

• 20 ton scale-up of LZS• Sited at 4850 ft Homestake single lab module, or expanded SNOLab.

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20 Tons hits fundamental neutrino limit

• LZD at 20 tons: 10-48 cm2 WIMP sensitivity• Atmospheric and diffuse supernova neutrinos set irreducible background

just beyond this• WIMP/supernova ratio independent of target

19Proven materials backgrounds; 99.5% discrimination; < 1 background n.r. event;

Nuclear Recoils Electron Recoils

T. Shutt - March 11 2009

Some comments on Xe microphysics, as it affects dark matter detection

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Xe microphysicsElectron recoils (Background)

Energy

Char

ge/L

ight

Nuclear recoils (Signal)

Char

ge/L

ight

Energy

• What determines band widths?• What determines band positions?• What is the best measure of

energy?• Why does discrimination improve

at low energy?Drift Field Dependence

nuclear recoils

electron recoils

T. Shut, LBNL Xe- Nov 16, 2009 22

Energy partitioning

• Excitations:— Ionization (Ni)— Recombined ions (r) -> photons — Excited atoms (Nex) -> photons

• Doke: predicts 6%

• New formalism:

•We find W=13.7±0.2.• Nuclear recoils have

additional factor: Lindhard

ne = (1! r) · Ni

n! = (ab

NexNi

+ r) · Ni

E = (ne + n!) · W

(ne + n!)er = EerW

(ne + n!)nr = L · EnrW

L = (ne+n!)nr

(ne+n!)er· Eer

Enr

Leff = n!nr

n!er· Eer

Enr, zero field

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Single and Dual Phase data

Charge

Ligh

t

Nuclear recoils: Lindhard • keV nuclear recoils move slower than electrons in atoms (vFermi).• Adiabatic interaction. Electronic excitation due to overlap of

shells.

• Equal masses: significant energy “lost” to recoils.— (Essentially absent for electron recoils).

• Result: less “electronic excitation” than for electron recoils.

• Described by Lindhard (Copenhagen school, 1960’s).— J. Lindhard et al., Mat. Fys. Medd. Dan. Vid. Selsk., vol. 33, no. 10, 1963.

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Recombination - based discrimination

Photons

Elec

trons

higher Field lower

Recombination

Electron recoils (Background)

Energy

Char

ge/L

ight

Nuclear recoils (Signal)

Char

ge/L

ight

Energy

T. Shut, LBNL Xe- Nov 16, 2009

Electron recoil band width

Recombination

Electron Recoils At 122 keV

(8891 total quanta)

Understanding Discrimination

Energy Dependence

Total

S1+S2

S1stat+inst

S2inst+stat

electron recoils nuclear recoilsNote: small recombination fluctuations

Predicted Discrimiantion

x2 light collection increase

XENON10

Recombination Modeling (Dahl, 2009)

• Long-standing puzzles: shapes of e.r. and n.r. bands, field independence of n.r. and low-energy e.r. bands

• Nuclear recoils have same recombination as electron recoils.— Discrimination based on enhanced direction-excitation light for nuclear recoils

• Qualitative (but not yet quantitative) understanding of recombination fluctuations

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20 keV

2 keV 20 keV

Nuclear recoils (Rival)4 keV

Electron recoils (Penelope)

Concluding comments• Two phase Xe is very powerful technology

— Large signal— Low intrinsic backgrounds— Multiple method of background rejection (discrimination, self-shielding, active

shielding)

• Low energy potential is high— Discrimination good near threshold— Factor of 2-3 better light collection might be achievable— S2 only data should extend to below ~keV— Calibration remains a challenge

• We should try to reach the neutrino limit

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