Possible merits of high pressure Xe gas for dark matter detection

C J Martoff (Temple) & P F Smith (RAL, Temple)

• most dark matter experiments use cryogenic solid or liquid targets

semiconductors

crystals

noble liquids

phonons

ionisation

scintillation

• Ionization & scintillation signals available also from gases at normal temperature. • Could provide reasonable size competitive detectors at 5-10 bar

Brief history of dark matter detectors

<-- 1986 --> 1990 2000

Bolometers for Cabrera, Fiorini, Stodolski, vonFeilitzsch, Seidel, Pretzl…

Semiconductors for Avignone, Caldwell, Boehm…

Crystal scintillators forparticle detection Owen (1959), Doll….

Liquid Ar &Xe studies for solar & rare events Rubbia, Cline, Aprile…..

Noble gas studies forcharged particles & M Suzuki, Saito, Flaks….. Edberg, Sadoulet……

Ionizaton/ bolometers

NaI, CsI pulse shape

Liquid Xe ioniz’n/scint’n

Liquid Ar ioniz’n/scint’n

Suggested high pressure noble gas DM exptsRAL, RAL/Ohio, HELLAZ, SIGN Funding for r&d relatively small

Balloon Xe gas X-ray telescopes SIGHT, AXEL

Low bkgrd Ge

scintillation

Signal options

pmts

electric field

electric field

ionization

both

Field reduces recombination scintillation output, but more measurements needed.

target gas

Assume scintillation only

Scintillation pulse shape discrimination in Xe gas(zero electric field)

0.001

0.01

0.1

0 500 1000 1500 2000 2500

time ns

light output/ns normalised to 1

gamma pulse

alpha pulse

M Suzuki 1982 NIM 192, 623

0.1

1

10

100

1000

10000

100000

1000000

0 1000 2000 3000

time cut t = t1 ns

number of events for t > t1

1000000 background gammas

20 nuclear recoils

Signal sensitivity versus total background

Thus nuclear recoil population of 20 recoils/year detectable for background 1E6 gammas/year

1.0

10.0

100.0

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+10

total background gamma events

significant number of recoils

90% significant number of recoils

Possible module sizes

R8778 pmts inside vessel 0.6m

copper vessel

Xe gas @ 10 bar = 34kg

2 m

2.5 m

0.75m

Xe gas @ 5 bar = 34kg

Neutron & Gamma/beta Backgrounds

muonmuon

rock U/Th/K

n

n

pmts U/Th

n,

n,

Cu detector vessel U/Th

Kr85 in Xe

water /n shield

n,

Low energy background levels

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

0 200 400 600

Water thickness cm

Events/kg/day

gammas from rock

neutrons from rock U/Th

neutrons from rock muons

neutrons from water muons

DM sensitivity 1E-8 pb

DM sensitivity 1E-10 pb

<------ gammas from Cu vessel 0.1ppb U/Th

<------ gammas from pmts 2 inch R8778

<------ n from metal vessel 0.1ppb U/Th-------

<----------- n from R8778 PMTs

possible WIMP sensitivity

Example 1 2m x 0.6m module at 10 bar (34kg) with 3m water shieldLight collection estimate gives energy threshold 2-3 keV

Total gamma background < 40 keV = 70/kg/d = 8E5/yearTotal neutron background < 40 keV = 4E-4/kg/d = 5/year

90% Poisson recoil limit / efficiency = (20+5)/year = 2E-3/kg/d

Equivalent WIMP proton cross section in 360 days = 2E-9pb

Example 212 modules (400kg) with n backgrounds reduced 1/10 by veto

90% Poisson recoil limit / efficiency = (28 +6)/year = 2.4E-4/kg/d

Equivalent WIMP proton cross section in 360 days = < 3E-10pb

Illustrative 1000 kg gas detector systemBased on individual Xe modules with local pmts

Some modules with Ar or Ar/Xe to confirm A2 dependence

water shield

30 x 2m x 0.6m (34kg) modules

End planes of pmts

Alternative large scale configuration suggested by F Calaprice (Princeton) - based on Borexino principles

Target vessels with transparent silica walls + wavelength shifters

water shield

surrounding water-shielded pmt array

optional scintillator veto region

Preliminary conclusions

• If nuclear recoils give pulse shapes similar to alphas then DM sensitivity levels in range 1E-8 - 1E-9pb possible for gas target masses 30-100kg, using scintillation only.

• Assessment of simultaneous ionization/scintillation signals requires new measurements.

• Sensitivity 1E-9 - 1E-10pb possible with 100-1000kg and existing pmt backgrounds. Target sizes reasonable for 5-10 bar gas pressure.

• Different gases easily compared for A-dependence.

• Room temperature simplifies design and operation. Large mass gas experiments could be lower-cost than mK bolometers and noble liquids for same sensitivity.