Current status of XMASS experiment

11th International Workshop on Low Temperature Detectors (LTD-11)

Takeda Hall, University of Tokyo, JAPAN

8/1, 2005 Hiroshi Ogawa (ICRR)

What’s XMASS

Xenon MASSive detector for solar neutrino (pp/7Be) Xenon neutrino MASS detector ( decay) Xenon detector for Weakly Interacting MASSive Particles (DM search)

Multi purpose low-background experiment with liq. Xe

Large Z (=54) Self-shielding effect Large photon yield (~42 photons/keV ~ NaI(Tl)) Low threshold High density (~3 g/cm3) Compact detector (10 ton: sphere with diameter of ~2m) Purification (distillation) No long life radioactive isotopeScintillation wavelength ~175 nm, detected directly by PMT Relative high temperature (~165 K)

Merit of using Liq. Xe:

100kg Prototype 800kg detector

10 ton detector

～ 30cm～ 80cm

～ 2.5m

R&DDark matter search

Multipurpose detector

(solar neutrino, …)

We are now here

Strategy of the scale-up

With light guide

Gamma ray shield

OFHC cubic chamber

Liq. Xe (31cm)3

MgF2 window

54 2-inch low BG PMTs

16% photo- coverage

Hamamatsu R8778

100 kg prototype detector

In theKamioka Mine

(near the Super-K)2,700 m.w.e

Vertex and energy reconstruction by fitter Vertex and energy calibration using radioactive sources 137Cs & 60Co Self shielding power are measured by 137Cs &60Co

Confirmation of estimated 800 kg detector performance

PMT are developed for VUV region (Q.E.~ 30%@175nm)PMT performance (Gain, Q.E.) in low temp. is similar to one in room temp.

z position distribution of the collimated ray source run

→ Data and MC agree wellγ

Demonstration of self shielding effect

Shelf shielding for real data and MC

Good agreement (< factor 2) Self shielding effect can be seen clearly. Very low background (10-2 /kg/day/keV@100-300 keV)

REAL DATA MC simulation

All volume20cm FV

10cm FV(3kg)

All volume20cm FV

10cm FV(3kg)

Miss-reconstruction due to dead-angle region from PMTs.Reduced in Light guide measurement & future 800kg detector.

Eve

nt r

ate

(/kg

/day

/keV

)

10-2/kg/day/keV

Aug. 04 runpreliminary

3.9dayslivetime

~1.6Hz, 4 fold, triggered by ~0.4p.e.

Kr = 3.3±1.1 ppt (by mass spectrometer) β687keV (99.563%) as BG

→ Achieved by distillation

U-chain = (33±7)x10-14 g/g (by prototype detector)

Th-chain < 23x10-14 g/g(90%CL) (by prototype detector)

1/2=164s (Q=3.3MeV) (7.7MeV)

214Bi 214Po 210Pb

1/2=299ns (Q=2.3MeV) (8.8MeV)

212Bi 212Po 208Po

Delayed coincidence search

Delayed coincidence search

(radiation equilibrium assumed)

(radiation equilibrium assumed)

Internal backgrounds in liq. Xe were measured

Main sources in liq. Xe are Kr, U-chain and Th-chain

XMASS succeeds to reduce Kr concentration in Xe

from ~3[ppb] to 3.3(±1.1)[ppt] with one cycle (~1/1000)

~3mLower

Higher

~1%

~99%

Purified Xe: 3.3±1.1 ppt Kr (measured)

Off gas Xe:330±100 ppb Kr(measured)

Raw Xe: ~3 ppb Kr

(preliminary)

(178K)

(180K)Operation@2atm

• Processing speed : 0.6 kg / hour• Design factor : 1/1000 Kr / 1 pass• Purified Xe : Off gas = 99:1

Xe purification system

Boiling point

(@2 atm)

Xe 178.1K

Kr 129.4K

800 kg detectorMain purpose: Dark Matter search

~800-2” PMTs immersed into liq. Xe 70% photo-coverage

~80cm diameter

~5 keVee threshold

External ray BG: 60cm, 346kg 40cm, 100kg

Expected dark matter signal(assuming 10-42 cm2, Q.F.=0.2 50GeV / 100GeV,)

pp & 7Be solar

Achieved

Expected sensitivity SI ~ 10-45 cm2 = 10-9 pb SD~ 10-39 cm2 = 10-3 pb

XMASS FV 0.5 ton yearEth = 5 keVee~25 p.e., 3 discoveryw/o any pulse shape info.

Detector design Geometry : 12 pentagons /

pentakisdodecahedronA tentative design (not final one)

Hamamatsu R8778MOD(hex)

12cm5.

4cm

5.8c

m(e

dg

e to

ed

ge)

0.3cm(rim)

Hexagonal quartz window Effective area: 50mm (min) QE <~25 % (target) Aiming for 1/10 lower background than R8778

Total 840 hex PMTs immersed into liq. Xe 70% photo-coverage Radius to inner face ~43cm

Now (prototype detector) Goal (800kg detector)

ray BG ~ 10-2 cpd/kg/keV 10-4 cpd/kg/keV

→ Increase volume for self shielding

→ Decrease radioactive impurities in PMTs (~1/10)

238U = (33±7)×10-14 g/g 1×10-14 g/g

→ Remove by filter

232Th < 23×10-14 g/g (90% C.L.) 2×10-14 g/g

→ Remove by filter (Only upper limit)

Kr = 3.3±1.1 ppt 1 ppt

→ Achieve by 2 purification pass

Very near to the target level!

1/100

1/33

1/12

1/3

Goal of BG for 800kg detector

Summary

• XMASS experiment: Multi purpose low-background experiment with large mass liq. Xe

• Performances required for 800 kg detector are confirmed : self shield effect, distillation and son on using 100kg prototype detector.

• 800 kg detector: Designed for dark matter search mainly, and 102 improvement of sensitivity above existing experiments is expected.