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Current status of XMASS experiment. 11 th International Workshop on Low Temperature Detectors (LTD-11) Takeda Hall, University of Tokyo, JAPAN. 8/1, 2005 Hiroshi Ogawa (ICRR). What’s XMASS. Multi purpose low-background experiment with liq. Xe. - PowerPoint PPT Presentation
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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.