XMASS experiment

1. XMASS experiment S. Moriyama for the XMASS collaboration Kamioka Observatory, ICRR, University of Tokyo 8th September 2004 @ IDM2004 Ultra-low BG, multi-purpose detector 3kg fiducial volume (FV) prototype detector 1ton (100kg FV) detector for DM Search

2. Ultra-low background multi-purpose detector ●XMASS ◎ Xenon MASSive detector for Solar neutrino (pp/7Be) ◎ Xenon neutrino MASS detector (double beta decay) ◎ Xenon detector for Weakly Interacting MASSive Particles (DM search) Solar neutrino Dark matter Double beta

3. Large self-shield effect Key idea: self shielding effect for low energy signals External g ray from U/Th-chain • Large Z makes detectors very compact • Large photon yield (42 photon/keV ~ NaI(Tl)) Liquid Xe is the most promising material. Single phase liquid Xe 23ton all volume 20cm wall cut 30cm wall cut (10ton FV) Volume for shielding Fiducial volume BG normalized by mass PMTs 0 1MeV 2MeV 3MeV

4. Strategy of the XMASS project ~1m ~2.5m ~30cm ~1 ton detector (FV 100kg) Dark matter search ~20 ton detector (FV 10ton) Solar neutrinos Dark matter search Prototype detector (FV 3kg) R&D Confirmation of feasibilities of the ~1ton detector Good results

5. 54 2-inch low BG PMTs Liq. Xe (31cm)3 MgF2 window 3kg FV prototype detector In the Kamioka Mine (near the Super-K) OFHC cubic chamber Hamamatsu R8778 Gamma ray shield 16% photo- coverage • Demonstration of reconstruction, • self shielding effect, and low background properties.

6. - m m n exp( ) å = Log( L ) Log( ) n ! PMT Vertex and energy reconstruction Reconstruction is performed by PMT charge pattern (not timing) Reconstructed here Calculate PMT acceptances from various vertices by Monte Carlo. Vtx.: compare acceptance map F(x,y,z,i) Ene.: calc. from obs. p.e. & total accept. QADC L: likelihood F(x,y,z,i) m : x total p.e. S F(x,y,z,i) n: observed number of p.e . FADC Hit timing F(x,y,z,i): acceptance for i-th PMT (MC) VUV photon characteristics: Lemit=42ph/keV tabs=100cm tscat=30cm === Background event sample === QADC, FADC, and hit timing information are available for analysis

7. + + + B A C Source run(g ray injection from collimators) I Collimator B Collimator C Collimator A • Very well reproduced. DATA MC

8. Source run(g ray injection from collimators) II 60Co: 1.17&1.33MeV DATA MC 137Cs: 662keV DATA MC • Impressive agreements. • Self shield works as expected. • Photo electron yield ~ 0.8p.e./keV for all volume 10-1 10-1 No energy cut, only saturation cut. BG subtracted r=2.884g/cc Arbitrary Unit 10-2 10-2 ~1/10 10-3 10-3 ~1/200 PMT Saturation region 10-4 10-4 10-5 10-5 -15 Reconstructed Z +15cm -15 Reconstructed Z +15cm Gamma rays Z= -15 Z= +15

9. All volume All volume 20cm FV 20cm FV 10cm FV (3kg) 10cm FV (3kg) Aug. 04 run preliminary Background data ~1.6Hz, 4 fold, triggered by ~0.4p.e. REAL DATA 3.9days livetime MC simulation Event rate (/kg/day/keV) • MC uses U/Th/K activity from PMTs, etc (meas. by HPGe). • Again, good agreement (< factor 2) • Self shield effect can be seen clearly. • Very low background (10-2 /kg/day/keV@100-300 keV) 10-2/kg/day/keV Miss-reconstruction due to dead-angle region from PMTs.

10. ? Miss reconstruction: dead angle from the PMTs (only for the prototype detector) • Scintillation light at the dead angle from PMTs give quite uniform 1p.e. level signal for PMTs. • This cause miss reconstruction as if the vertex is around the center of the detector. • Immersing PMTs into LXe and using spherical design solve this problem.  It will give low BG in the ROI for DM search HIT HIT HIT HIT HIT Inside the chamber is just copper surface.

11. Goal to look for DM by 1ton detector 1x10-14 g/g x33 2x10-14 g/g x32 1 ppt x5 Very near to the target level of U, Th Radon and Kr contamination. Preliminary Internal background activities • Current results • 238U(Bi/Po): = (33+-7)x10-14g/g • 232Th(Bi/Po): < 63x10-14g/g • Kr: < 5ppt NEW Factor ~30, but may decay out further Factor <~30 (under further study) NEW Achieved by distillation

12. Distillation to reduce Kr (1/1000 by 1 pass) • Very effective to reduce internal impurities (85Kr, etc.) • We have processed our Xe before the measurement. Original Xe: ~3 ppb Kr Off gas Xe: 330±100 ppb Kr (measured) Lower temp. ~1% ~3m Operation: 2 atm Processing speed: 0.6 kg / hour Design factor: 1/1000 Kr / 1 pass 13 stage of 2cmf Purified Xe: < 5 ppt Kr (measured after Kr-enrichment) Higher temp. ~99%

13. Achieved 1 ton (100kg FV) detector for DM Search • Solve the miss reconst. prob.  immerse PMTs into LXe • Ext. g BG: from PMT’s  Self-shield effect demonstrated • Int. BG: Kr (distillation), Radon  Almost achieved • Neutron: water or LS active shield (1/104) To be studied “Full” photo-sensitive,“Spherical” geometry detector external g ray (60cm, 346kg) external g ray: (40cm, 100kg ) 8x10-5/keV/kg/d /kg/day/keV Dark matter (10-6 pb, 50GeV, 100 GeV) Q.F. = 0.2 assumed 7Be 80cm dia. pp ~800-2” PMTs (1/10 Low BG) 70% photo-coverage ~5p.e./keVee 0 100 200 Energy(keVee)

14. Expected sensitivity XMASS FV 0.5ton year Eth=5keVee~25p.e., 3s discovery W/O any pulse shape info. 10-4 • Large improvements will be expected. SI ~ 10-45cm2 = 10-9pb SD~ 10-39cm2= 10-3pb 106 104 Edelweiss Al2O3 10-6 Tokyo LiF 102 Modane NaI Cross section to nucleon [pb] CRESST 1 UKDMC NaI 10-8 XMASS(Ann. Mod.) NAIAD 10-2 XMASS(Sepc.) 10-10 10-4 Plots except for XMASS: http://dmtools.berkeley.edu Gaitskell & Mandic

15. Summary • XMASS project: DM search with 100kg FV LXe. • R&D by 3kg FV prototype is well going Demonstration of reconstruction, self shield, and low BG properties. • Ext g rays: 10-2 /kg/day/keV  further self shield Kr: <5ppt  requirement almost achieved. Radon (U) (33+-7)10-14g/g (Th) < 63 10-14g/g  O(1/30) reduction is enough for 1ton det. • Further study for neutron is needed. Within a few years, we are planning to build the 100kg FV detector and start to search for DM!