The current status of ＸＭＡＳＳ

1. Cryodet 1 The current status of ＸＭＡＳＳ • Introduction • Current status of prototype detector • Next step • Summary Y.Koshio for XMASS collaboration (Kamioka observatory, ICRR)

2. Introduction External g ray from U/Th-chain 23ton Lxe all volume 20cm self-shielding 30cm self-shielding (10ton FV) Large self-shield effect Counts/(keV kg day) 0 1MeV 2MeV 3MeV Cryodet 2 What’s XMASS • Xenon MASSive detector for Solar neutrino (pp/7Be) • Xenon detector for Weakly Interacting MASSive Particles (Dark Matter search) • Xenon neutrino MASS detector (double beta decay) Why liquid xenon • Large photon yield (~42000photons/MeV) • Self-shield (large Z=54) • Purification (distillation) • Compact detector size (~3g/cm3 10ton=1.5m cubic) • No long life isotope • Scintillation wavelength (175nm, detect directly by PMT) • Relative high temperature (~165K)

3. Cryodet 3 10 ton detector Strategy of the scale-up 800kg detector 100kg Prototype With light guide ～30cm ～80cm ～2.5m R&D Dark matter search We are now here Multipurpose detector (solar neutrino, bb …)

4. Current status of 100kg prototype detector Cryodet 4 54 2-inch low BG PMTs Liq. Xe (30cm)3= 30L= 100Kg MgF2 window Photo coverage: 16%  0.6p.e./keV Set in Kamioka mine Menu of R&D • Stability check in low temp. • Low background setup • Vertex / energy reconstruction • Demonstration of self-shielding • Purification system • Particle ID • Attenuation length • Background study

5. Shield Cryodet 5 4p shield with door 1.0m Rn free air (~3mBq/m3) 1.9m

6. Energy and Vertex reconstruction - m m n exp( ) å = Log( L ) Log( ) n ! PMT L: likelihood m : F(x , y, z) × (total p.e ./total acceptance) n: observed number of p.e . Cryodet 6 Using photoelectron map made by MC (only charge,but no timing information) Vertex: MC hitmap Energy: Hitmap scale Maximum likelihood method is used. Reconstructed here QADC FADC Hit timing === Background event sample === QADC, FADC, and hit timing information are available for analysis Tuning of parameters correlated to photon tracking is important.

7. Results hole C hole A hole B DATA MC Cryodet 7 1. Performance of the vertex reconstruction Collimated g ray source run from 3 holes (137Cs, 662keV) + + + C A B → Vertex reconstruction works well

8. Cryodet 8 2. Performance of self shielding z position distribution of the collimated g ray source run → Data and MC agree well γ

9. All volume 20cm FV 10cm FV Cryodet 9 3. Performance of the energy reconstruction Collimated g ray source run from center hole 137Cs, 662keV s=65keV@peak(677keV) (s/E ~ 10%) Similar peak position in each fiducial. No position bias → Energy reconstruction works well

10. Cryodet 10 4. MC tune using source data • Input parameters: • Photon yield ~42000 photons/MeV • QE of PMT ~25% • Collection efficiency of PMT ~90% • Refractive index of liquid Xe 1.6 (measured) • Absorption • Scattering • Reflection on the surface of cupper • Using hit pattern of source data tune using total photo electron distribution 60cm 55cm 20% Tunable Very preliminary Fix abs.(100cm) and ref.(20%), e.g.) γ beam sensitive to scattering Change scattering 30cm 50cm 60cm γ beam front view side view

11. All volume 20cm FV 10cm FV Cryodet 11 5. Compare background shape b/w data and MC → Data and MC agree well → Self shield effect can be seen → Very low background ☆ MC (g ray background) ・ Outside of the shield ・ RI sources in PMTs ・ 210Pb in the lead shield 10-2 /kg/day/keV@100-300 keV

12. 214Bi 214Po 210Pb a (7.7MeV) b (Q=3.3MeV) t1/2=164ms 208Po 212Bi 212Po a (8.8MeV) b (Q=2.3MeV) t1/2=299ns Cryodet 12 6. Internal backgrounds in LXe were measured Main sources in liq. Xe are Kr, U-chain and Th-chain • Kr =3.3±1.1 ppt (by mass spectrometer) → 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) Delayed coincidence search (radiation equilibrium assumed) Delayed coincidence search (radiation equilibrium assumed)

13. Summary of BG measurement Cryodet 13 1/100 Now (prototype detector) Goal (800kg detector) • g 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 1/33 1/12 1/3 Very near to the target level!

14. All volume 20cm FV 10cm FV Cryodet 14 5. Compare background shape b/w data and MC True vertex Miss-reconstruction → Data and MC agree well → Self shield effect can be seen → Very low background ☆ MC (g ray background) ・ Outside of the shield ・ RI sources in PMTs ・ 210Pb in the lead shield 10-2 /kg/day/keV@100-300 keV

15. Wall effect HIT HIT HIT HIT HIT ? Cryodet 15 (expected only for the prototype detector) MC If true vertex is used for fiducial volume cut 1 Dead angle 10-1 10-2 • Scintillation lights at the dead angle from PMTs give quite uniform 1 p.e. signal for PMTs, and this cause miss reconstruction as if the vertex is around the center of detector 1000 2000 3000 0 Energy (keV) No wall effect This effect does not occur with the sphere shape 800 kg detector

16. With light guide Cryodet 16 Remove the wall effect Active veto BG simulation (light guide MC) PMT K PMT Th PMT U event rate (/keV/day/kg) Fiducial PTFE light guide (UV reflection) Fast neutron upper limit (90%C.L.) energy (keV)

17. Hole-B Cryodet 17 Comparisonof data w/ and w/o light guide Collimated g ray source from hole-B (137Cs,662keV) • with light guide • w/o light guide 10cm fiducial fiducial volume Counts Counts Energy [keV] Energy [keV] Reduce the events due to the wall effect Further analysis of Low energy background is under study

18. Next step Cryodet 18 800kg Detector :WIMP search External g ray BG: 60cm, 346kg 40cm, 100kg Achieved pp & 7Be solar n ~80cm diameter Expected dark matter signal (assuming 10-42 cm2, Q.F.=0.2 50GeV / 100GeV,) • 840-2” PMTs immersed into liq. Xe • 70% photo-coverage ~5 keVee threshold

19. Cryodet 19 Expected sensitivities XMASS FV 0.5 ton year Eth = 5 keVee~25 p.e., 3s discovery w/o any pulse shape info. 10-4 106 • Large improvements will be expected SI ~ 10-45 cm2 = 10-9 pb SD~ 10-39 cm2 = 10-3 pb 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

20. Detector design 5.8cm (edge to edge) 0.3cm (rim) 5.4cm 12cm Cryodet 20 Geometry : 12 pentagons / pentakisdodecahedron Hamamatsu R8778MOD(hex) A tentative design (not final one) • Hexagonal quartz window • Effective area: f50mm (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

21. Cryodet 21 Performance check by MC • Using Geant4 • intput parameter : Xe light yield =42000p.e. • abs. length =100cm, scat.length = 30cm • position recon. Method : same with 100kg prototype • (hitmap making & likelihood calculation) 10keV 5cm from center 10keV 35cm from center

22. Fiducial volume Position dependence of reconstructed position resolution Cryodet 22 5keV 10keV At division of F.V. (２５ｃｍ from center) 1MeV ~6mm 5keV ~80mm σ（real pos – recon pos）（ｍｍ） 50keV 500keV 100keV 1MeV Position dependence of Light yield & resolution Distance from center（ｍｍ） res％ Energy resolution Npe Light yield 1MeV 5keV Photo yield 4.2 p.e./keV （at center） Resolution: 1MeV 1.5% 5keV 20% 10keV 500keV 50keV 500keV 100keV 100keV 50keV 10keV 1MeV 5keV 0 100 200300 400 0 100 200 300 400 Distance from center（ｍｍ） Distance from center（ｍｍ）

23. Summary Cryodet 23 • XMASS experiment: • Large volume liquid Xe detector for multi-purpose • e.g. DM, pp/Be solar neutrino and bb measurement. • Key idea : Self shielding of liquid Xe • 100kg prototype detector: • The detector performance is as expected. • Most of the performances required for 800kg detector are confirmed. • 800kg next detector: • Mainly for dark matter search. • 2 orders improvement of sensitivity above existing experiments is expected.