Cryodet 1 The current status of ＸＭＡＳＳ Y.Koshio for XMASS collaboration (Kamioka observatory, ICRR) • • • • Introduction Current status of prototype detector Next step Summary Introduction Cryodet 2 What’s XMASS External g ray from U/Th-chain • Xenon MASSive detector for Solar neutrino (pp/7Be) • Xenon detector for Weakly Interacting MASSive Particles (Dark Matter search) 23ton Lxe all volume • Xenon neutrino MASS detector (double beta decay) Counts/(keV kg day) Why liquid xenon 20cm self-shielding 30cm self-shielding (10ton FV) • Large photon yield (~42000photons/MeV) Large self-shield • 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) 0 1MeV effect 2MeV 3MeV Cryodet 3 10 ton detector Strategy of the scale-up 100kg Prototype 800kg detector With light guide ～30cm ～80cm ～2.5m R&D Dark matter search We are now here Multipurpose detector (solar neutrino, bb …) 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 Shield Cryodet 5 4p shield with door material 1.0m 1.9m thickness Polyethylene 15cm Boron 5cm Lead 15cm EVOH sheets 30μm OF Cupper 5cm Rn free air (~3mBq/m3) Energy and Vertex reconstruction Reconstructed here Cryodet 6 Using photoelectron map made by MC (only charge,but no timing information) Vertex: Energy: MC hitmap Hitmap scale Maximum likelihood method is used. QADC exp(- m ) m n Log(L) = Log( ) n ! PMT L: likelihood m: F(x, y, z) × (total p.e ./total acceptance) n: observed number of p.e. 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. Results Cryodet 7 1. Performance of the vertex reconstruction Collimated g ray source run from 3 holes (137Cs, 662keV) hole C hole B hole A DATA + + + C BA MC → Vertex reconstruction works well Cryodet 8 2. Performance of self shielding z position distribution of the collimated g ray source run → Data and MC agree well γ Cryodet 9 3. Performance of the energy reconstruction Collimated g ray source run from center hole 137Cs, 662keV All volume 20cm FV 10cm FV s=65keV@peak(677keV) (s/E ~ 10%) Similar peak position in each fiducial. No position bias → Energy reconstruction works well Cryodet 10 4. MC tune using source data Input parameters: • Photon yield ~42000 photons/MeV tune using total photo • QE of PMT ~25% electron distribution • Collection efficiency of PMT ~90% • Refractive index of liquid Xe 1.6 (measured) • Absorption 60cm Tunable 55cm • Scattering • Reflection on the surface of cupper 20% Very preliminary Using hit pattern of source data e.g.) γ beam sensitive to scattering γ beam front view side view Fix abs.(100cm) and ref.(20%), Change scattering 30cm 50cm 60cm Cryodet 11 5. Compare background shape b/w data and MC All volume 20cm FV 10cm FV → Data and MC agree well ☆ MC (g ray background) ・ Outside of the shield → Self shield effect can be seen ・ RI sources in PMTs → Very low background 10-2 /kg/day/keV@100-300 keV 210 ・ Pb in the lead shield 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) Delayed coincidence search (radiation equilibrium assumed) 214Bi 214Po 210Pb a (7.7MeV) b (Q=3.3MeV) t1/2=164ms Th-chain < 23x10-14 g/g(90%CL) (by prototype detector) Delayed coincidence search (radiation equilibrium assumed) 208Po 212Bi 212Po a (8.8MeV) b (Q=2.3MeV) t1/2=299ns Summary of BG measurement Now (prototype detector) Goal (800kg detector) 1/100 g ray BG ~ 10-2 cpd/kg/keV → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10) 238U 232Th = (33±7)×10-14 g/g → Remove by filter < 23×10-14 g/g (90% C.L.) → Remove by filter (Only upper limit) Kr = 3.3±1.1 ppt → Achieve by 2 purification pass Cryodet 13 10-4 cpd/kg/keV 1/33 1/12 1/3 1×10-14 g/g 2×10-14 g/g 1 ppt Very near to the target level! 5. Compare background shape b/w data and MC Cryodet 14 All volume 20cm FV 10cm FV True vertex Miss-reconstruction → Data and MC agree well ☆ MC (g ray background) ・ Outside of the shield → Self shield effect can be seen ・ RI sources in PMTs → Very low background 10-2 /kg/day/keV@100-300 keV 210 ・ Pb in the lead shield Wall effect Cryodet 15 (expected only for the prototype detector) MC HIT HIT ? Dead angle 1 If true vertex is used for fiducial volume cut 10-1 HIT HIT HIT 10-2 Scintillation lights at the dead angle 0 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 Energy (keV) No wall effect This effect does not occur with the sphere shape 800 kg detector With light guide Cryodet 16 Remove the wall effect Fiducial PTFE light guide (UV reflection) event rate (/keV/day/kg) Active veto BG simulation PMT K (light guide MC) PMT Th PMT U Fast neutron upper limit (90%C.L.) energy (keV) Cryodet 17 Comparison of data w/ and w/o light guide Hole-B Collimated g ray source from hole-B (137Cs, 662keV) with light guide Counts Counts w/o light guide 10cm fiducial Energy [keV] fiducial volume Energy [keV] Reduce the events due to the wall effect Further analysis of Low energy background is under study Next step Cryodet 18 800kg Detector : WIMP search External g ray BG: 60cm, 346kg 40cm, 100kg Achieved pp & 7Be solar n ~80cm diameter 840-2” PMTs immersed into liq. Xe 70% photo-coverage ~5 keVee threshold Expected dark matter signal (assuming 10-42 cm2, Q.F.=0.2 50GeV / 100GeV,) Expected sensitivities Cross section to nucleon [pb] 10-4 10-6 10-8 10-10 Cryodet 19 XMASS FV 0.5 ton year Eth = 5 keVee~25 p.e., 3s discovery w/o any pulse shape info. 106 104 102 1 Edelweiss Al2O3 Tokyo LiF Modane NaI CRESST UKDMC NaI XMASS(Ann. Mod.) NAIAD 10-2 XMASS(Sepc.) 10-4 Large improvements will be expected SI ~ 10-45 cm2 = 10-9 pb SD~ 10-39 cm2 = 10-3 pb Plots except for XMASS: http://dmtools.berkeley.edu Gaitskell & Mandic Detector design Geometry : 12 pentagons / A tentative design pentakisdodecahedron Cryodet 20 Hamamatsu R8778MOD(hex) 5.8cm (edge to edge) (not final one) 5.4cm 0.3cm (rim) 12cm Total 840 hex PMTs immersed into liq. Xe 70% photo-coverage Radius to inner face ~43cm Hexagonal quartz window Effective area: f50mm (min) QE <~25 % (target) Aiming for 1/10 lower background than R8778 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 Cryodet 21 Fiducial volume At division of F.V. (２５ｃｍ from center) 1MeV ~6mm 5keV ~80mm Position dependence of Light yield & resolution Npe Photo yield 4.2 p.e./keV （at center） Resolution: 1MeV 1.5% 5keV 20% σ（real pos – recon pos）（ｍｍ） Position dependence of reconstructed position resolution Light yield Cryodet 22 5keV 10keV 50keV 500keV 1MeV 100keV Distance from center（ｍｍ） 1MeV res％ Energy resolution 5keV 10keV 500keV 100keV 50keV 10keV 5keV 0 100 200 300 400 Distance from center（ｍｍ） 50keV 500keV 1MeV 0 100 200 300 400 Distance from center（ｍｍ） 100keV Summary • 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. Cryodet 23
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