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XMASS experiment and its double beta decay option

XMASS experiment and its double beta decay option. 18 th Sep. 2005, HAW05 workshop, Double beta-decay and neutrino masses S. Moriyama, ICRR. XMASS experiment for dark matter search and low energy solar neutrino detection Double beta decay option. 1. Introduction. Solar neutrino.

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XMASS experiment and its double beta decay option

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  1. XMASS experiment and its double beta decay option 18th Sep. 2005, HAW05 workshop, Double beta-decay and neutrino masses S. Moriyama, ICRR XMASS experiment for dark matter search and low energy solar neutrino detection Double beta decay option

  2. 1. Introduction Solar neutrino • What’s XMASS Multi purpose low-background experiment with liq. Xe • Xenon MASSive detector for solar neutrino (pp/7Be) • Xenon detector for Weakly Interacting MASSive Particles (DM search) • Xenon neutrino MASS detector (bb decay) Dark matter Double beta

  3. Why Liquid Xenon? General properties: Large scintillation yield (~42000photons/MeV ~NaI(Tl)) Scintillation wavelength (175nm, direct read out by PMTs) Higher operation temperature (~165K, LNe~27K, LHe~4K) Compact (r=2.9g, 10t detector ~ 1.5m cubic) Not so expensive Well-known EW cross sections for neutrinos External gamma ray background: Self shielding (large Z=54) Internal background: Purification (distillation, etc) No long-life radio isotopes Isotope separation is relatively easy No 14C contamination (can measure low energy) Circulation

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

  5. 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 Double beta decay option? Good results

  6. 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.

  7. - 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

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

  9. Source run(g ray injection from collimators) II 60Co: 1.17&1.33MeV DATA MC 137Cs: 662keV DATA MC 10-1 10-1 No energy cut, only saturation cut. BG subtracted r=2.884g/cc • Good agreements. • Self shield works as expected. • Photo electron yield ~ 0.8p.e./keV for all volume 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

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

  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. Internal background activities • Current results • 238U(Bi/Po): = (33+-7)x10-14g/g • 232Th(Bi/Po): < 63x10-14g/g • Kr: < 5ppt Factor ~30, but may decay out further Factor <~30 (under further study) 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 “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. More detailed geometrical design • A tentative design (not final one) 12 pentagons / pentakisdodecahedron Hexagonal PMT ~50mm diameter Aiming for 1/10 lower BG than R8778 R8778: U 1.8±0.2x10-2 Bq Th 6.9±1.3x10-3 Bq 40K 1.4±0.2x10-1 Bq This geometry has been coded in a Geant 4 based simulator

  15. Expected sensitivity XMASS FV 0.5ton year Eth=5keVee~25p.e., 3s discovery W/O any pulse shape info. 10-4 106 • Large improvements expected. 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

  16. Double beta decay option

  17. 23ton 2.5m dia. sphere Event rate (keV-1kg-1y-1) 100% 136Xe 2nbb 8x1021yr 15ton 2.1m dia. 10ton (1.9m dia. ) 100% 136Xe 0nbb 1025yr <mn>~0.2-0.3eV BG for double beta decay signals with conventional XMASS detector • 2nbb not yet observed. NA 8.9% • Q=2.467MeV, just below 208Tl 2.615MeV g rays • Self shielding of liquid xenon is not very effective for high energy g rays. • g rays from rock & PMTs need to be shielded.

  18. Symbolically… One of possible solutions Put room temperature LXe into a thick, acrylic pressure vessel (~50atm). Wavelength shifter inside the vessel. We already have 10kg enriched 136Xe. Merit: Xe can be purified even after experiment starts!

  19. Expected sensitivity • Assume acrylic material U,Th~10-12g/g, no other bg. • Cylindrical geom. (4cm dia. LXe, 10cm dia. Vessel) • 10kg 136Xe • 42000photon/MeV but 50% scintillation yield, 90% eff. shifter, 80% water transp., 20% PMT coverage, 25% QE  57keVrms @ Qbb=2.48MeV 1yr, 10kg measurement 1.5 x 1025 yr  <mn>=0.2~0.3eV c.f. DAMA > 7 x 1023 yr (90%) U+Th normalized for 10kg, 1yr 57keVrms expected If U/Th ~ 10-16 g/g + larger mass  <mn>~0.02-0.03eV 2nbbwill not be BG thanks to high resolution!!

  20. - • Double focus detector • Cheap • Easy • Safe Water sheild Scintillation light Useful for any scintillators PMTｓ Pressure vessel R&D items c.f. wavelength shifter: M.A.Iqbal et al., NIMA 243(1986)459 L. Periale et al., NIMA 478(2002)377 D. N. McKinsey et al., NIMB 132 (1997) 351 • Pressure test • Wavelength shifter • Scintillation yield • Possible creep effect on acrylic material • Degas from acrylic surface • BG consideration (time anal., plastic scinti. vessel) • Detector design

  21. Pressure test vessel Test vessel held 80 atm water!! 120mm length valve water 50mm-dia., 50mm length ~98cc 110mm-dia.

  22. R&D study for wavelength shifter • DC light source: excimer xenon lamp • Vacuum vessel, signal PMT and monitor PMT Wavelength ~ LXe scintillation light 100 Arbitrary unit 0 l (nm) 160 170 190 180 172nm • Vacuum vessel ~80cm diameter • Signal and monitor PMTs • R8778 for XMASS • Sample fixed in 50mm dia. holder • Beam splitter: MgF2 tilted by 45 deg. sample monitor PMT PMT

  23. TPB in PS • Famous WLS for VUV lights TPB: Tetraphenyl butadiene  This measurement TPH: p-terphenyl DPS: Dephenyl stilbene Sodium salicylate • Doped in a polystyrene films 0.5, 1.0, 2.0, 4.0, 8.0, 16.0% (in weight) Ref. systematic study on doped films for 58nm and 74nm, D. N. McKinsey et al., NIMB, 132 (1997) 351-358 0.5% TPB doped PS, 100mm

  24. TPB 0.5% doped PS • Two measurements for systematic study (1) Gap between PS and PMT: PS n=1.59  Due to total reflection <39deg. light go into PMT • Solid angle11% Correction applied (2) Optical grease btw PS and PMT: grease n=1.47 • Light to orange region go into PMT (67deg., 39deg.) Solid angle 50% Correction applied • Efficiency 37+/-6% is obtained for 0.5% TPB PS. • However, 2% TPB PS does not give consistent results. Further careful study needed. PMT Quartz n=1.5-1.6 39deg. PMT 67deg.

  25. Background due to 8B solar neutrinos 8B solar neutrinos for double beta decay search A. A. Klimenko, hep-ph/0407156, 8B solar neutrinos will be background for 136Xe double beta decay search. 4.0x1027y  23meV Need Ba daughter tag, two track ID, or ?? Ge is safe because of its high energy res. T2n1/2 = 2.2x1022y T2n1/2 = 4.0x1027y FWHM=60keV

  26. Summary • XMASS experiment: dark matter, low energy solar neutrino, and double beta decay observation. • With 3kg FV R&D detector, we have demonstrated event reconstruction, self shielding, and low radioactive contamination in xenon. (1)238U(Bi/Po) = (33+-7)x10-14g/g (2) 232Th(Bi/Po) < 63x10-14g/g (3) Kr < 5ppt. (4) Background @ 200keV ~10-2/kg/keV/day • 100kg FV XMASS detector is expected to give ~100 improvement for current dark matter search. • For double beta decay option, another design is discussed. • Wavelength shifter (0.5% TPB in PS) gives 37+/-6% conversion efficiency for 172nm light. • Further R&D is ongoing for double beta decay option.

  27. Ge detector case • Ge case, 4.2x1027y is safe (FWHM 5keV)

  28. Estimation for the size of the water tank Back of an envelope estimation: • 208Tl 2.6MeV gamma rays from the rock • Compton scattered gamma by water does not contribute. • Gamma flux of 208Tl based on measured value in the mine. Attenuation coeff. of 2.614MeV in water ：4.27x10-2 cm-1 (1/70 by 1m) Gamma ray flux in the mine, 2.615MeV: 0.07cm2/sec Surface area of 10kg liquid xenon (24cm diameter): 2x103cm2 Probability to deposit 2.6MeV when the gamma ray incident：10% Water thickness: t (m) BG 1event/1yr（2x10-9!） 0.07 x 2x103 x 0.1 x exp(-4.27 t) x 86400 x 365 = 1  t =4.7m

  29. Water tank 3.3m 13m 10m Distance to the PMT’s • 20’’ PMT for Super-Kamiokande 208Tl gamma ray ~10/sec/PMT • Use 10 PMT’s To make 1event/1yr background, 10 x 10 x (0.122p)/4pt2 x exp(-4.27 t) x 86400 x 365 = 1  t=3.3m • Rough calculation gives: 13m in height 10m in diameter

  30. spectrum (light source) Wave length [nm]

  31. MgF2 transmittance Wave length [nm]

  32. Hamamatsu R8778MOD(hex) • Hexagonal quartz window • Effective area: f50mm (min) • QE ~30 % (target) • Aiming for 1/10 lower background than R8778 5.8cm (edge to edge) 0.3cm (rim) c.f. R8778 U 1.8±0.2x10-2 Bq Th 6.9±1.3x10-3 Bq 40K 1.4±0.2x10-1 Bq 5.4cm • Prototype has been manufactured already • Now, being tested 12cm

  33. McKinsey: Excitation by 58, 74nm

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