XMASS experiment and its double beta decay option

XMASS experiment and its double beta decay option 18 th Sep. 2005, HAW 05 workshop, Double beta-decay and neutrino masses S. Moriyama, ICRR XMASS experiment for dark matter search and low energy solar neutrino detection l Double beta decay option l

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

Why Liquid Xenon? General properties: Large scintillation yield (~42000 photons/Me. V ~Na. I(Tl)) Scintillation wavelength (175 nm, direct read out by PMTs) Higher operation temperature (~165 K, LNe~27 K, LHe~4 K) Compact (r=2. 9 g, 10 t detector ~ 1. 5 m 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) Circulation No long-life radio isotopes Isotope separation is relatively easy No 14 C contamination (can measure low energy)

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

Strategy of the XMASS project ~1 m ~30 cm Prototype detector (FV 3 kg) R&D Confirmation of feasibilities of the ~1 ton detector ~1 ton detector (FV 100 kg) Dark matter search ~2. 5 m ~20 ton detector (FV 10 ton) Solar neutrinos Dark matter search Double beta decay option? Good results

3 kg FV prototype detector In the Kamioka Mine (near the Super-K) OFHC cubic chamber 54 2 -inch low BG PMTs Gamma ray shield Mg. F 2 window • Demonstration of reconstruction, self shielding effect, and low background properties. Hamamatsu R 8778 Liq. Xe　 (31 cm)3 16% photocoverage

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. exp(- m ) m n Log(L) = å Log( ) n ! PMT QADC L: likelihood F(x, y, z, i) x total p. e. m: S F(x, y, z, i) n: observed number of p. e. F(x, y, z, i): acceptance for i-th PMT (MC) VUV photon characteristics: Lemit=42 ph/ke. V tabs=100 cm tscat=30 cm FADC Hit timing === Background event sample === QADC, FADC, and hit timing information are available for analysis

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

Arbitrary Unit Source run (g ray injection from collimators) II 137 Cs: 10 -1 60 Co: 662 ke. V DATA MC 10 -2 10 -3 ~1/200 10 -4 10 -5 -15 10 -5 Reconstructed Z +15 cm -15 Gamma rays l l l Z= -15 Z= +15 1. 17&1. 33 Me. V DATA MC ~1/10 No energy cut, only saturation cut. BG subtracted r=2. 884 g/cc PMT Saturation region Reconstructed Z +15 cm Good agreements. Self shield works as expected. Photo electron yield ~ 0. 8 p. e. /ke. V for all volume

Background data Aug. 04 run Event rate (/kg/day/ke. V) ~1. 6 Hz, 4 fold, triggered by ~0. 4 p. e. 3. 9 days livetime REAL DATA MC simulation All volume 20 cm FV 10 cm FV (3 kg) 10 -2/kg/day/ke. V Miss-reconstruction due to dead-angle region from PMTs. l l 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/ke. V@100 -300 ke. V)

Goal to look Internal background activities l for DM by 1 ton detector Current results • 238 U(Bi/Po): = (33+-7)x 10 -14 g/g Factor ~30, but may decay out further • 232 Th(Bi/Po): < 63 x 10 -14 g/g x 33 x 32 1 x 10 -14 g/g 2 x 10 -14 g/g Factor <~30 (under further study) • Kr: < 5 ppt Achieved by distillation x 5 1 ppt Very near to the target level of U, Th Radon and Kr contamination.

Distillation to reduce Kr (1/1000 by 1 pass) l l Very effective to reduce internal impurities (85 Kr, etc. ) We have processed our Xe before the measurement. Original Xe: ~3 ppb Kr Lower temp. ~1% ~3 m Boiling point (@1 atm) Xe 165 K Kr 120 K Off gas Xe: 330± 100 ppb Kr (measured) Operation: 2 atm Processing speed: 0. 6 kg / hour Design factor: 1/1000 Kr / 1 pass 13 stage of 2 cmf Higher temp. ~99% Purified Xe: < 5 ppt Kr (measured after Kr-enrichment)

1 ton (100 kg FV) detector for DM Search l l l 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 /kg/day/ke. V “Full” photo-sensitive, “Spherical” geometry detector external g ray (60 cm, 346 kg) external g ray: 8 x 10 -5/ke. V/kg/d (40 cm, 100 kg ) Achieved 7 Be 80 cm dia. ~800 -2” PMTs (1/10 Low BG) 70% photo-coverage ~5 p. e. /ke. Vee Dark matter (10 -6 pb, 50 Ge. V, 100 Ge. V) Q. F. = 0. 2 assumed pp 0 100 200 Energy(ke. Vee)

More detailed geometrical design l A tentative design (not final one) 12 pentagons / pentakisdodecahedron Hexagonal PMT ~50 mm diameter Aiming for 1/10 lower BG than R 8778: U 1. 8± 0. 2 x 10 -2 Bq Th 6. 9± 1. 3 x 10 -3 Bq 40 K 1. 4± 0. 2 x 10 -1 Bq This geometry has been coded in a Geant 4 based simulator

Expected sensitivity XMASS FV 0. 5 ton year Eth=5 ke. Vee~25 p. e. , 3 s discovery W/O any pulse shape info. Cross section to nucleon [pb] 10 -4 106 104 10 -6 102 1 10 -8 Edelweiss Al 2 O 3 Tokyo Li. F Modane Na. I CRESST UKDMC Na. I XMASS(Ann. Mod. ) NAIAD 10 -2 XMASS(Sepc. ) 10 -10 l 10 -4 Large improvements expected. Plots except for XMASS: http: //dmtools. berkeley. edu Gaitskell & Mandic

Double beta decay option

l l 2 nbb not yet observed. NA 8. 9% Q=2. 467 Me. V, just below 208 Tl 2. 615 Me. V g rays Self shielding of liquid xenon is not very effective for high energy g rays from rock & PMTs need to be shielded. Event rate (ke. V-1 kg-1 y-1) BG for double beta decay signals with conventional XMASS detector 23 ton 2. 5 m dia. sphere 100% 136 Xe 2 nbb 8 x 1021 yr 15 ton 2. 1 m dia. 10 ton (1. 9 m dia. ) 100% 136 Xe 0 nbb 1025 yr <mn>~0. 2 -0. 3 e. V

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

Expected sensitivity l l Assume acrylic material U, Th~10 -12 g/g, no other bg. Cylindrical geom. (4 cm dia. LXe, 10 cm dia. Vessel) 10 kg 136 Xe 42000 photon/Me. V but 50% scintillation yield, 90% eff. shifter, 80% water transp. , 20% PMT coverage, 25% QE 57 ke. Vrms @ Qbb=2. 48 Me. V 1 yr, 10 kg measurement U+Th normalized for 10 kg, 1 yr 1. 5 x 1025 yr <mn>=0. 2~0. 3 e. V c. f. DAMA > 7 x 1023 yr (90%) 57 ke. Vrms If U/Th ~ 10 -16 g/g + larger expected mass <mn>~0. 02 -0. 03 e. V 2 nbb will not be BG thanks to high resolution!!

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

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

R&D study for wavelength shifter l DC light source: excimer xenon lamp 100 Arbitrary unit Wavelength ~ LXe scintillation light 0 l 160 170 180 190 l (nm) Vacuum vessel, signal PMT and monitor PMT 172 nm sample monitor PMT • Vacuum vessel ~80 cm diameter • Signal and monitor PMTs 　　R 8778 for XMASS • Sample fixed in 50 mm dia. holder • Beam splitter: Mg. F 2 tilted by 45 deg.

TPB in PS Famous WLS for VUV lights TPB: Tetraphenyl butadiene This measurement TPH: p-terphenyl DPS: Dephenyl stilbene Sodium salicylate l 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 58 nm and 74 nm, D. N. Mc. Kinsey et al. , NIMB, 132 (1997) 351 -358 l 0. 5% TPB doped PS, 100 mm

TPB 0. 5% doped PS Two measurements for systematic study (1) Gap between PS and PMT: PS n=1. 59 Due to total reflection Quartz 39 deg. n=1. 5 -1. 6 <39 deg. light go into PMT Solid angle　11% Correction applied (2) Optical grease btw PS and PMT: grease n=1. 47 Light to orange region go into PMT (67 deg. , 39 deg. ) PMT 　Solid angle 50% 67 deg. Correction applied l Efficiency 37+/-6% is obtained for 0. 5% TPB PS. l However, 2% TPB PS does not give consistent results. Further careful study needed. l

Background due to 8 B solar neutrinos 8 B T 2 n 1/2 = 2. 2 x 1022 y T 2 n 1/2 = 4. 0 x 1027 y FWHM=60 ke. V solar neutrinos will be background for 136 Xe double beta decay search. 4. 0 x 1027 y 23 me. V Need Ba daughter tag, two track ID, or ? ? Ge is safe because of its high energy res. 8 B solar neutrinos for double beta decay search A. A. Klimenko, hep-ph/0407156,

Summary l l l XMASS experiment: dark matter, low energy solar neutrino, and double beta decay observation. With 3 kg FV R&D detector, we have demonstrated event reconstruction, self shielding, and low radioactive contamination in xenon. (1)238 U(Bi/Po) = (33+-7)x 10 -14 g/g (2) 232 Th(Bi/Po) < 63 x 10 -14 g/g (3) Kr < 5 ppt. (4) Background @ 200 ke. V ~10 -2/kg/ke. V/day 100 kg 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 172 nm light. Further R&D is ongoing for double beta decay option.

Ge detector case l Ge case, 4. 2 x 1027 y is safe (FWHM 5 ke. V)

Estimation for the size of the water tank Back of an envelope estimation: 208 Tl 2. 6 Me. V gamma rays from the rock 1. 2. Compton scattered gamma by water does not contribute. 3. Gamma flux of 208 Tl based on measured value in the mine. Attenuation coeff. of 2. 614 Me. V in water ： 4. 27 x 10 -2 cm-1 (1/70 by 1 m) Gamma ray flux in the mine, 2. 615 Me. V: 0. 07 cm 2/sec Surface area of 10 kg liquid xenon (24 cm diameter): 2 x 103 cm 2 Probability to deposit 2. 6 Me. V when the gamma ray incident： 10% Water thickness: t (m) BG 1 event/1 yr（2 x 10 -9!） 0. 07 x 2 x 103 x 0. 1 x exp(-4. 27 t) x 86400 x 365 = 1 t =4. 7 m

Distance to the PMT’s 20’’ PMT for Super-Kamiokande 　208 Tl gamma ray ~10/sec/PMT l Use 10 PMT’s Water tank l 　To make 1 event/1 yr background, 10 x (0. 122 p)/4 pt 2 x exp(-4. 27 t) x 86400 x 365 = 1 t=3. 3 m l Rough calculation gives: 13 m in height 10 m in diameter 3. 3 m 10 m

spectrum (light source) Wave length [nm]

Mg. F 2 transmittance Wave length [nm]

5. 8 cm (edge to edge) Hamamatsu R 8778 MOD(hex) l Hexagonal quartz window l Effective area: f 50 mm (min) l QE ~30 % (target) l Aiming for 1/10 lower background than R 8778 5. 4 cm 0. 3 cm (rim) 12 cm c. f. R 8778　 U 1. 8± 0. 2 x 10 -2 Bq Th 6. 9± 1. 3 x 10 -3 Bq 40 K 1. 4± 0. 2 x 10 -1 Bq l Prototype has been manufactured already l Now, being tested

Mc. Kinsey: Excitation by 58, 74 nm