XMASS experiment IDM 2006 Rhodes Greece 15 th

XMASS experiment IDM 2006, Rhodes, Greece 15 th Sep. 2006 A. Takeda for the XMASS collaboration B. Kamioka Observatory, ICRR, C. University of Tokyo 1. Introduction 2. 800 kg detector design 3. Summary 1

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 neutrino MASS detector (bb decay) l Xenon detector for Weakly Interacting MASSive Particles (DM search) Solar neutrino Dark matter Double beta 2

Ø Why liquid xenon l Large Z (=54) Self-shielding effect l Large photon yield (~42 photons/ke. V ~ Na. I(Tl)) Low threshold l High density (~3 g/cm 3) Compact detector (10 ton: sphere with diameter of ~2 m) l Purification (distillation) l No long life radioactive isotope l Scintillation wavelength (175 nm, detected directly by PMT) l Relative high temperature (~165 K) 3

Ø Key idea: self-shielding effect for low energy events U-chain gamma rays Blue : γ tracking Pink : whole liquid xenon Deep pink : fiducial volume External g ray from U/Th-chain BG normalized by mass g tracking MC from external to Xenon All volume 20 cm wall cut 30 cm wall cut (10 ton FV) Large self-shield effect 0 1 Me. V 2 Me. V 3 Me. V Background are widely reduced in < 500 ke. V low energy region 4

Ø Strategy of the scale-up 100 kg Prototype 10 ton detector 800 kg detector With light guide ～ 30 cm ～ 80 cm ～ 2. 5 m R&D ü Vertex & energy reconstruction ü Self shielding power ü BG level We are here ! Dark matter search Multipurpose detector (solar neutrino, bb …) 5

Trend of Dark matter (WIMPs) direct searches l Recoiled nuclei are mainly observed by 3 ways Scintillation Na. I, Xe, Ca. F 2, etc. Phonon Ge, Te. O 2, Al 2 O 3, Li. F, etc Ge, Si Ionization Ge l Taking two type of signals simultaneously is recent trend CDMS, EDELWEISS: phonon + ionization u g ray reduction owing to powerful particle ID u However, seems to be difficult to realize a large and uniform detector due to complicated technique 6

Strategy chosen by XMASS l Make large mass and uniform detector (with liq. Xe) Same style as successful experiments of Super-K, SNO, Kam. LAND, etc. l Reduce g ray BG by fiducial volume cut (self shielding) Super-K SNO Kam. LAND 7

2. 800 kg detector design Main purpose: Dark Matter search External g ray BG: 60 cm, 346 kg 40 cm, 100 kg Achieved pp & 7 Be solar n ~80 cm diameter l 812 -2” PMTs immersed into liq. Xe l 70% photo-coverage 0 ~4 p. e. / ke. V 100 200 300 Energy [ke. V] Expected dark matter signal (assuming 10 -6 pb, Q. F. =0. 2 50 Ge. V / 100 Ge. V, ) 8

Expected sensitivities Cross section to nucleon [pb] 10 -4 XMASS FV 0. 5 ton year Eth = 5 ke. Vee~25 p. e. , 3 s discovery w/o any pulse shape info. 106 104 10 -6 10 -8 102 1 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 10 -4 l Large improvements will be expected SI ~ 10 -45 cm 2 = 10 -9 pb SD~ 10 -39 cm 2 = 10 -3 pb Plots except for XMASS: http: //dmtools. berkeley. edu Gaitskell & Mandic 9

Ø Status of 800 kg detector l Basic performances have been already confirmed using 100 kg prototype detector ü Vertex and energy reconstruction by fitter ü Self shielding power ü BG level l Detector design is going using MC ü Structure and PMT arrangement (812 PMTs) ü Event reconstruction ü BG estimation l New excavation will be done soon ü Necessary size of shielding around the chamber 10

Ø Structure of 800 kg detector 12 pentagons / pentakisdodecahedron Hamamatsu R 8778 MOD 1 PMT Hex agonal quarts window m c 1 3 6 2 1 3 5 4 31 cm 7 8 34 cm 10 PMTs per 1 triangle 9 10 5 triangles make pentagon 11

l Total 812 hex PMTs (10 PMTs/triangle× 60 + 212 @gap) immersed into liq. Xe l ~70% photo-coverage l Radius to inner face ~44 cm Each rim of a PMT overlaps to maximize coverage 12

Ø Event reconstruction @Boundary of fiducial volume l Position resolution 10 ke. V ~ 3. 2 cm 5 ke. V ~ 5. 3 cm s (reconstructed) [cm] 60 Generated R = 31 cm 50 E = 10 ke. V Events 40 30 20 s = 2. 3 cm 10 0 22 26 30 34 38 Reconstructed position [cm] 12 10 Fiducial volume 8 5 ke. V 6 10 ke. V 4 50 ke. V 2 0 100 ke. V 500 ke. V 1 Me. V 0 10 20 30 40 Distance from the center [cm] 13

R_reconstructed(cm) 50 45 40 35 30 25 20 15 10 5 5 ke. V ~ 1 Me. V l Generated VS reconstructed • Up to <~40 cm, events are well reconstructed with position resolution of ~2~5 cm • Out of 42 cm, grid whose most similar distribution is selected because of no grid data • In the 40 cm~44 cm region, reconstructed events are concentrated around 42 cm, but they are not mistaken for those occurred in the center • No wall effect • Out of 45 cm, some events occurring behind the PMT are miss reconstructed 0 5 10 15 20 25 30 35 40 45 50 Distance from the center [cm] 14

Light leak events Some scintillation lights generated behind the PMT enter the inner region It is not problem if light shield is installed PMT PMT hit map 15

Ø 800 kg BG study Achieved (measured by prototype detector) Goal (800 kg detector) l g ray from PMTs ~ 10 -2 cpd/kg/ke. V 1/100 10 -4 cpd/kg/ke. V → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10) l 238 U l 232 Th = (33± 7)× 10 -14 g/g → Remove by filter < 23× 10 -14 g/g (90% C. L. ) → Remove by filter (Only upper limit) l Kr = 3. 3± 1. 1 ppt → Achieve by 2 purification pass 1/33 1× 10 -14 g/g 1/12 2× 10 -14 g/g 1/3 1 ppt 16

Ø Estimation of g ray BG from PMTs Counts/ke. V/day/kg All volume R<39. 5 cm R<34. 5 cm R<24. 5 cm • U-chain • 1/10 lower BG PMT than R 8778 Statistics: 2. 1 days All volume R<39. 5 cm R<34. 5 cm R<24. 5 cm No event is found below 100 ke. V after fiducial cut (R<24. 5 cm) < 1× 10 -4 cpd/kg/ke. V can be achieved (Now, more statistics is accumulating) Energy [ke. V] 17

Ø Water shield for ambient g and fast neutron Necessary shielding was estimated for the estimation of the size of the new excavation Generation point of g or neutron wa Liq. Xe water Configuration of the estimation l Put 80 cm diameter liquid Xe ball l Assume several size of water shield 50, 100, 150, and 200 cm thickness l Assume copper vessel (2 cm thickness) for liquid Xe MC geometry 18

Detected/generated*surface [cm 2] Ø g attenuation Initial energy spectrum from the rock 104 g attenuation by water shield 103 102 Deposit energy spectrum (200 cm) 10 1 10 -1 PMT BG level 10 -2 0 100 200 300 Distance from LXe [cm] More than 200 cm water is needed to reduce the BG to the PMT BG level 19

Ø fast neutron attenuation water: 200 cm, n: 10 Me. V • Fast n flux @Kamioka mine: (1. 15± 0. 12) × 10 -5 /cm 2/sec • Assuming all the energies are 10 Me. V very conservatively water < 2× 10 -2 counts/day/kg Liq. Xe No event is found from the generated neutron of 105 200 cm water is enough to reduce the BG to the PMT BG level BG caused by thermal neutron is now under estimation 20

Ø New excavation @Kamioka mine New excavation for XMASS and other underground experiment will be made soon ~5 m ~20 m ~15 m 21

3. Summary l XMASS experiment: Multi purpose low-background experiment with large mass liq. Xe l 800 kg detector: Designed for dark matter search mainly, and 102 improvement of sensitivity above existing experiments is expected l Detector design of 800 kg detector is going ü BG estimation ü Shielding ü New excavation 22

Backup 23

800 kg detector: Main purpose: Dark Matter search ~80 cm diameter External g ray BG: 60 cm, 346 kg 40 cm, 100 kg Achieved 5 ke. V pp & 7 Be solar n 10 ke. V Photoelectrons (p. e. ) l ~800 -2” PMTs immersed into liq. Xe l 70% photo-coverage ~4 p. e. /ke. V Expected dark matter signal (assuming 10 -42 cm 2, Q. F. =0. 2 50 Ge. V / 100 Ge. V, ) 24

XMASS collaboration • ICRR, Kamioka Y. Suzuki, M. Nakahata, S. Moriyama, M. Shiozawa, Y. Takeuchi , M. Miura, Y. Koshio, K. Abe, H. Sekiya, A. Takeda, H. Ogawa, A. Minamino, T. Iida, K. Ueshima • ICRR, RCNN T. Kajita, K. Kaneyuki • Saga Univ. H. Ohsumi • Tokai Univ. K. Nishijima, T. Maruyama, Y. Sakurai • Gifu Univ. S. Tasaka • Waseda Univ. S. Suzuki, J. Kikuchi, T. Doke, A. Ota, Y. Ebizuka • Yokohama National Univ. S. Nakamura, Y. Uchida, M, Kikuchi, K. Tomita, Y. Ozaki, T. Nagase, T. Kamei, M. Shibasaki, T. Ogiwara • Miyagi Univ. of Education Y. Fukuda, T. Sato • Nagoya ST Y. Itow • Seoul National Univ. Soo-Bong Kim • INR-Kiev O. Ponkratenko • Sejong univ. Y. D. Kim, J. I. Lee, S. H. Moon 25

5. 8 cm (edge to edge) Hamamatsu R 8778 MOD(hex) l Hexagonal quartz window l Effective area: f 50 mm (min) l QE <~25 % (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 26

c. f. R 8778 (used for 100 kg chamber) 　 U 1. 8± 0. 2 x 10 -2 Bq Th 6. 9± 1. 3 x 10 -3 Bq 40 K -1 Th U 1. 4± 0. 2 x 10 Bq 40 K ※measured by HPGe detector in Kamioka Ceramic dielectric parts to support dynodes For R 8778 mod 　using quartz Glass parts for feed through & containment For R 8778 mod 　Reduce glass material Improvement result will be coming soon! 27

Ø BG levels Events/kg/ke. V/day DAMA Na. I ZEPLIN before PSD cut Kamioka Ge Current XMASS (new improvement!) XMASS 800 kg CDMS II After PID DM signal for LXe 100 Ge. V 10 -6 pb Heidelberg Moscow Kam. LAND (>0. 8 Me. V) Super-K 28

R&D status using prototype detector 100 kg prototype Ø Main purpose l Confirmation of estimated 800 kg detector performance ~30 cm cube 3 kg fiducial With light guide version u Vertex and energy reconstruction by fitter u Miss fitting due to dead angle of the cubic detector (“wall effect”, will be explained later) can be removed with light guide u Self shielding power l BG study u Understanding of the source of BG u Measuring photon yield and its attenuation length 29

Ø 100 kg prototype detector In the Kamioka Mine (near the Super-K) 2, 700 m. w. e. OFHC cubic chamber 54 2 -inch low BG PMTs Hamamatsu R 8778 16% photocoverage Liq. Xe (31 cm)3 Gamma ray shield Mg. F 2 window 30

4 p shield with door 1. 0 m 1. 9 m material thickness 　　Polyethylene 15 cm 　　Boron 5 cm 　　Lead 15 cm 　　EVOH sheets 30μm 　　OF Cupper 5 cm Rn free air (~3 m. Bq/m 3) 31

100 kg Run summary l 1 st run (Dec. 2003) u Confirmed performances of vertex & energy reconstruction u Confirmed self shielding power for external g rays u Measured the internal background concentration l 2 nd run (Aug. 2004) u Succeeded to reduce Kr from Xe by distillation u Photo electron yield is increased u Measured Rn concentration inside the shield l 3 rd run (Mar. 2005) with light guide u Confirmed the miss fitting (only for the prototype detector) was removed u Now, BG data is under analysis 32

Ø 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 33 information are available for analysis

Performance of the vertex reconstruction Collimated g ray source run from 3 holes (137 Cs, 662 ke. V) hole C hole B hole A DATA MC + + + C BA → Vertex reconstruction works well 34

Performance of the energy reconstruction Collimated g ray source run from center hole (137 Cs, 662 ke. V) All volume 20 cm FV 10 cm FV s=65 ke. V@peak (s/E ~ 10%) Similar peak position in each fiducial. No position bias → Energy reconstruction works well 35

Demonstration of self shielding effect z position distribution of the collimated g ray source run → Data and MC agree well γ 36

Event rate (/kg/day/ke. V) Shelf shielding for real data and MC ~1. 6 Hz, 4 fold, triggered by ~0. 4 p. e. 3. 9 days livetime REAL DATA Aug. 04 run preliminary 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 Good agreement (< factor 2) l Self shielding effect can be seen clearly. l Very low background (10 -2 /kg/day/ke. V@100 -300 ke. V) 37

Ø Internal backgrounds in liq. Xe were measured Main sources in liq. Xe are Kr, U-chain and Th-chain l Kr = 3. 3± 1. 1 ppt (by mass spectrometer) → Achieved by distillation l U-chain = (33± 7)x 10 -14 g/g (by prototype detector) Delayed coincidence search (radiation equilibrium assumed) 214 Bi 214 Po 210 Pb a (7. 7 Me. V) b (Q=3. 3 Me. V) t 1/2=164 ms l Th-chain < 23 x 10 -14 g/g(90%CL) (by prototype detector) Delayed coincidence search (radiation equilibrium assumed) 208 Po 212 Bi 212 Po a (8. 8 Me. V) b (Q=2. 3 Me. V) t 1/2=299 ns 38

Kr concentration in Xe cpd/kg/ke. V 　　　　 l 85 Kr makes BG in low enegy region 102 Target = Xe Kr 0. 1 ppm 1 10 -2 10 -4 DM signal (10 -6 pb, 50 Ge. V, 100 Ge. V) 10 -6 0 200 400 600 800 l Kr can easily mix with Xe energy (ke. V) because both Kr and Xe are rare　　　　　　　 gas l Commercial Xe contains a few ppb Kr 39

Xe purification system l XMASS succeeds to reduce Kr concentration in Xe from ~3[ppb] to 3. 3(± 1. 1)[ppt] with one cycle (~1/1000) • Processing speed : 0. 6 kg / hour Boiling point (@2 atm) • Design factor : 1/1000 Kr / 1 pass • Purified Xe : Off gas = 99: 1 Raw Xe: ~3 ppb Kr Lower (178 K) ~3 m ~1% Xe 178. 1 K Kr 129. 4 K Off gas Xe: 330± 100 ppb Kr (measured) Purified Xe: Operation@2 atm Higher (180 K) ~99% 3. 3± 1. 1 ppt Kr (measured) (preliminary) 40

Ø Remaining problem: wall effect (only for the prototype detector) HIT 1 ? Dead angle MC If true vertex is used for fiducial volume cut 10 -1 HIT HIT 10 -2 l 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 (ke. V) No wall effect This effect does not occur with the sphere shape 800 kg detector 41

Ø 800 kg detector 42