The XMASS experiment Seminar Kamioka Oct 12 2007

The XMASS experiment Seminar @ Kamioka Oct 12, 2007 H. Ogawa for XMASS collaborator From this year, the XMASS experiment starts the new project, 800 kg xenon detector for dark matter search. Today I talk about this new project with these contents : 1. Introduction 2. Schedule & progress 3. Summary

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

Strategy of XMASS experiment 100 kg Prototype FV 3 kg PMT are mounted At outside of detector 23 ton detector, FV 10 t 800 kg detector FV 100 kg PMT are mounted At inside of detector ～ 30 cm ※Basic design ～ 2. 5 m ～ 80 cm 2~3 yr construction from Further study on DM 2007& >3 yr obs. Solar neutrino phys. With reflector & shield R&D Dark matter search Multipurpose detector (solar neutrino, bb …)

Major method of DM direct detection CDMS EDELWEISS ionization Direct detection＝ Nuclear recoil heat Ge detector etc Tokyo Li. F, Na. F SIMPLE etc CRESST etc XENON ZEPLIN-II etc Scintillation light XMASS, DAMA, ZEPLIN-I etc The method of “ionization”, “heat” and “scintillation light” are progressed. These method are combinated for each other or used as large size one to reduce the background.

DM search for XMASS uses scintillation light detection with large volume xenon Dark matter Xenon nucleus Scintillation light Q value ~25 e. V Xe Ge Si Xe Si Ge The merit of Xenon using： Large mass (Z=54) - Dominates to obs. Rate ∝ A 2 -external BG reduction by Self shield Large photon yield (42 p. e. /ke. V~Na. I)。 Scintillation wavelength ~ UV 170 nm, =>detected directly by PMT High density ~3 g/cm 3 =>compact detector （d~80 cm for ~ton)。 Purification No long life time for RI Red: differential, Blue: integrated R. J. Gaitskell, Ann. Rev. Part. Sci. , 54 (2004) 315.

feature：Vertex reconstruction Large photoelectron Yield can be obtained ~5 p. e. /ke. V Accurate vertex Reconstruction based on light pattern possible 54 2 -inch low BG PMTs Hamamatsu R 8778 Confirmation with a prototype detector done. Liq. Xe (31 cm)3 16% photo. Mg. F 2 window coverage

Arbitrary Unit Demonstration with a prototype detector 137 Cs: 10 -1 60 Co: 662 ke. V DATA MC 10 -2 10 -3 ~1/200 PMT saturation 10 -4 10 -5 1. 17&1. 33 Me. V DATA Gamma rays MC ~1/10 -15 Reconstructed Z 10 -5 +15 cm -15 Gamma rays Z= -15 Z= +15 Reconstructed Z +15 cm Confirmed Self shielding >2 orders of magnitude agreements

ØPower of self shield effect g tracking MC from external to Xenon U-chain gamma rays Dark matter search Blue : g tracking Pink : whole liquid xenon Deep pink : fiducial volume 1. With liquid xenon ~1 ton, reduce BG below 100 ke. V to -4/day/ke. V/kg by self shielding. 2. Search the signal from dark matter in low energy region. 10

Sensitivity of the 800 kg detector Cross section to nucleon (pb) XMASS FV 0. 5 ton year (100 kg, 5 yr) 3 s discovery 10 -4 SI DAMA 10 -6 10 -8 104 XENON 10 102 XMASS 800 kg 1 XENON 10+ Super. CDM 10 -10 S Phase A 10 106 WARP 140 kg 10 J Ellis et al. benchmark 10 102 1033 WIMP mass(Ge. V) -2 10 -4 x ~100 sensitive than XENON 10 Plots exept for XMASS http: //dmtools. berkeley. edu Gaitskell & Mandic

2. Schedule & progress component 4 -6 7 -9 10 -12 month detector Xenon line PMT 2009 2008 2007 1 -3 design 4 -6 7 -9 production 10 -12 1 -3 Installation production installation production Development of breeder • MC study, electronics etc are also progressed. • Today contents for 800 kg detector: – detector structure – PMT test – BG reduction study • ~2 yrs are taken for detector construction • DM search will start from ~ 2009 (expectation)

Ø Structure of 800 kg detector 12 pyramids / 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 pentagonal pyramid

prototype of triangle mockup to study the PMT holding, attachment Hexagonal PMT

l PMT test within liquid xenon Liquid Xenon hex PMT 1 Co-57 15 mm PTFE light guide Blue LED (122 ke. V) const. (p 0) : 74. 8 mean (p 1) : 2088. 7 sigma (p 2) : 49. 9 (136 ke. V) const. (p 3) : 6. 1 mean (p 4) : 2267. 2 sigma (p 5) : 73. 9 hex PMT 2 PTFE light guide PMT 1 + PMT 2 resolution : 2. 4 % @122 ke. V Study of PMT stability in Xenon is progressed.

Water shielding and a new experimental hall 20 inch PMTs for muon veto Kam. LAND 15 m SK 21 m Sep/12/2007 New Halls • n prod. in PMTs is estimated < g. BG • Passive for n from rock, active for n from CR m • Excavation started from 7 th Aug. and finish Feb. 2008.

BG reduction for 800 kg detector PMT g n RI in LXe • Origins of Background PMT g rays: low BG PMT, self shielding Neutrons: passive/active water shield Radioactive impurities in LXe : 1) RI into LXe 2) Radon emanation from PMTs Target BG ~10 -4 events/kg/d/ke. V(109 pb): U<1 e-14 g/g, Th<2 e-14 g/g, filtering Rn 222~0. 05 dacay/day/kg, Rn 220~0. 02 dacay/day/kg Kr<1 ppt distillation

– – 238 U chain 1. 8 x 10 -3 Bq/PMT 232 Th chain 6. 9 x 10 -4 Bq/PMT 60 Co 5. 5 x 10 -3 Bq/PMT 40 K 1. 4 x 10 -2 Bq/PMT • Below 300 ke. V number of events in the 25 cm fiducial volume decreases rapidly. • Below 100 ke. V remaining events are few. • Below 300 ke. V, <10 -4 dru BG level. • Below 100 ke. V, BG becomes <10 -5 level. dru • Target of PMT activity dru ØEstimated PMT ＢＧ All volume 40 cm Fiducial Volume 35 cm Fiducial Volume 25 cm Fiducial Volume

Water shield tank. Reduce the external gammas and fast neutron Path length of fast neutron at water G 4 MC : Generation: 107 water: 200 cm, energy: 10 Me. V • Fast n flux @Kamioka mine: (1. 15+0. 12) x 10 -5 /cm 2/sec - Z [cm] ~200 cm water shield case, Rate of neutron into xenon with energy deposit are < 2 x 10 -4 counts/day/kg Liq. Xe ~10 m X [cm] The detail design of water shield tank is ~1. 5 m for detector progressed. ~4. 25 m

Radioactive impurities in LXe Target values to achieve our goal 238 U: < 1 x 10 -14 g/g (~1 decay/100 kg/d) 232 Th: < 2 x 10 -14 g/g (~1 decay/100 kg/d) 85 Kr: < 1 ppt from reactors Measured with the prototype detector (9± 6) x 10 -14 g/g Further reduction by filter < 23 x 10 -14 g/g Upper limit, use filter 3. 3± 1. 1 ppt by a prototype distillation tower U, Th, Kr near to the goal. Within reach.

Xenon distillation 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)

Xenon circulation line Purpose : Xenon liquation, installation to detector, purification • • line： – Refrigerator: pulse tube type – Liquid circulation unit： xenon purification with filtering – Reserve tank: keeping by liquid – Piping, monitors etc… Filtering： – Charcoal：absorbs the radon. Low radon emanation itself is required. – Molecular sieve : remove water from xenon. Need to reduce RI&radon emanation in MS. Basic xenon line Xenon gas in Getter Distillation Refrigerator filtering Filtering 1) Charcoal 2) molecular sieve Main detector ~1 ton Liquid circulation unit Reserve tank

Summary • DM search for XMASS – Scintillation light from nuclear recoil is detected. • XMASS 800 kg detector – 1 ton liquid xenon, 90 cm diameter, 60 triangles, 812 PMTs – Dark matter search ~10 -9 pb • Schedule – Budget funded in this year. – Design & planning phase to construct the detector now.