The XMASS experiment

K. Abe for the XMASS collaboration Kamioka Observatory, ICRR, University of Tokyo The XMASS Collaboration Kamioka Observatory, ICRR, Univ. of Tokyo： Y. Suzuki, M. Nakahata, S. Moriyama, M. Yamashita, Y. Koshio, A. Takeda, K. Abe, H. Sekiya, H. Ogawa, K. Kobayashi, K. Hiraide, K. Ueshima, A. Shinozaki, H. Nishiie, S. Hirano IPMU, University of Tokyo： K. Martens, J.Liu Kobe University: Y. Takeuchi, K. Otsuka Saga University: H. Ohsumi Tokai University: K. Nishijima, D. Motoki Gifu University： S. Tasaka Waseda University： S. Suzuki Yokohama National University： S. Nakamura, I. Murayama, K. Fujii Miyagi University of Education： Y. Fukuda STEL, Nagoya University： Y. Itow, K. Masuda, H. Uchida, Y. Nishitani Seoul National University： S.B. Kim Sejong University： Y.D. Kim KRISS: Y.H. Kim, M.K. Lee, K. B. Lee, J.S. Lee 1. XMASS experiment  What’s XMASS Multi purpose low-background experiment with liq. Xe Y. Suzuki et al., hep-ph/0008296  Xenon MASSive detector for solar neutrino (pp/7Be)  Xenon neutrino MASS detector (ββ decay)  Xenon detector for Weakly Interacting MASSive Particles (DM search)

Solar neutrino Dark matter

As a 1st phase, an 800kg detector for dark matter search is under

Double beta construction. 3  800kg detector Calibration system Elec. hut Inner and Outer Water tank OFHC copper vessel Refrigerator

11m 857kg xenon

642 10m PMTs ~ 1.2m 72 20inch PMTs (veto)  Structure of the PMT holder

Single phase liquid Xenon detector  857kg of liquid xenon, 100kg in the fiducial volume  642 PMTs (630 hex +12 round)  Q.E. : 28-39% Hex: R10789-11  Photo coverage: 62.4%  3D event reconstruction  5keVee threshold with 4.4pe/keVee 58.4

×60 Round: R10789-11MOD

310mm Φ47 Φ1113mm Expected sensitivity ‐45 2 Spin Independent Case σχp>2x10 cm for 50‐100GeV WIMP, 90%C.L. 1yr exposure, 100kg FV, BG: 1x10‐4 /keV/day/kg Scinllaon efficiency: 0.2 Expected energy spectrum 1 year exposure -44 2 σχp=10 cm 50GeV WIMP Black:signal+BG Red:BG 2. Background reduction

(1) BG from detector materials  642 PMTs: Main BG source although RI level is 1/100 of ordinary PMT.  OFHC copper for structure: Bring in the mine < 1month after electroformed (Mitsubishi Material Co.)  Other materials: All the components were selected with HPGe and ICP-MS. (>250 samples were measured) The total RI level is much lower than PMT BG.

We developed new ultra low RI PMT with Hamamatsu. (1/100 of ordinary one). γ tracking  < 10 Self-shieldingforBGfromPMTs -4 n contribution <2.2x10 60Co 40K Th chain U chain /keV/day/kg (100kgF.V.) 2.92 +/-0.16 < 5.10 1.51 +/-0.31 0.70 +/-0.28 BG/PMT [mBq] -5 /d/kg

Counts/day/kg/keV Counts/day/kg/keV keV keV (2) External BG (γ, n) from rock

 γ and n from rock are sufficiently reduced by a 2m thickness pure water tank: γ < γ from PMT, n << 10-4 /day/kg  10m dia. and 11m height water tank for future extensions.  72 20’’ PMTs for active veto for CR µ.

107 n’s γ Att. vs. n

thickness y [cm] 11m water

Liq. Xe PMT BG level  2m needed Reduction of gamma rays X [cm] 10m 0 1 2 3 (m) 5m dia. = ~2m wall thickness (3) Internal BG (1) : Kr

85  Kr ( Kr: Qβ=687keV, τ=10.8y) can be reduced by distillation.  Our goal: Kr < 1ppt ( <10-5 /day/keV/kg)  5 order of magnitude reduction with 4.7kg/hr processing time was achieved. K. Abe et al. for XMASS collab., Astropart. Phys. 31 (2009) 290  Target value can be achieved in 10 days for 1ton xenon. (0.1ppm 1ppt) commercial

LXe GKr 4m intake outlet

Boiling point (@0.2MPa) Kr LXe Xe 178 K outlet Kr 140~150 K Distillation tower (4) Internal BG (2) : Rn

 Measured Rn emanation rate from all materials is < 15mBq  Our goal: 222Rn < 0.6 mBq/ton (<10-5 /kev/day/kg)  Continuous Rn removal with xenon circulation is needed.  Filtering: by gas and liquid. Under study.

Charcoal Filter GXe <30 liter‐GXe/m

LXe ~5 liter‐LXe/m 3. Status of experiment

• Construction of detector main parts have been finished. – PMT holder. – Inner Vessel chamber. – Outer vessel chamber. – Water tank. • Circulation line – Almost finished • Calibration system – Just finished (1)Experimental hall, water shield, and gas handling syst. Calibraon system Electronics hut

GXe reservoir Distillation 10m3 x 2, <1MPa LXe tank Tower

Clean booth at the entrance GXe compressor (2)The PMT holder: construction from Nov. 2009

3.7 4.0 m m

3.0 m The PMT holder: PMT installation • 642 PMTs are attached during 13 days. • 200g/PMT • ~200kg for all PMTs The PMT holder: PMT installation

• All PMTs attached, except boundary 30. • Boundary 30 PMTs were attached after connection of upper and lower half. The PMT holder: Connection of two halves The PMT holder: Filler attachment. Total 2.8ton: end of Feb. 2010 (3)Manufacturing detector vessel • A challenge: Manufacturing a large flange with soft OFHC copper. Inside: Electropolished • Due to insufficient strength of its neck part, it needed to be reinforced by adding ribs. • It took four months.

(4)Inner vessel chamber (5)Outer vessel chamber

(6)Circulation system

Gas circulation <30L/min gas emergency gas pump pump 100L/min Cable line Calibration line filters

Water 10 m3 tank evaporator x 2 Condenser filters 857kg 360W gas storage Outer vacuum liquid 700L pump Liq. Liquid circulation Storage ~5L/min 700 L Liquid xenon reservoir 10m3 emergency reservoir and compressor.

Compressor

• 700L can be contained. • Collect xenon with liquid and keep. • In the case of emergency we can • Vacuum insulation collect xenon gas by compressor. • PTR 180W @-100 degree • 1ton, 170m3 xenon gas with 0.9MPa. • Liquid nitrogen line. • 100 L/min flow rate. • Liquid pump • Max 5L/ • Condenser min • 2 PTR 180W@-100degre e • Estimated heat invasion to the detector is ~50W. • Liquid nitrogen line is also equipped. Gas circulation <30L/min gas emergency gas Cable line pump pump Calibration filters 100L/min line Water 10 m3 tank evaporato x 2 Condenser r 857kg 360W filters gas Outer vacuum liquid 700L storage pump Liq. Liquid circulation Storage ~5L/min • Gas pump • Evaporator – Max 30L/min – To keep Max • Construction of • Gas filter 30L/min gas circulation line – To remove Rn, flow. is almost use charcoal completed. – PTR • Distillated 1.2 ton liquid xenon is already filled Gas circulation <30L/min gas emergency gas in 700L liquid pump pump Cable line reservoir. 100L/min Calibration line filters • Commissioning

Water 10 m3 will be start tank evaporator x 2 soon. Condenser 857kg 360W filters gas reservoir Outer vacuum liquid 700L pump Liq. Liquid circulation reservoir ~5L/min (7)Calibration system • Gamma source – To check • Position reconstruction • Energy resolution • Xenon parameter check, transparency, photo yield – From inside and outside of the detector. – Construction of System is already finished. • LED – PMT Gain (1pe) – 8 LEDs are attached to the PMT holder. – LED calibration at room temperature was done at last month. • Laser + diffuser – PMT Timing – Already finished. Calibration system for inside Linear drive detector + stepping moter  To introduce calibration source to inside detector Linear and rotary drive source  Operate from the water tank top, 5m above detector

 Move top PMT and make window Gate valve through which source can enter Liquid xenon ~5m  Introduce source to inside of the detector.

 Open/close of the window can be checked by optical fiber scope.

 Source can be changed even during observation. 29 Top PMT Inside of detector Constant  ＰＭＴ drive system force Tank top (atmosphere) spring Linear and rotary drive handle Rotate 90 degree

Lift up 150mm

Control rod

Inside detector Top PMT 30 (xenon) Calibration source rod

 φ12mm, length 1560mm,1.54kg  Lift up and down by φ0.3mm stainless steel wire  Calibration source is attached at the end of rod.

Source ＋holder Adaptor OFHC copper (exchangeable) (Stainless steel) 31 Prepared source

nuclide energy [keV] strength diameter package [Hz] [mm] material (1) Fe-55 5.9 350 5 brass (2) Cd-109 22, 25, 88 800 5 brass (3) Am-241 59.5 485 0.15 SUS (4) Co-57 122 100 0.21 SUS

 (1) Fe-55 and (2) Cd-109 are φ5mm Fe sealed in 50µm thickness brass tube  Pressurized test (1MPa)  Leak check at low temperature. 50mm • First task of this system is to check liquid xenon quality during first introduction of liquid xenon scheduled in middle of this month. 4. XMASS schedule in near future

9月 10月 11月

Disllaon Introduce liquid to get rid xenon, circulaon of Kr and purificaon Circulaon line construcon Calibraon Check detector 20’’ＰＭＴ response and xenon Installaon quality Start observaon in water tank.

• Distillation and installation of 20’’ PMT have been finished. • Observation will start after confirmation of xenon quality and detector performance by calibration. Summary • XMASS 800kg detector – Use 1ton liquid xenon. – Single phase detector. – Target sensitivity 10-45cm2 – BG level 10-4/keV/kg/day • Background – From detector, material selection and self shielding. – External gamma and neutron, water shield. – Internal, Kr and Rn, distillation and purification. • Construction – Most parts finished. PMT holder, IVC, OVC, circulation and calibration system. • Schedule – Distillation and installation of 20’’ PMTs finished. – Finishing circulation line. – Introduction of liquid xenon and calibration will be start soon. – After confirmation of detector performance, observation will start.

 Radioactive contamination in LXe

Internal origin of background Measured with the Target values to achieve our goal prototype detector

238U: < 1x10-14 g/g (9±6) x10-14 g/g (~1decay/100kg/d) Further reduction by filter 232Th: < 2x10-14 g/g < 23 x10-14 g/g (~1decay/100kg/d) Upper limit, use filter 85Kr: < 1ppt 3.3±1.1 ppt by a prototype distillation tower

U, Th, Kr near to the goal. Within reach. 較正装置の設置 (水タンク上)

ステッピングモータ、 巻取りドラム

ワイヤー (SUS301, 直径0.3mm)

較正源棒 3m (無酸素銅、直径12mm,長さ1560mm)

較正源交換窓 ゲートバルブ (内径40mm)

PMT操作 水タンク上 ハンドル

較正用配管 較正装置の設置 (検出器内)

較正源棒ガイドパイプ

検出器内 着座センサー

PMTホルダー

較正源棒 39 (検出器最下部位置)

R10789（中性子） 9e-10 SUS U-chain Kovar 7e-10 SiO2 Glass 5e-10 Alumina All 3e-10 1.98e-8 [neutrons/sec/PMT]

[/sec/MeV/PMT] 1e-10 R10789のU,Th含有量 0 2 4 6 8 10 Neutron Energy [MeV] からSOURCES*を使い （ 、 ）及び 由来の中 3e-10 SUS α n SF Th-chain Kovar 性子を計算 SiO 2e-10 2 Glass Alumina All 1e-10 [/sec/MeV/PMT] 6.77e-9 [neutrons/sec/PMT]

0 2 4 6 8 10 * V. Tomasello et.al. NIM A 595 Neutron Energy [MeV] (2008) 431・ 438