Status of the XMASS experiment

S. Moriyama for the XMASS collaborationKamioka observatory, Institute for Cosmic Ray Research,

Univ. of Tokyo, July 27, 2010, IDM2010

XMASS project

• A large scale, multi-purpose, low background detector for astro-particle physics. 　 Scalability and LowBG

• As a first phase, an 800kg detector for dark matter search is under construction.

●XMASS ◎ Xenon MASSive detector for Solar neutrino (pp/7Be) ◎ Xenon neutrino MASS detector (double beta decay) ◎ Xenon detector for Weakly Interacting MASSive Particles (DM search)

Y. Suzuki et al., hep-ph/0008296

10ton FV (24ton) 2.5mSolar n, 0nbb, DM 100kg FV (800kg)

0.8m, DM

Structure of the 800kg detector• Single phase liquid Xenon detector

– 857kg of liquid xenon, 100kg in the fiducial volume– 630 hex +12 round PMTs with 28-39% Q.E.– photocathode > 62% inner surface– Pentakis dodecahedron– 3D event reconstruction– 5keVee threshold with 4.4pe/keVee

Developed with Hamamatsu

Background reduction 1: g/n from det. partsg /n from detector parts can be reduced by:1. Reduction of RI contamination

I. PMTs: ~1/10 of prev. PMT achievedII. OFHC: brought in the mine <1month

after electrorefining (Mitsubishi Material Co.)

III. Material selection: >250 parts were measured by HPGe, ICPMS, Rn det.

2. Self shielding < 10-4 /keV/d/kg

keV

Cou

nts/

day/

kg/k

eV

g into LXe sphereMC simulation

, g n

n contribution < 2x10-5/keV/d/kg

BG/PMT in mBq with base parts

U chain 0.70 +/- 0.28

Th chain 1.5 +/- 0.31

40K < 5.1

60Co 2.9 +/- 0.16

Liq. Xe

water

X [cm]

107 n’s

n

Background reduction 2: g and n from rockg and n from rock (a,n) will be reduced by apure water tank g << g from PMT, n<<10-4 /d/kg- 11m high and 10m diameter, 72 PMTs (20’’).- First example for dark matter experiments.- Active veto for CR m, passive for g and n.- Applicable for future extensions.- Reduction of n by m’s in rock will be

studied.

31 20 (m)

Att. vs.thickness

PMT BG level 2m needed

, g n

g

y [c

m]

Redu

ction

of g

amm

a ra

ys

>4m2m

2m

Background reduction 3: internal radioactiv.

Kr

LXeintake

Kr (Qb=687keV) and Rn can be reduced by:1. Distillation: Kr has lower boiling point.

I. 5 orders of magnitude reduction (0.1ppm 1ppt ) can be done with 4.7kg/hr: 10days for 1ton before filling into the detector.

K. Abe et al. for XMASS collab., Astropart. Phys. 31 (2009) 290

2. Filtering: by gas and liquid. Under study.

, , a b g

LXeoutlet

GKroutlet

Kr Rn LXe ~a few liter-LXe/m

CharcoalFilter

GXe <30 liter-GXe/m

Expected sensitivityscp>2x10-45 cm2

for 50-100GeV WIMP, 90%C.L.

1yr exposure, 100kg FV,BG: 1x10-4 /keV/d/kgScintillation efficiency: 0.2

XENON10

CDMSII

XMASS 1yr

Black:signal+BGRed:BG

Expected energy spectrum

1 year exposurescp=10-44 cm2

50GeV WIMP

Spin Independent Case

Experimental hall, water shield, and gas handling syst.

Clean boothat the entrance

LXe tank

GXe buffer tank 10m3 x 2, <10bar

GXe compressor

DistillationTower

Electronics hutCalibration system

water tankKamioka

Observatory

Detector Construction

Construction of the PMT holder: Nov. 2009

PMT installation: 311 for each half, 40 for boundary

Cabling: 642 pairs of a coax and HV cable, each 13m length

All the work is under Rn free air and clean environment.

Joining two halves

Filler attachment. Total 2.8ton: end of Feb. 2010

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. On the 29th of July, it will be delivered.

Electronics• Analog Timing Modules (ATM) used for Super-

Kamiokande record charge and timing of PMTs.• FADCs record waveform of PMTs with 500MHz.

Water purification system ~5m3/hour <2mBq-Rn/m3

Rn free air generator ~20m3/hour ~a few mBq-Rn/m3

Schedule• August: Installation of the vessel and PMT

holders at the center of the water tank. Cabling and connection of liquid/gas system will be done.

• Sep.-Oct.: Distillation of xenon to reduce Kr (0.1ppm 1ppt). Evacuation of the detector, water filling test, and liquid xenon filling.

• Oct.-: gas and liquid circulation starts. Reducing contamination in LXe. Data taking will start.

Summary

• The XMASS project aims to observe pp solar neutrinos, neutrinoless double beta decay, and dark matter signals. The 800kg detector is the first phase of the project.

• It is expected to have low background of 1x10-4 keV-1 d-

1kg-1 in the 100kg FV and sensitivity for SI down to 2x10-

45cm2 with one year operation.• All the parts of the 800kg detector are ready and their

assembly work will be finished by the end of August.• Data taking for WIMP search will be started in this year.