Recent Status of XMASS experimentS. Moriyama Institute for Cosmic Ray Research, University of TokyoFor the XMASS collaborationJune 3rd, 2011 @ SPCS2011 Shanghai, China

The XMASS collaborations

Kamioka Observatory, ICRR, Univ. of Tokyo ：Y. Suzuki, M. Nakahata, S. Moriyama, M. Yamashita, Y. Kishimoto,Y. Koshio, A. Takeda, K. Abe, H. Sekiya, H. Ogawa, K. Kobayashi,K. Hiraide, A. Shinozaki, S. Hirano, D. Umemoto, O. Takachio, K. Hieda

IPMU, University of Tokyo ： K. Martens, J.LiuKobe University: Y. Takeuchi, K. Otsuka, K. Hosokawa, A. MurataTokai University: K. Nishijima, D. Motoki, F. KusabaGifu University ： S. TasakaYokohama National University ： S. Nakamura, I. Murayama, K. FujiiMiyagi University of Education ： Y. FukudaSTEL, Nagoya University ： Y. Itow, K. Masuda, H. Uchida, Y. Nishitani,

H. TakiyaSejong University ： Y.D. KimKRISS: Y.H. Kim, M.K. Lee, K. B. Lee, J.S. Lee 41 collaborators,

10 institutes

Kamioka Observatory

• 1000m under a mountain = 2700m water equiv.

• 360m above the sea• Horizontal access• Super-K for n physics and other experiments in

deep underground• KamLAND (Tohoku U.)

By courtesy of Dr. Miyoki

XMASS experiment●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)

• It was proposed that Liquid xenon was a good candidate to satisfy scalability and low background.

• As the first phase, an 800kg detector for a dark matter search was constructed.

Y. Suzuki, hep-ph/0008296

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

in future

100kg FV (800kg) 0.8m, DMFirst phase

Structure of the 800kg detector• Single phase liquid Xenon (-100oC, ~0.065MPa) scintillator

– 835kg of liquid xenon, 100kg in the fiducial volume– 630 hex +12 round PMTs with 28-39% Q.E. are in LXe.– photocathode > 62% inner surface– Pentakis dodecahedron– Interaction position reconstruction– 5keVelectron equiv. (~25keVnuclear recoil) thre.

Developed with Hamamatsu 1.2m diameter

Self shielding effect

• Photo electric effect starts to dominate @500keV: strong self shielding effect is expected for low energy radiations.

• Large photon yield (>~NaI(Tl)) provides a good event reconstruction

E (keV)

Atte

nuat

ion

leng

th (c

m)

water

~O(50keV)

~O(500keV)

PhotoElectricEffect

Comptoneffect10cm

1cm LXe

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

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

<1month after electrorefining.III. Material selection with a HPGe det.

2. Self shielding < 10-4 /keV/day/kg c.f. XENON100 FV before PID: ~10-2/keV/day/kg

keV

Cou

nts/

keV

/day

/kgg into LXe sphere

MC simulation

g, n

n<1.2x10-5/keV/d/kg @5-10keVV.Tomasello, et al., NIMA595(2008)431

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 rock

g and n from rock will be reduced by apure water tank g << g from PMT, n<<10-4/d/kg- 11m high, 10m diameter, and 72 PMTs

(20’’).- First example for dark matter experiments.- Active veto for CR m, passive for g and n.- Applicable for future extensions.

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.

5 orders of magnitude reduction was expected to have been done (0.1ppm1ppt) with 4.7kg/hr: 10days 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

XENON100

CDMSII

XMASS 1yr

Black:signal+BGRed:BG

Expected energy spectrum

1 year exposurescp=10-44 cm2

50GeV WIMP

Spin Independent

Detector Construction

Assembly of PMT holder and installation of PMTs

Joining two halves

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P-01As of Sep. 2010

Evaluation of detector performance and internal

background

Demonstration of the detector performance

• Calibration system– Introduction of radioactive sources

into the detector.– <1mm accuracy along the Z axis.– Thin wire source for some low energy g rays to avoid shadowing effect.

– 57Co, 241Am, 109Cd, 55Fe, 137Cs..

SteppingMotorLinearMotionFeed-through

Topphototube

~5m Gatevalve

4mmf

0.15mmf for 57Co sourceSource rod with a dummy source

Photoelectron dist. at the center of the detector

• 57Co source at the center of the detector gives a typical response of the detector.

• High p.e. yield 15.1+/-1.2p.e./keV was obtained.

• The photo electron yield distribution was reproduced by a simulation well.

Total photo electron distribution

real datasimulation

Reconstructed energy distribution

real datasimulation

122keV

136keV59.3keV of W

~4% rms

Performance of the vertex reconstruction• Reconstructed

vertices for various positioning of the source overlayid.

• Position resolution was as expected by a simulation:

1.4cm RMS (0cm) 1cm RMS (+-20cm) for 122keV g rays

Real Data Simulation

Evaluation of internal background sources• External background (gammas and neutrons) can be

effectively reduced by the water tank and outer part of LXe. Radioactive contaminations (internal background) need to be reduced by other means.

• Radon 222, Radon 220 and Krypton 85 are our concern because they give low energy BG (~flat at signal region).

U chainRn222

Th chainRn220

Radon 222 in LXe• 214Po decays with 164ms half life.• Identified by a short time coincidence. • Gain of 321 PMTs (1/2 of all PMTs) are

reduced to have larger dynamic range.• 8.2+/-0.5mBq in the inner volume

100 500 1000Time difference (ms)

Fitting withan expecteddecay curve

1st event (214Bi b)2nd event (214Po a)

Tail due tosaturation

Radon 220 in LXe• 216Po decays with 0.14sec half life• Identified by two a’s with short coinc.• Result was consistent with chance

coincidence of daughters of 222Rn.• Upper limit <0.28mBq (90%C.L.)

0 500 1000Time difference (ms)

Consistent withchance coincidenceand null hypothesisof Radon 220.

Kr in LXe• Gas chrom. and APIMS (atmospheric pressure ionization

mass spectroscopy) started to work. More sensitive measurement (<1ppt) using a cold trap is under prep.

• Data analysis to look for delayed coincidence events is under study.

Calibration sampleKr=2.7+/-0.6ppb

0-2 -1 1-3 Time (min)Cal x 1/270 (10ppt equiv)

Background (He)

101

100

10-1

10-2

10-3

Ion

curr

ent (

nA)

Sensitivity~10pptachieved

Summary on the internal BG• Our target BG level in FV at signal region is in total 2x10-

4/kg/day/keV. Half of the budget is for the PMT gamma rays and another half is for the internal BG.

• Target for each internal BG corresponds to 1/10 of total:– 222Rn: target 1.0mBq, observed 8.2+-0.5mBq– 220Rn: target 0.43mBq, observed <0.28mBq (90%C.L.)– Kr: target 2ppt, will be evaluated soon

• Low 222Rn, 220Rn, and 85Kr levels are realized in our detector and close to our target numbers.

• 222Rn would be reduced by a cooled charcoal column in gas phase. 85Kr can be reduced by a second distillation process if necessary.

Conclusion

• The XMASS 800kg detector aims to detect dark matter with the sensitivity 2x10-45cm2 (spin independent case).

• It utilizes a single phase of LXe target.• Construction of the 800kg detector finished last winter.• Commissioning runs are on going to confirm the detector

performance and low background properties.– Energy resolution and vertex resolution were as expected. ~1cm

position resolution and ~4% energy resolution for 122keV g.– Radon background are close to the target values and Kr

contamination will be evaluated soon.