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1. Introduction2. R&D status using prototype detector3. Summary

XMASS experiment8th June 2005

A. Takeda for the XMASS collaborationKamioka Observatory, ICRR,University of Tokyo

WIN05

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1. Introduction

Dark matter

Double beta

Solar neutrino

What’s XMASS

Xenon MASSive detector for solar neutrino (pp/7Be) Xenon neutrino MASS detector ( decay) Xenon detector for Weakly Interacting MASSive Particles (DM search)

Multi purpose low-background experiment with liq. Xe

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Why liquid xenon

Large Z (=54) Self-shielding effect Large photon yield (~42 photons/keV ~ NaI(Tl)) Low threshold High density (~3 g/cm3) Compact detector (10 ton: sphere with diameter of ~2m) Purification (distillation)

No long life radioactive isotope Scintillation wavelength (175 nm, detected directly by PMT) Relative high temperature (~165 K)

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PMTs

Single phaseliquid Xe Volume for shielding

Fiducial volume

23ton all volume20cm wall cut30cm wall cut (10ton FV)

BG

nor

ma

lize

d b

y m

ass

1MeV0 2MeV 3MeV

Large self-shield effect

External ray from U/Th-chain

Key idea: self-shielding effect for low energy events

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100kg Prototype 800kg detector

10 ton detector

～ 30cm～ 80cm

～ 2.5m

R&DDark matter search

Multipurpose detector

(solar neutrino, …)

We are now here

Strategy of the scale-up

With light guide

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Trend of Dark matter (WIMPs) direct searches

Scintillation

Phonon Ionization

Ge, TeO2, Al2O3, LiF, etc

NaI, Xe, CaF2, etc.

Ge

Recoiled nuclei are mainly observed by 3 ways

CDMS, EDELWEISS: phonon + ionization

Taking two type of signals simultaneously is recent trend

ray reduction owing to powerful particle ID

However, seems to be difficult to realize a large and uniform detector due to complicated technique

Ge, Si

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Strategy chosen by XMASS

Make large mass and uniform detector (with liq. Xe)

Reduce ray BG by fiducial volume cut (self shielding)

Same style as successful experiments of Super-K, SNO, KamLAND, etc.

Super-K KamLANDSNO

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800 kg detector

Main purpose: Dark Matter search

~800-2” PMTs immersed into liq. Xe 70% photo-coverage

~80cm diameter

~5 keVee threshold

External ray BG: 60cm, 346kg 40cm, 100kg

Expected dark matter signal(assuming 10-42 cm2, Q.F.=0.2 50GeV / 100GeV,)

pp & 7Be solar

Achieved

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Geometry design A tentative design (not final one)

Total 840 hex PMTs immersed into liq. Xe 70% photo-coverage Radius to inner face ~43cm

12 pentagons / pentakisdodecahedron

This geometry has been coded in a Geant 4 based simulator

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Hamamatsu R8778MOD(hex)

12cm

5.4c

m5.

8cm

(ed

ge

to e

dg

e)

0.3cm(rim)

c.f. R8778 　 U 1.8±0.2x10-2 Bq Th 6.9±1.3x10-3 Bq40K 1.4±0.2x10-1 Bq

Hexagonal quartz window Effective area: 50mm (min) QE <~25 % (target) Aiming for 1/10 lower background than R8778

Prototype has been manufactured already Now, being tested

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Expected sensitivities

Large improvements will be expected SI ~ 10-45 cm2 = 10-9 pb SD~ 10-39 cm2 = 10-3 pb

XMASS(Ann. Mod.)

XMASS(Sepc.)

Edelweiss Al2O3

Tokyo LiF

Modane NaI

CRESST

UKDMC NaI

NAIAD

XMASS FV 0.5 ton yearEth = 5 keVee~25 p.e., 3 discoveryw/o any pulse shape info.

10-6

10-4

10-8

10-10

Cro

ss s

ectio

n to

nuc

leon

[p

b]

10-4

10-2

1

102

104

106

Plots except for XMASS: http://dmtools.berkeley.eduGaitskell & Mandic

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100kg prototype

With light guide version

2. R&D status using prototype detector

Main purpose

Confirmation of estimated 800 kg detector performance

BG study

Vertex and energy reconstruction by fitter Miss fitting due to dead angle of the cubic detector (“wall effect”, will be explained later) can be removed with light guide Self shielding power

Understanding of the source of BG Measuring photon yield and its attenuation length

~30 cm cube3 kg fiducial

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In theKamioka Mine

(near the Super-K)

Gamma ray shield

OFHC cubic chamber

Liq. Xe (31cm)3

MgF2 window

54 2-inch low BG PMTs

16% photo- coverage

Hamamatsu R8778

2,700 m.w.e.

100 kg prototype detector

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1.0m

1.9mRn free air (~3mBq/m3)

material thickness

　　 Polyethylene 15cm

　　 Boron 5cm

　　 Lead 15cm

　　 EVOH sheets 30μm

　　 OF Cupper 5cm

4 shield with door

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Progress so far

1st run (Dec. 2003)

2nd run (Aug. 2004)

3rd run (Mar. 2005) with light guide

Confirmed performances of vertex & energy reconstruction Confirmed self shielding power for external rays Measured the internal background concentration

Succeeded to reduce Kr from Xe by distillation Photo electron yield is increased Measured Rn concentration inside the shield

Confirmed the miss fitting (only for the prototype detector) was removed Now, BG data is under analysis

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PMT

n

nL )

!

)exp(Log()Log(

L: likelihood

: F(x,y,z,i)

x total p.e. F(x,y,z,i)

n: observed number of p.e.

=== Background event sample === QADC, FADC, and hit timing information are available for analysis

F(x,y,z,i): acceptance for i-th PMT (MC)VUV photon characteristics:

Lemit=42ph/keVabs=100cm

scat=30cm

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.

Reconstructedhere

FADC Hit timing

QADC

Reconstruction is performed by PMT charge pattern (not timing)

Vertex and energy reconstruction

17→ Vertex reconstruction works well

1. Performance of the vertex reconstruction Collimated ray source run from 3 holes (137Cs, 662keV)

DATA

MC

hole Ahole Bhole C

ABC+++

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65keV@peak(/E ~ 10%)

Similar peak position in each fiducial.No position bias

2. Performance of the energy reconstruction Collimated ray source run from center hole

137Cs, 662keV

→ Energy reconstruction

works well

All volume20cm FV10cm FV

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z position distribution of the collimated ray source run

→ Data and MC agree wellγ

Demonstration of self shielding effect

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Shelf shielding for real data and MC

Good agreement (< factor 2) Self shielding effect can be seen clearly. Very low background (10-2 /kg/day/keV@100-300 keV)

REAL DATA MC simulation

All volume20cm FV

10cm FV(3kg)

All volume20cm FV

10cm FV(3kg)

Miss-reconstruction due to dead-angle region from PMTs.

Eve

nt r

ate

(/kg

/day

/keV

)

10-2/kg/day/keV

Aug. 04 runpreliminary

3.9dayslivetime

~1.6Hz, 4 fold, triggered by ~0.4p.e.

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Kr = 3.3±1.1 ppt (by mass spectrometer)

→ Achieved by distillation

U-chain = (33±7)x10-14 g/g (by prototype detector)

Th-chain < 23x10-14 g/g(90%CL) (by prototype detector)

1/2=164s (Q=3.3MeV) (7.7MeV)

214Bi 214Po 210Pb

1/2=299ns (Q=2.3MeV) (8.8MeV)

212Bi 212Po 208Po

Delayed coincidence search

Delayed coincidence search

(radiation equilibrium assumed)

(radiation equilibrium assumed)

Internal backgrounds in liq. Xe were measured

Main sources in liq. Xe are Kr, U-chain and Th-chain

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Kr 0.1ppm

DM signal(10-6 pb, 50GeV,100 GeV)

0 200 400 600 800

1

102

10-2

10-4

10-6

cpd/

kg/k

eV

energy (keV)

Target = Xe

Kr concentration in Xe

85Kr makes BG in low enegy region

Kr can easily mix with Xe because both Kr and Xe are rare gas

Commercial Xe contains a few ppb Kr

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XMASS succeeds to reduce Kr concentration in Xe

from ~3[ppb] to 3.3(±1.1)[ppt] with one cycle (~1/1000)

~3mLower

Higher

~1%

~99%

Purified Xe: 3.3±1.1 ppt Kr (measured)

Off gas Xe:330±100 ppb Kr(measured)

Raw Xe: ~3 ppb Kr

(preliminary)

(178K)

(180K)Operation@2atm

• Processing speed : 0.6 kg / hour• Design factor : 1/1000 Kr / 1 pass• Purified Xe : Off gas = 99:1

Xe purification system

Boiling point

(@2 atm)

Xe 178.1K

Kr 129.4K

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Now (prototype detector) Goal (800kg detector)

ray BG ~ 10-2 cpd/kg/keV 10-4 cpd/kg/keV

→ Increase volume for self shielding

→ Decrease radioactive impurities in PMTs (~1/10)

238U = (33±7)×10-14 g/g 1×10-14 g/g

→ Remove by filter

232Th < 23×10-14 g/g (90% C.L.) 2×10-14 g/g

→ Remove by filter (Only upper limit)

Kr = 3.3±1.1 ppt 1 ppt

→ Achieve by 2 purification pass

Very near to the target level!

1/100

1/33

1/12

1/3

Summary of BG measurement

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HIT

HIT

HIT

HIT

HIT

?

MCIf true vertex is usedfor fiducial volume cut

10-2

1

10-1

Energy (keV)0 1000 2000 3000

No wall effect

Remaining problem: wall effect (only for the prototype detector)

Scintillation lights at the dead angle 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

Deadangle

This effect does not occur with the sphere shape 800 kg detector

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PTFE light guide(UV reflection)

Active veto

Fiducial

Prototype detector with light guide

10cm

10cm10cm

Purpose: remove the wall effect and understand the source of BG in the DM region

6 pieces

10X10X10cm3

(~3 kg Xe)

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222Rn decays (210Pb 64 keV endpoint) implanted in PTFE surfaces

might make the dominant BG

air

PTFE

222Rn

218Po

214Pb

210Pb

α

α

α

Position distribution of 210Pb (in NaI)

Light guide setup

Implanted to ~0.1μm

N.J.T. Smith et al., Phys. Lett. B 485 (2000) 9

We edged the PTFE inside ~10μm

Recoil process implants30% of the original surface Rn decays

Edging of PTFE surfaces

0 0.05 0.10 0.15Z [m]

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・ MC simulation was done with GEANT3

0 20 40 60 80

10-2

10-4

1

1001000 20 40 60 800

0.4

0.2

0.6

0.8

1.0

Eff

icie

ncy

cpd/

keV

/kg

energy(keV) energy(keV)

Efficiency curve BG spectrum

Signal window

(10-15keV)

Efficiency~30% @10keV Expected BG~10-2cpd/keV/kg

Fast neutron BG

(90% C.L. upper limit)

0.48p.e./keV PMT KPMT ThPMT U

→ Very low BG ~ 10-2 cpd/keV/kg @ <100keV

Expected BG spectrum

29Energy [keV]

Co

unt

s

10cm fiducial

fiducialvolumeC

ou

nts

Energy [keV]

w/o light guide

Reduce the events due to the wall effect

Hole-B

Result 1: comparing the data taken with and without light guide

with light guide

Collimated ray source run from hole-B (137Cs, 662keV)

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Outside the light guide(Data)

　　

　　

　　

　　

　　

　　

　　

　

　　　　　　　　　　　　　　　　　0 1000 2000 300010-4

10-3

10-2

10-1

1

10

energy(keV)

even

ts/k

eV/k

g/da

y

Live time (3.3days)

Outside light guide(MC)

　　

　　

　　

　　

　　

　　

　　

　

0 1000 2000 300010-4

10-3

10-2

10-1

1

10

energy(keV)ev

ents

/keV

/kg/

day

Result 2: Obtained energy spectrum outside the light guide

Good agreement (< factor 2) Trigger rate is same as the measurement witout guide (Aug. 2004)

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3. Summary XMASS experiment: Multi purpose low-background experiment with large mass liq. Xe

800 kg detector: Designed for dark matter shearch mainly, and 102 improvement of sensitivity above existing experiments is expected

R&D with the 100 kg prototype detector Most of the performances required for 800 kg detector are confirmed

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XMASS collaboration• ICRR, Kamioka Y. Suzuki, M. Nakahata, Y. Itow, S. Moriyama, M. Shiozawa, Y. Takeuchi , M. Miura, Y. Koshio, K. Ishihara, K. Abe, A. Takeda, T. Namba, H. Ogawa, S. Fukuda, Y. Ashie, A. Minamino, R. Nambu, J. Hosaka, K. Taki, T. Iida, K. Ueshima• ICRR, RCNN T. Kajita, K. Kaneyuki• Saga Univ. H. Ohsumi, Y. Iimori,• Tokai Univ. K. Nishijima, T. Hashimoto, Y. Nakajima, Y. Sakurai• Gifu Univ. S. Tasaka• Waseda Univ. S. Suzuki, K. Kawasaki, J. Kikuchi, T. Doke, A. O

ta• Yokohama National Univ. S. Nakamura, T.Fukuda, S. Oda, N. Kobayashi, A. Hashimoto• Miyagi Univ. of Education Y. Fukuda, T. Sato• Seoul National Univ. Soo-Bong Kim, In-Seok Kang• INR-Kiev Y. Zdesenko, O. Ponkratenko• UCI H. Sobel, M. Smy, M. Vagins, P.Cravens• Sejong univ. Y. Kim • Ewha Womans Univ. K. Lim• Indiana Univ. M. Ishitsuka

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Electronics of light guide run (Mar. 2005)

ADC

Triggermodule

Gategenerator

SumAmp

delayPMT

PMT

PMT

PMT

Fan-out

x8

SumAmp

x8

Discri(VME)Discri(NIM)~1/4p.e.thres.FADC(2ch/250MHz) 16μs

ID sumOD sum

ID sumOD sum

ID x8

FADC(8ch/500MHz) 1μs

Gain : 8.25×106

ID 2hit≧

OD 4hit≧

400ns

InnerPMT×6

OuterPMT×48

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measurement➢Data taking : 3/9 ~ 3/19, 2005➢Background run : 3.7days runtime, 3.3days livetime➢Trigger rate 1.5Hz (inner 0.2Hz, outer 1.4Hz) ➢Calibration run : 137Cs / 60Co / 57Co / 133Ba source run

ID hit event

FADC ID sumTDC

OD hit event

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Background study

keV0 1000 2000 3000

1

10-1

10-2

cpd/

kg/k

eV

Outside of the shield

238U in PMTs232Th in PMTs

40K in PMTs

210Pb in lead shield

Outside of the shield 0.71 cm-2 s-1 (>500keV)

RI sources in PMTs 238U : 1.8×10-2 Bq/PMT 232Th : 6.9×10-3 Bq/PMT 40K : 1.4×10-1 Bq/PMT

210Pb in the lead shield 250Bq/kg

Expected spectra in all volume

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• Low energy calibration source (1)

Attenuation length of 20 keV x-ray in liq. Xe is short ~ 50 μm The overall size of the source itself should be small not to block the scintillation photons

Irradiate neutrons to natural Pd wire of 10 μm diameter102Pd(n,γ) 103Pd EC decay of 103Pd produce 20 keV x-ray

X-ray

Scintillation photons

D

• EC decaying nuclei preferable X-rays• Candidates : 71Ge(463d), 153Gd(263d), 103Pd(17d)

Liq. Xe

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• Low energy calibration source (2) ☆ 125I(X-ray source) : 27.5 keV (59.9day)

☆ Temperature Range : -200 ~ +100 in Centigrade

☆ Overall source diameter < 20 mm

☆ Weak source ~ a few kBq

Length60mm

125I (1kBq)Electrodeposition

Material AF= 10mm

CoatingMaterial BThick=3mm

5mm

Sourceposition

Liq. Xe

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• Plan of prototype detector ☆ Introduce RI source(103Pd, 125I,…) inside the

chamber

→ Source driving system is ready

→ Detailed study of the energy and vertex fitter

Low energy source

(103Pd, 125I,…)

Wire

Motor

Position accuracy is within 1mm

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