The current status of ＸＭＡＳＳ

• Introduction

• Current status of prototype detector

• Next step

• Summary

Cryodet1

Y.Koshiofor XMASS collaboration(Kamioka observatory, ICRR)

What’s XMASS

• Xenon MASSive detector for Solar neutrino (pp/7Be)• Xenon detector for Weakly Interacting MASSive Particles (Dark Matter search)• Xenon neutrino MASS detector (double beta decay)

Why liquid xenon• Large photon yield (~42000photons/MeV)• Self-shield (large Z=54)• Purification (distillation)• Compact detector size (~3g/cm3 10ton=1.5m cubic)• No long life isotope• Scintillation wavelength (175nm, detect directly by PMT)• Relative high temperature (~165K)

Introduction

Co

unt

s/(k

eV

kg

da

y)

23ton Lxe all volume20cm self-shielding30cm self-shielding (10ton FV)

1MeV0 2MeV 3MeV

Large self-shield effect

External ray from U/Th-chain

Cryodet2

100kg Prototype 800kg detector

10 ton detector

～ 30cm～ 80cm

～ 2.5m

R&DDark matter search

Multipurpose detector

(solar neutrino, …)

We are now here

With light guide

Strategy of the scale-up

Cryodet3

Current status of 100kg prototype detector

54 2-inch low BG PMTs

Liq. Xe 　(30cm)3= 30L= 100Kg

MgF2 window

• Stability check in low temp.• Low background setup• Vertex / energy reconstruction• Demonstration of self-shielding

• Purification system• Particle ID• Attenuation length• Background study

Menu of R&D

Photo coverage: 16% 0.6p.e./keV

Set in Kamioka mine

Cryodet4

1.0m

1.9m

Shield

Rn free air (~3mBq/m3)

material thickness

　　 Polyethylene 15cm

　　 Boron 5cm

　　 Lead 15cm

　　 EVOH sheets 30μm

　　 OF Cupper 5cm

4 shield with doorCryodet

5

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

Reconstructedhere

FADC Hit timing

QADC

Energy and Vertex reconstruction

Using photoelectron map made by MC (only charge,but no timing information)

Vertex: MC hitmapEnergy: Hitmap scale

Maximum likelihood method is used.

PMT

n

nL )

!

)exp(Log()Log(

L: likelihood: F(x , y, z) ×

(total p.e./total acceptance)n: observed number of p.e.

Tuning of parameters correlated to photon tracking is important.

Cryodet6

Results

→ 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+++

Cryodet7

2. Performance of self shielding z position distribution of the collimated ray source run

→ Data and MC agree well γ

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65keV@peak(677keV)(/E ~ 10%)

Similar peak position in each fiducial.No position bias

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

137Cs, 662keV

→ Energy reconstruction

works well

All volume20cm FV10cm FV

Cryodet9

4. MC tune using source data

Input parameters:• Photon yield ~42000 photons/MeV• QE of PMT ~25%• Collection efficiency of PMT ~90%• Refractive index of liquid Xe 1.6 (measured)• Absorption• Scattering• Reflection on the surface of cupper

Using hit pattern of source data

tune using total photo electron distribution

Tunable

e.g.) γ beam

front view

30cm 50cm 60cm

side view

γ beam

sensitive to scattering

Change scattering

Fix abs.(100cm) and ref.(20%),

60cm55cm20%

Very preliminaryVery preliminary

Cryodet10

5. Compare background shape b/w data and MC

☆ MC ( ray background) ・ Outside of the shield ・ RI sources in PMTs ・ 210Pb in the lead shield

→ Data and MC agree well

→ Self shield effect can be seen

→ Very low background

All volume20cm FV10cm FV

10-2 /kg/day/keV@100-300 keV

Cryodet11

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)

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

6. Internal backgrounds in LXe were measuredCryodet

12

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 Cryodet13

5. Compare background shape b/w data and MC

☆ MC ( ray background) ・ Outside of the shield ・ RI sources in PMTs ・ 210Pb in the lead shield

→ Data and MC agree well

→ Self shield effect can be seen

→ Very low background

Miss-reconstruction

All volume20cm FV10cm FV

10-2 /kg/day/keV@100-300 keV

True vertex

Cryodet14

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

(expected 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

Wall effect Cryodet15

With light guide

Remove the wall effect

PTFE light guide(UV reflection)

Active veto

Fiduciale

ven

t ra

te (

/keV

/day

/kg

)

PMT KPMT ThPMT U

BG simulation(light guide MC)

energy (keV)

Fast neutron upper limit (90%C.L.)

Cryodet16

Energy [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

Comparison of data w/ and w/o light guide

with light guide

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

Further analysis of Low energy background is under study

Cryodet17

800kg Detector : WIMP search

840-2” PMTs immersed into liq. Xe 70% photo-coverage

~80cm diameter

~5 keVee threshold

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

pp & 7Be solar

Achieved

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

Next step Cryodet18

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

Cryodet19

Detector design Geometry : 12 pentagons /

pentakisdodecahedronA tentative design (not final one)

Hamamatsu R8778MOD(hex)

12cm5.

4cm

5.8c

m(e

dg

e to

ed

ge)

0.3cm(rim)

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

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

Cryodet20

Performance check by MC Using Geant4 intput parameter : Xe light yield =42000p.e.

abs. length =100cm, scat.length = 30cm position recon. Method : same with 100kg prototype

(hitmap making & likelihood calculation)

10keV 5cm from center

10keV 35cm from center

Cryodet21

5keV

10keV

100keV 500keV

1MeV

50keV

Fiducial volume

Distance from center （ｍｍ）

σ（

real

po

s –

rec

on

po

s）

（ｍ

ｍ）

0 100 　 200 　 300 400

0 100 200 300 400

5keV

10keV

100keV 500keV

1MeV

50keV

5keV10keV

100keV

500keV

1MeV

50keV

Npe Light yield Energy resolution

Distance from center （ｍｍ）

res ％

Distance from center （ｍｍ）

Position dependence of reconstructed position resolution

Position dependence of Light yield & resolution

At division of F.V. ( ２５ｃｍ from center)1MeV ~6mm 5keV ~80mm

Photo yield 4.2 p.e./keV

（ at center ）Resolution:

1MeV 1.5% 5keV 20%

Cryodet22

Summary

• XMASS experiment: Large volume liquid Xe detector for multi-purpose e.g. DM, pp/Be solar neutrino and measurement. Key idea : Self shielding of liquid Xe

• 100kg prototype detector: The detector performance is as expected. Most of the performances required for 800kg detector are confirmed.

• 800kg next detector: Mainly for dark matter search. 2 orders improvement of sensitivity above existing experiments is expected.

Cryodet23