Status XMASS experiment

M. Nakahata for XMASS collaboration Kamioka

Observatory, ICRR, University of Tokyo

Dark2007, September 28, 2007

Dark matter

Double beta

Solar neutrino

What’s XMASS

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

Multi purpose low-background and low-energy threshold experiment with liq. Xe

Strategy of the XMASS project

800kg detector(FV 100kg)

Dark matter search

~20 ton detector(FV 10ton)Solar neutrinosDark matter search

Prototype detector (FV 3kg) R&D

~2.5m~1m~30cm

Confirmation of feasibility of the ~1ton detector

Funded from 2007

Double beta decay optionSo far

Concept of background reductionSelf-shielding

PMTs

Single phaseliquid Xe Volume for shielding

Fiducial volume

23ton all volume (d=240cm)

20cm 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

Low Background region near the center of the fiducial volume

pp, 7Be solar

3kg FV prototype detector

In theKamioka

Mine(near the

Super-K)

Liq. Xe 　(31cm)3

MgF2 window

54 2-inch low BG PMTs

ray/neutron shield

OFHC cubic chamber

• Demonstration of reconstruction, self shielding effect, and low background properties.

16% photo-coverage

HamamatsuR8778

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.

Vertex and energy reconstruction

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)

Source run( ray injection from collimators)

DATA

MC

Collimator A Collimator B Collimator C

A B C

+++Well reproduced.

Source run( ray injection from collimators)

Self shield works as expected.

Photo electron yield ~ 0.8p.e./keV for all volume

Gamma rays

Z= +15Z= -15

137Cs: 662keVDATAMC

PMTSaturationregion

-15 +15cm -15 +15cm

~1/200

~1/10

Reconstructed Z Reconstructed Z

Arb

itrar

y U

nit

10-1

10-2

10-3

10-4

10-5

10-1

10-2

10-3

10-4

10-5

60Co: 1.17&1.33MeV DATA MC

No energycut, onlysaturation cut.BG subtracted=2.884g/cc

Background data

MC uses U/Th/K activity from PMTs, etc (meas. by HPGe). Self shield effect can be clearly seen. Very low background (10-2 /kg/day/keV@50-300 keV)

REAL DATA MC simulation

All volume20cm FV

10cm FV(3kg)

All volume20cm FV

10cm FV(3kg)

10-2/kg/day/keV

3.9days livetime

Eve

nt r

ate

(/kg

/day

/keV

)

　　　　　　　　　　　　　　　　　　　　

　　

　　

　　

　　

　　

　

　

　　　　　　　　　　　　　　　　　　　　

　　

　　

　　

　　

　　

　　

　　

0 20001000

10-2

1

3000keV

10-1

0 20001000 3000keV

10-2

1

10-1

Distillation to reduce kryptonA distillation system was made and tested.System specification:

Boiling point (@1 atm)

Xe 165K

Kr 120K

~3m

13 stage of Operation: 2 atm(abs.)Processed speed: 0.6 kg / hourDesign factor: 1/1000 Kr / 1 pass

Lower temp.

Higher temp.

~1%

2cm

~99%

Purified Xe: (3.3±1.1) x 10-12 Kr (by a high sensitivity measurement method)

Off gas Xe:330±100 x 10-9 Kr(measured)

Original Xe: ~3 x 10-9 Kr

178±2K in tower

Process speed: 0.6kg Xe/hour Collection efficiency: > 99% Kr concentration after process: < 1/1000

Radioactive contamination in LXe(internal background)

Achieved with the prototype detector

U, Th, Kr near to the goal. Within reach.

Requirement for 800kg detector

238U: < 1x10-14 g/g (~1decay/100kg/d)232Th: < 2x10-14 g/g (~1decay/100kg/d)85Kr: < 1ppt

238U:(9±6) x10-14 g/g Further reduction by filter232Th: < 23 x10-14 g/g Upper limit, use filter85Kr: 3.3±1.1 ppt by a prototype distillation tower

800kg(100kg FV) detector for DM search Low energy threshold immerse PMTs into LXe Ext. BG: from PMT’s Self-shield effect (demonstrated) Internal BG: Kr, radon Removed by distillation, filters…

“Full” photo-sensitive,“Spherical” geometry detector

80cm diameter812 hexagonal PMTs

70% photo-coverage ~5p.e./keVee

ray BG from PMTs: 60cm, 346kg 40cm, 100kg

Achieved

pp & 7Be solar

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

More detailed geometrical design A tentative design (not final one)

12 pentagons / pentakisdodecahedron

This geometry has been coded in a Geant 4 based simulator

Hexagonal PMT~50mm diameter

Aiming for 1/10 lower BG than R8778 R8778: U (1.8±0.2)x10-2 Bq Th (6.9±1.3)x10-3 Bq 40K (1.4±0.2)x10-1 Bq

10keVR=5cm

800kg reconstruction and BG study

Extensive study to optimize the detector ongoing

5 keV

10 keV

50 keV100 keV500 keV1 MeV

Fiducial volume

2010 300 40

8

10

6

4

2

0

12

Distance from the center [cm]

(r

econ

stru

cted

) [c

m]

10keVR=35cm

Photon tracking: absorption, scattering, and reflection are taken into account.

Vertex resolution

5keV(~25p.e.) threshold

Estimated PMT ＢＧ

• Activity of PMT 　– 238U chain 1.8x10-3 Bq/PMT

– 232Th chain 6.9x10-4 Bq/PMT

– 60Co 5.5x10-3 Bq/PMT

– 40K 1.4x10-2 Bq/PMT

• Below 300 keV number of events in the 25cm fiducial volume decreases rapidly.

• Below 300keV, <10-4 /kg/day/keV BG level.

• Below 100 keV,

<10-5 /kg/day/keV BG level.

All volume 40cm Fiducial Volume35cm Fiducial Volume25cm Fiducial Volume

(/kg

/day

/keV

)(/

kg/d

ay/k

eV)

Sensitivity of the 800kg detector

10-45cm2 (10-9pb) for SI and 10-39cm2 (10-3pb) for SD

0.5ton ・ year exposure (100kg, 5yr)3 discovery

Plots exept for XMASShttp://dmtools.berkeley.eduGaitskell & Mandic

103

XENON10

DAMA

SuperCDMS Phase A

XMASS800kg

WARP140kg

J Ellis et al. benchmark

WIMP mass(GeV)10 102 103

10-42

10-44

10-46

10-40

Cro

ss s

ectio

n to

nu

cle

on

(cm

2)

Spin Independent(SI)

10-38

10-40

10-34

10-36

10-30

10-32

Spin Dependent(SD)

SI

CDMS-II

Sep/12/2007

• Shield environmental gammas and neutrons by pure water (available from SK system).

• Active shield by water Cherenkov detector for muon induced background.

Water shield for environmental background

~10m diameter, ~10m highpure water tank.

20-inch PMTs for veto counter.

800kg detector

Simulation of environmental and fast neutron

wa

water

Liq. Xe

Generation point of and neutron

MC geometry

Configuration of the estimation Put 80cm diameter liquid Xe ball Assume copper vessel (2cm thickness) Assume several size of water shield 50, 100, 150, and 200cm thickness for liquid Xe

External gamma and neutron are another large background sources.

To reduce these background, use thick water shield.

Thickness of shield is estimated by a simulation.

background

PMT BG level

attenuation by water shield

0 100 200 300Thickness of shield [cm]

10

104

De

tect

ed/g

ene

rate

d*s

urf

ace

[cm

2]

103

102

1

10-1

10-2More than 200cm water is needed to reduce the BG to the PMT BG level

Initial energy spectrum from the rock

Deposit energy spectrum (200cm)

water: 200cm, energy: 10MeV

Liq. Xewater

< 2 x 10-4 counts/day/kg

Fast neutron background

• Fast n flux @Kamioka mine: (1.15+0.12) x10-5 /cm2/sec

• Assuming all neutron’s energies are 10 MeV very conservatively

200cm of water is enoughto reduce the fast neutron

Generat:107 MC events, no event in Liq.Xe volume

X [cm]

Z [

cm]

Reach points before thermalized-

New experimental halls at Kamioka

KamLAND

SK

New Halls

21m

15m

15m

The halls will be ready by summer 2008.

170m

Sep/12/2007 taup2007

Excavation status

As of end of September 2007.The halls will be ready by summer 2008.Hall for XMASS

PMT1

PTFEfiller

PMT2

200 mm

40 mm

Liquid Xe

15 mm

Blue LED

Co-57

Test PMT immersed inLiquid Xe

PTFE light guide

• Two PMTs immersed in liquid Xe• Co-57 (122 keV, 136 keV) source between the PMTs

SUS304 chamber

Cold cap

100 mm

PMT 1

PMT 2

PMT immerse test: pulse height distribution

(122keV) const. (p0) : 74.8 mean (p1) : 2088.7 sigma (p2) : 49.9 (136keV) const. (p3) : 6.1 mean (p4) : 2267.2 sigma (p5) : 73.9

Resolution: 2.4 % @122 keV

pe1: p.e. of PMT1pe2: p.e. of PMT2

selected

Light yield: 17.1 p.e. / keV

122keV peak

136 keV peak

Summary

• R&D by the prototype detector– Demonstrated vertex reconstruction method and

self-shield– Developed background reduction methods

• 800 kg detector– Sensitivity of 10-45 cm2 for spin independent– Budget funded in this year and construction

started. – Plan to construct detector within 2-3 years.