XMASS

・　 Physics motivation of XMASS

・　 R&D using １００ｋｇ detector

・　 Next step: 800 kg detector

Kamioka observatory, ICRR, Univ. of Tokyo

M. Nakahata

for XMASS collaboration

LowNu 2003, May 20

XMASS collaboration• ICRR, Kamioka observatory Y. Suzuki, M. Nakahata, Y. Itow, M. Shiozawa, Y. Takeuchi, S. Moriyama, T. Namba, M. Miura,Y. Koshio, Y. Fukuda, S. Fukuda, Y. Ashie, A. Minamino, R. Nambu• ICRR, RCNNT. Kajita, K. Kaneyuki, A. Okada, M. Ishitsuka• Saga Univ.T. Tsukamoto*, H. Ohsumi, Y. Iimori, • Niigata Univ.K. Miyano, K. Ito• Tokai Univ.K. Nishijima, T. Hashimoto, Y. Nakajima• Gifu Univ.S. Tasaka,• Waseda Univ.M. Yamashita, S. Suzuki, K. Kawasaki, J. Kikuchi, T. Doke,• TITY. Watanabe, K. Ishino, • Yokohama National Univ.S. Nakamura, T.Fukuda, J.Hosaka, • Seoul National Univ.Soo-Bong Kim, In-Seok Kang,• INR-KievY. Zdesenko, O. Ponkratenko,• UCIH. Sobel, M. Smy, M. Vagins, P.Cravens

XMASS

Scintillation detection by PMTs

Dark M: Xenon detector for weakly interacting MASSive Particles : Xenon neutrino MASS detectorSolar : Xenon MASsive detector for Solar neutrinos

10 ton class Liq. Xe

Dark matter search

Eth = 20keV; 30 events/dayEth = 5keV; 2000 events/day

Expected signal of dark matter

Sensitivity of dark matter search 10 ton Liq. Xe

Spin independent

Spectrum only

Seasonal variation

10-6 pb

Double deta decay

e-

e-

e-

e-

Majorana mass of neutrinos

It is quite important to investigate whether the origin of the neutrino mass is Majorana type

See-saw mechanism is correct ?

136Xe(natural abundance 8.87%), double beta nuclei

22 02

background against

1/2 theory=8 x 1021 y

Resolution by photon statistics

42000potons/MeV x 2.48MeV x 30%Q.E.

31000 p.e. 0.6 % rms

10ton natural, 5 year

2600 keV24002200

Signal region

1/2 theory=8 x 1021 y

background against

Sensitivity of

10ton natural, 5 year

Both ＋－　sigma

Only ＋ sigma

mMajorana sensitivity: 0.01 – 0.02 eV

0 1 2 3 4 5 6Resolution at 2.5 MeV

0 1 2 3 4 5 6Resolution at 2.5 MeV

02 lifetime sensitivity: several ×1027 years

10t Liq. Xe 5yr

(Cf. SK-I, ~13 events/day)

Solar neutrinospp: 10ev/day7Be: 5ev/day (> 50keV, SSM)

Expected spectrum of solar neutrinos

+e + e

~1% stat. error measurement of pp flux

Quite High statistics

KamLAND and pp solar neutrinos will determine precise oscillation parameters

Precise measurement of oscillation parameters

• Expected region using pp neutrinos (90 % C.L.) : 10 ton Liq. Xe e scattering 5 years data Statistical error and SSM pre

diction error(1%) • Accuracy of mixing angle: sin22

= 0.77 0.03(stat.+SSM)

Fiducial volume

rays from 238U chain

Background rejection

6 orders of magnitude reduction for gamma rays below 500keV

(U/Th/K/Co/Cs/…)

neutron

rays from 85

Kr, 42,39Ar, U/Th

Self-shield

No long life isotope

H2O, paraffinDistillation

• Self-shield of external gamma rays

Development of low background PMTs• New type 2-inch PMT(developed with Hamamatsu Co.)

Q.E. ~ 30% @ 175nmCollection efficiency ~ 90%Quartz windowMetal tube (low background)Parts were selected using HPGe

Single p.e. distribution

Bq/PMTequiv.10-3 10-210-4

New 2-inch PMTMetal forPMT bulb

capacitor (film)Chip resistorQuartz (PMT)CablesPCB(PTFE)

Usual PMTs

Resistor

capacitor

Glass Epoxy PCB

Selection of parts for the new type PMT

Low BG PMT 238U 1.8x10-2Bq232Th 6.9x10-3Bq40K 1.4x10-1Bq60Co 5.5x10-3Bq

Bulb for PMT

100kg detectorpurpose

MgF2 window

54 2-inch PMTs

Liq. Xe 　(30cm)3=30L=100Kg

(1) Low background effort with new PMTs(2) Vertex/energy reconstruction(3) Demonstration of self shielding(4) e/separation(5) R&D of purification system(6) Possible detection of 2 decay

100kg detector

MgF2 window

Supur-insulation

100kg detector: cooling

Vacuum chamber (this time stainless steel)

ＧＭ refrigerator （ 100W @170K ）

Cooling and liq.Xe supply, recovery test in February using only 6 PMTs

100kg detector cooling

PMT mouting

One PMT per plane

Temperature monitor

PMTPMT HV ON

8 days

detector

Supply liq. Xe

100kg detector: Gas line

Getter

Original Xe

Recovery Xe

Emergency tank(9m3)

Vacuum chamber

Emergency recovery line

rupture disk

Emergency valve

Liq. N2

Level Monitor

4 days

Supply, recovery port

100kg detector: Liq.Xe supply and recovery

Supply (2 days) recovery(0.85 days)

L

ML

M

HM

H

We found that the PMTs work even at -90℃

60Co source on the detector

６ PMT test data

Limit vertex position using( of 6PMTs)/ (average of 6PMTs)

100kg detector: Test data Observed spectrum

SimulationThe observed spectrum is consistent with the simulation

All<1

All<1

Shield of １００ｋｇ detector

• OFHC vacuum chamber, OFHC shield(5cm thick), Lead shield (15cm), Boron (10cm), Polyethylene(15 cm), Radon shield (EVOH sheet)

Installation in May, June

Installation of ShieldDoor part is already installed. (May, 2003)

Moved into clean room in Kamioka mine.

Lead shield (15cm thick)OFHC shield(5cm)

Full shield will be installed by the end of June, 2003.

Vertex/energy reconstruction (MC simulation)• Make PMT hitmaps

• Maximize the likelihood:

F(x, y, z; i): acceptance of scintillation light for i-th PMT from (x, y, z). GEANT based simulation gives F(xg, yg, zg, i) on a grid with 2.5cm spacing. F(x, y, z, i) is linearly interpolated by using F(xg, yg, zg, i).

PMT

n

nL )

!

)exp(Log()Log(

L: likelihood: F(x, y, z) x (total p.e./total acceptance)n: observed number of p.e.

Typical 1MeV alpha ray

Red: true vertexGreen: reconstructed

0.88cm (68%)

1MeV 300keV

1.40cm (68%)

=62keV(gauss fit)

=33keV(gauss fit)

Vertex/energy resolution (MC)

Uniformly distributed source in the fv (20cm cube)

2 phase detector(S.Suzuki 　 et al.)

100kg detector: background level（ expectation by simulation ）

0.1ppm 85Kr

85Kr~0.01ppm

/kg/

day

/keV

• Gamma rays from 238U, 232Th, 40K outside the chamber.238U: 2.4Bq, 232Th: 1.4Bq, 40K: 0.86Bq, determined by a HPGe.• Events were reconstructed by the reconstruction tool.

100kg all volume estimated

100kg 20cm cube, ½ PMT cut

Efficiency: 60% @ 100keV 2.4% @ 10keV

Purification by distillation• R/D of purification system Simple distillation system for Kr Theoretically, 1/1000 reduction Process power: 0.6kg/hr

Buffer tank

Compressor

Heatexchanger

Purified Xe

Xeinput

Refrigerators

Purification tower ~3m13 stages

Heater

Getter

GXe flow

LXe

Kr

Tower

Boiling point: Xe=165K, Kr=120K, Ar=87K

Test this purification system in 2003.

Next step: 800kg detector

• 800kg liquid Xe

• 642 2-in PMTs

• 77% photo-coverage

• ~5 p.e./keV

80cm diameter

/kg/

day

/keV

pp

7Be

, 8x1021 yr

External background in 800kg detector• Dominant contribution is from PMT

• Assuming further 1/10 reduction of PMTs BG

external ray (60cm, 346kg)

external ray (40cm, 100kg )

Dark matter (10-8 pb, 50GeV, 100 GeV)

Dark matter sensitivity800kg detector

Spin independent Spin dependent

spectrumSeasonal variation

Requirements in background 　 for 800kg detector

ExternalEnvironmental rays: 15cm Pb + 5cm OFHC or water shield

Fast neutron recoil at 10 keV : < 1/10000 of environment

Thermal neutron absorption: < 1/400 of environment

Internal

85Kr: < 0.35 ppt of Kr (by distillation)42Ar, 39Ar: < 0.1 ppm Ar (by distillation)

Radon: < 1microBq/m3

U/Th in Liq.Xe: < 10-14 g/g

~2m of water shield

Conclusion• Large volume liquid Xe detector is quite effective fo

r dark matter, double beta decay and low energy solar neutrinos.

• R&D is going on using 100 kg detector.• Using the 100 kg detector, we will test vertex and en

ergy reconstruction, self-shielding, and purification of liquid Xe.

• The next step is 800 kg detector. The 800 kg detector is able to search for dark matter with 4 orders of magnitude better sensitivity than current experiments.

• Final goal of the XMASS project is 10 ton class detector for multi-purpose astro-particle physics.