CsI crystal for WIMP Search

CsI crystal for WIMP SearchSun Kee Kim

Seoul National University

For the KIMS Collaboration

Seoul National Univ. : J.M.Choi, R.K.Jain, S.C.Kim, S.K.Kim*, T.Y.Kim, H.S.Lee, H.Park I.H.Park, E.Won, H.S.Won, H.Y.Yang, M.S.YangSejong Univ. : W.K.Kang, J.I. Lee, D.S.Lim, Y.D.Kim Yonsei Univ. : J.Hwang, H.J.Kim, J.H.Lee, Y.J.Kwon, Iwha Womans Univ. : I.S.Hahn, E.K.Lee SeongKyunKwan Univ. : I.S.Cho, D.H.Choi, S.H.Noh, I.Yu Chonbuk National Univ. : S.Y.Choi KAIST : P.Ko Univ. of Maryland : M.H.Lee, E.S.Seo, National Taiwan Univ., :H.B.Li, C.H.Tang, M.Z.Wang Academia Cinica :W.P.Lai, H.T. Wong , Inst. Of High Energy Physics : J.Li, Y.Liu, Q.Yue Inst. Of Atomic Energy : B.Xin, Z.Y.Zhou, Tsing Hua University : J.J.Zhu

Korea Invisible Mass Search

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)

Brief History of KIMS

• 1997 Summer : First discussion on WIMP search in Korea

• 1997 Fall : Started R&D on CsI(Tl) for WIMP search

• 1998 Summer : Demonstrated that CsI(Tl) can be used

as a WIMP detector - presented at ICHEP98

• 1999 Spring : Started background measurement at Cheongphyung

• 2000 Summer : Project funded, KIMS collaboration formed

• 2001 August : TDR, Project Review Committee


CsI(Tl) Crystal

CsI(Tl) NaI(Tl)Density(g/cm3) 4.53 3.67Decay Time(ns) ~1050 ~230Peak emission(nm) 550 415Hygroscopicity slight strong

Advantages High light yield ~50,000/MeV Pulse shape discrimination Easy fabrication and handling High mass number(both Cs and I)

Disadvantages Emission spectra does not match with normal bialkali PMT => effectively reduce light yield 137Cs(1/2 ~30y) ,134Cs(1/2 ~2y) may be problematic


Photoelectron yield

Full size crystal (7x7x30) : ~ 4 p.e./keVSmall crystal(3x3x3) : 5~7 p.e./keV

RbCs(green extended) PMT(D726UK)70% larger p.e. than the normal bialkali PMT


Neutron Beam Test

KIGAM(Korea Institute of Geology and Material)3.2 MeV p : 3H(p,n)3He 2.62 MeV n


Quenching factor

QF = Emeasured / Erecoiln n


Pulse Shape


Mean Time

i

ii

A

tAt


Quality factor

2)(

)1(

KS

B

S B

cut

Ideal detector ~ 1, ~ 0

K << 1

MT

BKS


CsI(Na)

1st measurement of CsI(Na)


~ 2 hours from SNU

Power plant

Homyung Mt.( 虎鳴山 )

Reservoir

Laboratory

Access tunnel(1.4km)350mPukhan River( 北漢江 )

Underground Lab. at CheongpyungStorage Water Power Plant

~1000 mwe


External background

Cosmic rays : ~ 10-4 relative to the sea level Expected at ~400 m underground The rock composition : 238U ~ 4.8 ppm, 232Th ~ 6 ppm 40K ~ 4 ppm (ICP-Mass)

Gamma background With a shielding of 15cm Pb + 10cm Cu(OFHC) Can be controlled < 0.005 cpd (counts/keV/kg/day)

Neutron background Photonuclear interaction of cosmic ray muon Fission of 238U, (alpha,n) reaction With 30cm polyethylene + 20 cm Liquid scintillator Can be controlled <0.001 cpd


W/o shielding

10cm Pb + 10 cm Cu(16 days data taking)

100% HPGe installed in CPL

Gamma Background

222Rn_up(226Ra)/222Rn_down(214PB,214Bi)=0.22+-0.06


Neutron Background

~ 4x10-5 /cm2/sec

Input to the GEANT4 simulation

Measured with 0.5 liter BC501A liquid scintillator


Internal background

137Cs : 1/2= 30.07 year decay to 137Ba* (Q = 1175.6 keV) 2 min life time, emitting 661.6 keV gamma Hard to reject => serious background at low energy

134Cs : 1/2 = 2.065 year decsy to 134Ba* (Q=2058.7 keV) immediate gamma emission Can be rejected easily => not a serious problem

Radioisotopes in the crystal - most important background after the proper shielding

87Rb : 1/2 half life = 4.75 x 1010 year (27.8% natural abundance) Beta deacy to 87Sr (Q=282.3 keV) no gamma emission Hard to reject => potentially a serious problem => reduction technique in material is known


Internal background

In a crystal currently available

rays andrays of 87Rb, 134Cs, and 137Cs

+ GEANT

60 cpd

137Cs : 13.3mBq/kg, 134Cs : 54.2mBq/kg, 87Rb :~203ppbAnother crystal

137Cs : ~155mBq/kg, 134Cs : ~35mBq/kg, 87Rb : 3.9 ppb

0.63 cpd/1ppb 87Rb0.3 cpd/1mBq 137Cs0.005 cpd/1mBq 134Cs


238U, 232Th in the crystal

10~20 alphas/kg/day ~3 ppt (U+Th)


Measurement of Internal Background

Crystal 137Cs, 134Cs : direct measurement (fit to the MC prediction) Rb : direct measurement + chemical analysis U, Th : direct measurement of alphas ~ 15 ppt

Powder 137Cs, 134Cs : 100% HPGe detector Rb : chemical analysis (ICP Mass)

CsOH, CsNO3, Pollucite, Water 137Cs, 134Cs : 100% HPGe detector

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140 160 180 200137Cs(mBq/kg)

134Cs(mBq/kg)


Background measurement of powders

0 50 100 150

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16Crystals

CsMnO4

~ 3mBq/kg

CsI Powders

137Cs ~14mBq/kgRb ~ 21 ppb

CsOHCsNO3


137Cs in Pollucite ?

In CsI powder : 10~100 mBq/kg In Pollucite : < 5.8 mBq/kgProcessing water : 10.1 4.8 mBq/literExtraction of CsMnO4 from Pollucite with pure water 2.05 3.16 mBq/kg

< 5.8 mBq/kg


CsI (30-50 keV) : 14.3 kg daysNaI(Tl) (35-40 keV) : 330 kg days

Surface effect ?

V.A. Kudryavtsev et al., hep-ex/0109013

62 kg days exposure


Shielding Design


Detector Design

CsI(Tl) 9cm x 9cm x 30cm ~ 11 kg

PMT (3 inch RbCs)

5x5 = 25 crystals ~ 275 kg

Quartz

20cm BC501A tagging efficiency > 75%


Sensitivity

After 1 year data takingwith 100 kg CsI(Tl) 2 keV threshold


Summary & Prospect

Underground Lab. at CheongPyung is established Environmental background : low enough Comfortable place for long term experiment

Extensive R&D on CsI(Tl) crystal has been carried out Low energy threshold ~ 2keV Pulse shape discrimination of -rays : promising Intrinsic background : almost understood

Shielding design/Simulation/Electronics/DAQ Design almost at final stage

Prospect ~100 kg CsI(Tl) crystal within this year 1 year data taking will cover DAMA region

CsI crystal for WIMP Search

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