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

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

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

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

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

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

Neutron Beam Test

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

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

Quenching factor

QF = Emeasured / Erecoiln n

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

Pulse Shape

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

Mean Time

i

ii

A

tAt

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

Quality factor

2)(

)1(

KS

B

S B

cut

Ideal detector ~ 1, ~ 0

K << 1

MT

BKS

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

CsI(Na)

1st measurement of CsI(Na)

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

~ 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

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

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

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

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

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

Neutron Background

~ 4x10-5 /cm2/sec

Input to the GEANT4 simulation

Measured with 0.5 liter BC501A liquid scintillator

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

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

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

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

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

238U, 232Th in the crystal

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

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

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)

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

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

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

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

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

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

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

Shielding Design

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

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%

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

Sensitivity

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

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

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