A CsI(Tl) Dark Matter Search Experiment- KIMS -

Yeongduk Kim

Sejong University, Seoul, Korea

IDM 2002 meeting, 2002. Sep 5

Korean Invisible Mass Search

Collaborators

Seoul National Univ. : J.M.Choi, R.K.Jain, S.C.Kim, S.K.Kim*, T.Y.Kim, H.S. Lee, S.E. Lee, H..Park, 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, Y.J.Kwon Iwha Womans Univ. : I.S.Han, E.K.Lee, I.H. Park SeongKyunKwan Univ. : 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 Tsinghua University : J. Zhu* PI

Outline

• CsI(Tl) crystals

• Underground site

• Studies on background reduction

• Perspectives

• Summary

Why CsI(Tl) Crystal ?

CsI(Tl) NaI(Tl)Density(g/cm3) 4.53 3.67Decay Time(ns) ~1000 ~230Peak emission(nm) 550 415Hygroscopicity slight strong

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

Disadvantages Emission spectra does not match with normal bi-alkali PMT 137Cs(1/2 ~30y) ,134Cs(1/2 ~2y) may be problematic

Low energy signal with CsI(Tl)

Large crystal (7x7x30cm) :~ 4.5 p.e./keV

Small crystal(3x3x3cm) :~ 6 p.e./keV

3” Green Extended RbCs PMT(Electron Tubes) Digital Oscilloscope with 10ns bin

• CsI(Na) has spurious events due to surface effect• 2 keV threshold ~ 10 keV recoil energy

Response of CsI(Tl) with elastically scattered neutron

Pulse shape discrimination at ~ keV energy

i

ii

A

tAt

• Nuclear recoil vs gamma events• Mean time for each events

for each photoelectrons in an event

4<E<10 keV

Comparison of PSD power

2)(

)1(

K

S

B

S B

cut

Ideal detector ~ 1, ~ 0

K << 1

NaI(Tl)

CsI(Tl)

• Location : minimum 350 m underground

Power plantLaboratory

Access tunnel(1.4km)

350m

Underground Site

Background of CsI(Tl)

• 137Cs (artificial)

• 134Cs (artificial+133Cs(n,gamma))

• 87Rb (natural)

87Rb 0.63 cpd/1ppb HR ICP-MASS137Cs 0.35 cpd/1mBq/kg HPGe134Cs 0.07 cpd/1mBq/kg “

Single Crystal (~10 kg) background @ ~10keV

Pollucite(raw material for Cs) contains < 8 mBq/kg of 137Cs

134Cs~35mBq/kg

87Rb3.9 ppb

(ICP-MASS)

137Cs 155mBq/kg

Geant 4 Simulation8.9 kg day data

Crystals w/o selection of CsI powder (1)

137Cs Dominating crystal

CsI(Tl) from IHEP(China)

Crystals w/o selection of CsI powder (2)

87Rb Dominating crystal

134Cs54.2 mBq/kg

87Rb203 ppb

(ICP-MASS)

137Cs 13.3mBq/kg

Selection of CsI powder from various vendors

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 87Rb

Selected

Small samples

Chemetall

Crystals with selection of CsI powder

Powder Crystal137Cs 15.5 ± 2.6 19.8 ±2.5 mBq/kg134Cs 27.4 ± 4.6 34.0 ± 4.487Rb 20.0 ppb 23.2 (?)BG(~10 keV) 20.0 cpd 21 cpd

1st Demonstration of Reducing Bacground of CsI(Tl) by selecting powder.

Should reduce further.

Water Samples

Water samples with HPGe – Precipitation with AMP (Ammonium Molybdophosphate)

“Normal”

“Purified”

“Ultra-pure”

137Cs(“Normal water”) >>137Cs(“Purified”)~ 20 times

Water is main source !

A large amount of water used for extractionOf Cs (Chemetall)

CsI powder with “Purified” water

• CsI powder with only “purified” water in a production scale.

CsI powder Crystal“Normal” water 15.5 ±2.6 7 cpd (5.4 cpd expected)“Purified” water 5.3 ± 1.0 2.4 cpd(Expected)

• Factor 3 reduction of 137Cs with “Purified” water

Rb reduction by Recrystallization

• CsI solubility in water is very high.

• Recrystallization is done at slightly lower temperature from saturation point.

• 20 ppb powder ~ 1 ppb (< 1cpd)

Crystal growing by Bridgmann reduced Rb by about 25%

Crystals W/OSelection

Summary of Internal Background Reduction

Normal Water

Purified WaterP C

P C

W

External background

Cosmic rays : ~ 10-4 relative to the sea level

The rock composition (ICP-MASS) 238U ~ 4.8 ppm, 232Th ~ 6 ppm, 40K ~ 4 ppm With a shielding of 15cm Pb(Boliden) + 10cm Cu(OFHC) Can be controlled < 0.005 cpd based a MC simulation study (GEANT4)

Neutron Background at underground

Neutron Flux ~ 4x10-5 /cm2/secMainly from (alpha,n) reaction

GEANT4 simulation

Can be controlled <0.001 cpd 30cm LSC (Outside Shielding) + 20cm LSC(Inside Shielding)

BC501A liquid scintillator

Shielding Structure

Cosmic Muon Veto

Neutron detector inside Copper shielding

20cm BC501A Neutron tagging efficiency > 75%

Po-Be neutron source

Sensitivity (Spin-Independent)

After 1 year data takingwith 100 kg CsI(Tl) 2 keV threshold 3 count/(kev kg day)

DAMA

CDMSLimit

Summary

Extensive R&D on CsI(Tl) crystal has been carried out• Pulse shape discrimination from -rays• Main source of 137Cs contamination due to impure water.• Rb reduction down to ~1ppb achieved.

< 5cpd from internal background.

Shielding capable of 250 kg of CsI(Tl) under construction.• Environmental background : small enough• Large (n,gamma) separable LSC inside shielding is

tested.

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