1. A CsI(Tl) Dark Matter Search Experiment - KIMS -
Korean Invisible Mass Search
Yeongduk Kim
Sejong University, Seoul, Korea
IDM 2002 meeting, Sep 5

2. 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.Yang
Sejong 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

3. Outline
CsI(Tl) crystals
Underground site
Studies on background reduction
Perspectives
Summary

4. Why CsI(Tl) Crystal ? Advantage Disadvantages
High light yield ~50,000/MeV
Pulse shape discrimination
Easy fabrication and handling
High mass number(both Cs and I)
SI + SD
CsI(Tl) NaI(Tl)
Density(g/cm3)
Decay Time(ns) ~ ~230
Peak emission(nm)
Hygroscopicity slight strong
Disadvantages
Emission spectra does not match with normal bi-alkali PMT
137Cs(t1/2 ~30y) ,134Cs(t1/2 ~2y) may be problematic

5. Low energy signal with CsI(Tl)
3” Green Extended RbCs PMT
(Electron Tubes)
Digital Oscilloscope with 10ns bin
Large crystal (7x7x30cm) :
~ 4.5 p.e./keV
Small crystal(3x3x3cm) :
~ 6 p.e./keV

6. Response of CsI(Tl) with elastically scattered neutron
CsI(Na) has spurious events due to surface effect
2 keV threshold  ~ 10 keV recoil energy

7. Pulse shape discrimination at ~ keV energy
Nuclear recoil vs gamma events
Mean time for each events
for each photoelectrons in an event
4<E<10 keV

8. Comparison of PSD power
NaI(Tl)
CsI(Tl)
Ideal detector
 ~ 1,  ~ 0
K << 1 S B S B
cut

9. Underground Site Location : minimum 350 m underground
Access tunnel(1.4km)
350m
Laboratory
Power plant

11. 134Cs (artificial+133Cs(n,gamma)) 87Rb (natural)
Background of CsI(Tl)
137Cs (artificial)
134Cs (artificial+133Cs(n,gamma))
87Rb (natural)
Single Crystal (~10 kg) ~10keV
87Rb cpd/1ppb HR ICP-MASS
137Cs cpd/1mBq/kg HPGe
134Cs cpd/1mBq/kg “
Pollucite(raw material for Cs) contains < 8 mBq/kg of 137Cs

12. Crystals w/o selection of CsI powder (1)
137Cs Dominating crystal
8.9 kg day data
Geant 4 Simulation
137Cs
155mBq/kg
134Cs
~35mBq/kg
87Rb
3.9 ppb
(ICP-MASS)

13. CsI(Tl) from IHEP(China)
Crystals w/o selection of CsI powder (2)
87Rb Dominating crystal
CsI(Tl) from IHEP(China)
137Cs
13.3mBq/kg
134Cs
54.2 mBq/kg
87Rb
203 ppb
(ICP-MASS)

14. Selection of CsI powder from various vendors
Crystals
CsOH
CsNO3
CsMnO4
~ 3mBq/kg
CsI Powders
Small
samples
137Cs ~14mBq/kg
Rb ~ 21 ppb
Chemetall
Selected
137Cs
87Rb

15. Crystals with selection of CsI powder
1st Demonstration of
Reducing Bacground of
CsI(Tl) by selecting powder.
Should reduce further.
Powder  Crystal
137Cs ± ±2.5 mBq/kg
134Cs ± ± 4.4
87Rb ppb (?)
BG(~10 keV) 20.0 cpd 21 cpd

17. Water Samples “Purified” “Normal” Water is main source ! “Ultra-pure”
A large amount of water used for extraction
Of Cs (Chemetall)
Water samples with HPGe – Precipitation with AMP (Ammonium Molybdophosphate)
137Cs(“Normal water”) >>
137Cs(“Purified”)
~ 20 times
“Purified”
“Normal”
Water is
main source !
“Ultra-pure”

18. CsI powder with “Purified” water
CsI powder with only “purified” water in a production scale.
CsI powder Crystal
“Normal” water  ±  7 cpd (5.4 cpd expected)
“Purified” water  ±  cpd(Expected)
Factor 3 reduction of 137Cs with “Purified” water

19. 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%

20. Summary of Internal Background ReductionW P C
Crystals W/O
Selection
Purified Water P C
Normal Water

21. Cosmic rays : ~ 10-4 relative to the sea level
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 < cpd
based a MC simulation study (GEANT4)

22. Neutron Background at underground BC501A liquid scintillator
Neutron Flux ~ 4x10-5 /cm2/sec
Mainly from (alpha,n) reaction
GEANT4 simulation
 Can be controlled <0.001 cpd
30cm LSC (Outside Shielding)
+ 20cm LSC(Inside Shielding)

23. Shielding Structure
Cosmic Muon Veto

24. Neutron detector inside Copper shielding
Po-Be neutron source
20cm BC501A
 Neutron tagging efficiency > 75%

25. Sensitivity (Spin-Independent)
After 1 year data taking
with 100 kg CsI(Tl)
2 keV threshold
3 count/(kev kg day)
CDMS
Limit
DAMA

26. 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