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