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

Korean Invisible Mass Search Yeongduk Kim Sejong University, Seoul, Korea IDM 2002 meeting, 2002. Sep 5

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

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

Why CsI(Tl) Crystal ?

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

Density(g/cm3) CsI(Tl) 4.53

Decay Time(ns) ~1000 Peak emission(nm) 550 Hygroscopicity

slight SI + SD

NaI(Tl) 3.67

~230 415 strong

Disadvantages Emission spectra does not match with normal bi-alkali PMT 137 Cs(

t

1/2 ~30y) , 134 Cs(

t

1/2 ~2y) may be problematic

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

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

Pulse shape discrimination at ~ keV energy

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

t

  

A i A i t i

for each photoelectrons in an event 4

Comparison of PSD power

K

  (  ( 1     ) 2 ) Ideal detector  ~ 1,  ~ 0 K << 1 cut B S  S  B CsI(Tl ) NaI(Tl)

Underground Site

• Location : minimum 350 m underground

Access tunnel(1.4km) Power plant 350m Laboratory

Background of CsI(Tl) • • •

137 134 Cs (artificial) Cs (artificial+ 133 Cs(n,gamma)) 87 Rb (natural)

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

87 Rb 137 Cs 134 Cs 0.63 cpd/1ppb 0.35 cpd/1mBq/kg 0.07 cpd/1mBq/kg HR ICP-MASS HPGe “

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

Crystals w/o selection of CsI powder (1) 137 Cs Dominating crystal

 Geant 4 Simulation

155mBq/kg ~35mBq/kg 3.9 ppb (ICP-MASS) 137 Cs 134 Cs 87 Rb

8.9 kg day data

Crystals w/o selection of CsI powder (2) 87 Rb Dominating crystal



137 Cs 13.3mBq/kg 134 Cs 54.2 mBq/kg 87 Rb 203 ppb (ICP-MASS)

CsI(Tl) from IHEP(China)

Selection of CsI powder from various vendors

CsOH CsNO Small samples 3 Chemetall 9 8 3 2 1 7 6 5 4 16 15 14 13 12 11 10 0 50 137 Cs 100 87 Rb 150 Crystals CsMnO 4 ~ 3mBq/kg CsI Powders

137 Cs ~14mBq/kg Rb ~ 21 ppb

Selected

Crystals with selection of CsI powder 1 st Demonstration of Reducing Bacground of CsI(Tl) by selecting powder. Should reduce further.

137 Cs 134 Cs 87 Rb BG(~10 keV) Powder  15.5 ± 2.6

27.4 ± 4.6

20.0 ppb 20.0 cpd Crystal 19.8 ± 2.5 mBq/kg 34.0 ± 4.4

23.2 (?) 21 cpd

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

CsI powder with “Purified” water

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

“Normal” water  “Purified” water  CsI powder 15.5 ± 2.6  5.3 ± 1.0  Crystal 7 cpd (5.4 cpd expected) 2.4 cpd(Expected) • Factor 3 reduction of 137 Cs 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%

Summary of Internal Background Reduction

Crystals W/O Selection W P C Purified Water P C Normal Water

External background

Cosmic rays : ~ 10 -4 relative to the sea level The rock composition (ICP-MASS) 238 U ~ 4.8 ppm, 232 Th ~ 6 ppm, 40 K ~ 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 BC501A liquid scintillator

Neutron Flux ~ 4x10 -5 /cm 2 /sec Mainly from (alpha,n) reaction GEANT4 simulation  Can be controlled <0.001 cpd 30cm LSC (Outside Shielding) + 20cm LSC(Inside Shielding)

Cosmic Muon Veto Shielding Structure

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

Sensitivity (Spin-Independent)

DAMA CDMS Limit

After 1 year data taking with 100 kg CsI(Tl)

2 keV threshold 3 count/(kev kg day)

Summary

 • • •

Extensive R&D on CsI(Tl) crystal has been carried out Pulse shape discrimination from



-rays Main source of

Rb reduction down to ~1ppb achieved. 

137 Cs

contamination due to impure water.

< 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