Transcript Status of KIMS experiment WIMP search

Production of low radioactivity CsI(Tl)
crystals for WIMP searches
- a case of artificial radioisotope LRT2004, Dec. 12, 2004
Yeongduk Kim
Sejong University
For the KIMS Collaboration
WIMP search with CsI(Tl) Crystal
KIMS
Neutron shield / Muon det.(30cm)
Lead shield(15cm)
Polyethylene(5cm)
Copper shield(10cm)
Goal :
25 CsI(Tl)-216kg Crystals
With <2 cpd backgrounds
CsI(Tl) crystal
Low background HPGe meassurement facility
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100% Coaxial HPGe(Ortec)
10cm OFHC + 15cm Pb
4 liter/min N2 Flow
~ 0.2 cpd @ 1MeV
Background spectrum of CsI(Tl)
Crystals on the shelf (Belle CsI(Tl) crystals)
: Budker Institute of Nuclear physics (Ukraine)
: Shanghai Institute of Ceramics (China)
: Crismatec (France)
Internal background of CsI Crystal
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137Cs
(artificial) : serious background at low energy
134Cs (artificial+133Cs(n,gamma))
87Rb (natural) : Hard to reject
 reduction technique in material is known
CPD
137Cs
: 10 mBq/kg
134Cs : 20 mBq/kg
87Rb : 10 ppb
87Rb
137Cs
0.35 cpd/(mBq/kg)
134Cs 0.07 cpd/(mBq/kg)
87Rb 1.07 cpd/ppb
Geant Simulation
137Cs
134Cs
keV
1. Mechanism and reduction of
• Origin of
137Cs
137Cs
contamination
: artificial(fission product)
Bomb test + Chernobyl
Close to Chernobyl
~ 0.1 Bq/liter
• “Typically”, surface sea(river) water contains a few mBq/liter.
• naturally can suspect water may be the source.
• the pollucite(Cs ore) is not contaminated (<5.8 mBq/kg).
137Cs
in surface soil
Blagoeva, J. Environ. Racioactivity (1995)
Production of CsI powder in Chemetall Company (Germany)
Power plant
70 liters/
1kg of CsI
Cartridge
Polisher
SK-water
purification system
Processing
water
Pure
water
~ 0.5 MΩ
River
water
Measurements of
Precipitation
(24hours)
Water : 20 liter
+ HNO3(70% 1cc/liter)
+AMP(0.5g/liter)
*AMP :
Ammonium Molybdophosphate
137Cs
in Water
(mBq/liter)
Processing
0.35-1.16
Pure
0.055 ± 0.017
Ultra-pure
< 0.02
137Cs
Decanting
&Drying
Gamma
Detection
in Water
Production of low background crystals
• 137Cs contamination is well controlled.
• Up to < 1 cpd level, it is dominated by impure water
• No serious contamination is observed in
Crystallization process
Expected vs measured activities
• 134Cs is not observed in water
• 134Cs level is ~ 20 – 40 mBq/kg
regardless of water quality
1cpd
2.
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87Rb
Reduction
Mainly reduced in production stage of High Purity CsI powder production
Further reduction is observed in crystallization process (Bridgmann)
Measured by HR(High Resolution) ICP-MASS in Old Dominion U. (USA)
CsI samples are diluted by factor 500
Detection Limit of HR ICP-MASS for Rb is 0.2 ppt(0.1ppb for sample)
~1 ppb level obtained  1 cpd level background
Laboratory for Isotope and Trace Element Research
(LITER) @ Old Dominion U.
1.7 tons of pure CsI powder produced
• 2mBq/kg of 137Cs
• 1.37 – 2.52 ppb of Rb in powder
 ~ 1ppb Rb expected by crystallization
3. U,Th background :
U Chain :
218Po214Pb(145
  Coincidences
msec)
T1/2=4.23+-1.10 min
(T1/2 = 3.10 min)
After subtraction of Th232
Th Chain :
216Po212Pb(145
294.8 +/- 49.4 events
msec)
U : 1.16 +/- 0.19 ppt
T1/2 = 0.138+/-0.012 sec
(T1/2 = 0.145 sec)
Th : 0.411 +/- 0.078 ppt
of Th232 if it is equilibrium
Accidentals
It is low enough compared with
other backgrounds.
Background spectra of current crystal.
cpd
237 kg days data
6cpd
Contributions
1. Cs137 ~2.9 cpd
2. Cs134 ~ 1.8 cpd
137Cs
& Rb87
 full size
crystal
~ 5Reduction
p.e./keV
 Trigger Threshold less than 2 keV
( 5 p.e. within 2 ms )
3. Rb87 ~ 1.0 cpd
Total ~ 6 cpd @ 10keV
Summary and prospects
• The mechanism of 137Cs in Cs compounds are well understood, and reduction
of the contamination is successfully proved.
• At powder level, the contribution is < 1 cpd.
• Rb reduction is tried at company site, and crystallization process further
reduces down to 1 ppb level.
• Expect to be sensitive 1 order of magnitude below the DAMA signal center
• Confirmation of < 2cpd crystal.
• Construction of 216 kg crystal array
• Further purification will be decided depending
on the results of 216 kg crystal data.
Log Mean Time distribution of Neutron &
137Cs Compton & Data
3~4 keV
4 ~5 keV
5~6 keV
6~7 keV
7~8 keV
8~9 keV
9~10 keV
10~11 keV
11~12 keV
Production of CsI powder in Company
Pollucite Ore
Digestion
Power plant
Quartz
Rb reduction possible
Purification of Cesium salt solution
Waste 1
Waste 2
Conversion to CsOH solution
Evaporation
CsOH solution(50%)
Carbonization
CsOH solution + HI
Pure technical CsI Salts
Deionized water
Rb reduction
VE-water
< 10 μS
~ 2 μS = 0.5 MΩ
Filtration
Cs2CO3 Solution
CsHCO3 +HI
High Purity CsI Salts
Recrystallization
in the same tank