Production of low radioactivity CsITl crystals for WIMP searches a case of artificial radioisotope

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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

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WIMP search with CsI(Tl) Crystal
Goal 25 CsI(Tl)-216kg Crystals With lt2 cpd
backgrounds
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Low background HPGe meassurement facility

• 100 Coaxial HPGe(Ortec)
• 10cm OFHC 15cm Pb
• 4 liter/min N2 Flow
• 0.2 cpd _at_ 1MeV

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Background spectrum of CsI(Tl)
Crystals on the shelf (Belle CsI(Tl) crystals)
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Internal background of CsI Crystal

• 137Cs (artificial) serious background at low
  energy
• 134Cs (artificial133Cs(n,gamma))
• 87Rb (natural) Hard to reject
• ? reduction technique in material is known

CPD
137Cs 0.35 cpd/(mBq/kg) 134Cs 0.07
cpd/(mBq/kg) 87Rb 1.07 cpd/ppb
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1. Mechanism and reduction of 137Cs contamination

• Origin of 137Cs artificial(fission product)

Bomb test Chernobyl
Close to Chernobyl 0.1 Bq/liter
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• 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 (lt5.8
  mBq/kg).

137Cs in surface soil
Blagoeva, J. Environ. Racioactivity (1995)
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Production of CsI powder in Chemetall Company
(Germany)
Power plant
70 liters/ 1kg of CsI
Processing water
River water
Cartridge Polisher
Pure water 0.5 M?
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Measurements of 137Cs in Water
AMP Ammonium Molybdophosphate
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Production of low background crystals

• 137Cs contamination is well controlled.
• Up to lt 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
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2. 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) _at_ 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

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3. U,Th background Coincidences
U Chain 218Po?214Pb(145 msec)
T1/24.23-1.10 min (T1/2 3.10 min)
After subtraction of Th232 294.8 /- 49.4 events
Th Chain 216Po?212Pb(145 msec)
U 1.16 /- 0.19 ppt
Accidentals
It is low enough compared with other backgrounds.
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Background spectra of current crystal.
237 kg days data

• Contributions
• Cs137 2.9 cpd
• Cs134 1.8 cpd
• Rb87 1.0 cpd
• Total 6 cpd _at_ 10keV

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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 lt 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 lt 2cpd crystal.
• Construction of 216 kg crystal array
• Further purification will be decided depending
• on the results of 216 kg crystal data.

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Log Mean Time distribution of Neutron 137Cs
Compton Data
34 keV
4 5 keV
56 keV
89 keV
67 keV
78 keV
1112 keV
1011 keV
910 keV
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Production of CsI powder in Company
Pollucite Ore
Power plant
Digestion
Quartz
Rb reduction possible
Purification of Cesium salt solution
Waste 1
Deionized water
Waste 2
Conversion to CsOH solution
VE-water lt 10 µS 2 µS 0.5 M?
Evaporation
Rb reduction
CsOH solution(50)
Carbonization
Filtration
Cs2CO3 Solution
CsOH solution HI
CsHCO3 HI
Pure technical CsI Salts
High Purity CsI Salts
Recrystallization in the same tank