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 CsI(Tl) crystal Neutron shield / Muon det.(30cm) Lead shield(15cm) Polyethylene(5cm) Copper shield(10cm) Goal : 25 CsI(Tl)-216kg Crystals With <2 cpd backgrounds
Low background HPGe meassurement facility • 100% Coaxial HPGe(Ortec) • 10cm OFHC + 15cm Pb • 4 liter/min N2 Flow • ~ 0.2 cpd @ 1MeV
Background spectrum of CsI(Tl) : Budker Institute of Nuclear physics (Ukraine) : Shanghai Institute of Ceramics (China) : Crismatec (France) Crystals on the shelf (Belle CsI(Tl) crystals)
137Cs : 10 mBq/kg 134Cs : 20 mBq/kg 87Rb : 10 ppb 87Rb Geant Simulation 137Cs 134Cs keV Internal background of CsI Crystal • 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 0.35 cpd/(mBq/kg) 134Cs 0.07 cpd/(mBq/kg) 87Rb 1.07 cpd/ppb
1. Mechanism and reduction of 137Cs contamination • Origin of 137Cs : 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)
SK-water purification system 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 MΩ
Decanting &Drying Precipitation (24hours) Gamma Detection Water : 20 liter + HNO3(70% 1cc/liter) +AMP(0.5g/liter) Measurements of 137Cs inWater *AMP : Ammonium Molybdophosphate
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. 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
T1/2 = 0.138+/-0.012 sec (T1/2 = 0.145 sec) Th : 0.411 +/- 0.078 ppt of Th232 if it is equilibrium 3. U,Th background : Coincidences U Chain : 218Po214Pb(145 msec) T1/2=4.23+-1.10 min (T1/2 = 3.10 min) After subtraction of Th232 294.8 +/- 49.4 events Th Chain : 216Po212Pb(145 msec) U : 1.16 +/- 0.19 ppt Accidentals It is low enough compared with other backgrounds.
cpd 6cpd 137Cs & Rb87 Reduction • full size crystal ~ 5 p.e./keV • Trigger Threshold less than 2 keV ( 5 p.e. within 2 ms ) 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 @ 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 ofNeutron & 137Cs Compton & Data 3~4 keV 4 ~5 keV 5~6 keV 6~7 keV 8~9 keV 7~8 keV 9~10 keV 10~11 keV 11~12 keV
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 < 10 μS ~ 2 μS = 0.5 MΩ 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