a design study for a cobra upgrade to - Institut für Kern- und ...

5.3 Monte Carlo studies of the designed upgrade 73
Figure 5.9: Energy deposition of about 6.5×10 5 coincident 116 Cd events
in one CZT and one or two CsI detector, with marked 5σ interval
around the 1294 keV entry in the CsI detectors.
5.3.3 Background contamination in CsI crystal
Several radioactive isotopes can be found in CsI crystals. The KIMS collaboration
[42] investigated the contamination levels of commercially
available CsI(Tl) crystals. The main contamination were 137 Cs, 134 Cs
and 87 Rb, whereas isotopes of K and Na were negligible [43]. Lee et al.
[42] describe the reduction of internal background of CsI:Tl crystals and
achieved contamination levels listed blow [55].
Contamination levels for 238 U and 232 Th of only 0.75 ppt and 0.38 ppt
isotope commercial con- first reduced con- second reduced contamination
level tamination level [42] tamination level [55]
137 Cs 18-210 mBq/kg 15.5±2.6 mBq/kg 7.5 mBq/kg
134 Cs 38-586 mBq/kg 27.4±4.6 mBq/kg 14.1 mBq/kg
87 Rb 3.2-816 ppb 20±1 ppb 11.8 ppb
Table 5.7: Contamination levels of commercially available CsI:Tl detectors and
of achieved resolution in 2006 and 2007 [42, 55].