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

74 5 Scintillator upgrade
were reported [55]. Of these contamination only 134 Cs and events of
the U and Th decay chains have high enough energy for concealing the
double beta signal. The influence of 137 Cs and 87 Rb were considered
nevertheless, because their signals can pile up with other background
sources.
134 CS is an anthropogenic isotope with a half life of 2.06 years, but can
also be produced by the 133 Cs(n, γ) 134 Cs reaction with a thermal neutron
capture cross section of 25.8 barn [56]. The beta decay of 134 Cs
(Q=2.059 MeV) can be observed by detecting correlated 604.7, 795.8 keV
γ rays and an electron with the endpoint energy of 658.1 keV. Other decay
modes are also possible, see figure 5.10.
Figure 5.10: Decay scheme of 134 Cs [16]
Lee et al. [55] measured a contamination level of 14.1 mBq/kg, leading
to 1.2×10 7 134 Cs events in the proposed COBRA upgrade during the
first year.
The entries in the CZT detectors occur only in 3.16 % of all 134 Cs decays.
In the case of coincident events in the CZT and the CsI detectors,
mainly one CZT and one (0.43 %), two (1.08 %) or three (0.79 %) CsI detectors
are involved. The CZT crystals have full energy peaks for the
604.7 keV and 795.8 keV gamma ray and entries up to 2000 keV. The energy
deposition in the CsI detectors shows mainly the prominent two