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5.3 Monte Carlo studies of the designed upgrade 79
For calculations, 238 U and 232 Th decay chains in equilibrium were assumed.
For the case of CsI crystals, this is not likely to be, as the different
chemical and physical properties of the elements of the Th and U
decay chain will lead to a broken equilibrium or single impurities and
their short-lived progenies. However, as this is a background estimation,
it was considered to be sufficient as an upper limit.
An analysis, considering the background reduction of 99 % of all
events, emitting alpha particles in the scintillator, with a pulse shape
discrimination was not used here, owing to the already small event
rates. Such analyses should consider the effect of δ-electrons, which are
produced by alpha particles, but lead to an electron-event like pulse
shape.
137 CS is an anthropogenic radioactive isotope. It is a beta-emitter,
decaying with 94.4 % branching ratio and a half live of 30.07 years, to
the metastable state of 137m Ba. The emitted electron has an endpoint
energy of 514 keV. After half life of 2.55 min 137m Ba deexcites to the
ground state under emission of a 661.7 keV γ. Both particles are not
coincident in time. In 5.6 % 137 Cs decays directly to the ground state
of 137 Ba, emitting an electron with an endpoint energy of 1175.6 keV.
Deniz et al. [57] announced to reduce the 137 Cs contamination in CsI:Tl
powder by using purified water instead of usual processing water
which contained 0.1 mBq/l.
Figure 5.13: Decay scheme of 137 Cs [16]
The 137 Cs contamination is no serious background for the double beta
signature. The main decay channel produces a noncoincident electron