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76 5 Scintillator upgrade
Figure 5.11: Energy deposition of about 1.8×10 5 coincident 134 Cs
events in one or two CZT and CsI detectors, with marked 5σ interval
around the 1294 keV entry in the CsI detectors.
The background contribution of 134 Cs for events with more than two
involved detectors can be reduced by considering the angle correlation
of the subsequent gamma rays. After a 134 Cs decay, the majority
of gamma rays are emitted at a 4 + → 2 + → 0 + transition
with a correlation of the emitting angle between both photons of
W (θ) ∝ 1 + 1
8 cos2 θ + 1
24 cos4 θ. This favours emission in the same or op-
posite directions. Up to now, the only information of the interaction
position is the involved detector number. An analysis, considering the
angles between the in one event involved detectors is, owing to the big
dimensions of the CsI crystals in comparison to the small distance between
different detectors, not possible. However, the distribution of
hits in neighbouring and not neighbouring CsI crystals was studied.
It showed, that the regarded double beta decay leads to more hits in
neighbouring and next neighbouring CsI crystals (for events, involving
one CZT and two CsI crystals) than 134 Cs decays. Four categorisations
are distinguished, being neighbouring crystals, which have a joint
surface (N), next neighbouring crystals with a joint edge (NN), crystals
at opposite sides of the Nest, having joint surfaces with the Nest (O)
and crystals, being not in the former categorisation (R). The distribu-