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5.3 Monte Carlo studies of the designed upgrade 71
Figure 5.8: Distribution of the number of involved CZT and CsI crystals
for 116 Cd decays into the first excited state of 116 Sn.
One million decays were simulated in the CZT crystals. The spectrum
of deposited energy in all CZT crystals (left side of figure 5.7) shows
the energy deposition of both electrons in the source CZT crystal up to
1511 keV, with the maximum around the expected 0.32 × Q. The full
energy peak of the 1294 keV gamma ray, absorbed in a second CZT,
and the deposition of the electron and a part of the gamma energy in
the source crystal is visible. The CsI sum spectrum (right side of figure
5.7) shows the 1294 keV full energy peak and a Compton spectrum up
to 1080.7 keV, overlaid with the backscatter peak at 213.3 keV. It can be
assumed, that hardly any electrons leave the CZT crystal.
The distribution of events, depositing energy in the CZT and CsI crystals
is plotted in figure 5.8. 23.2 % of all events are not detected in the
CsI crystals. This correspond with the from section 5.1.1—“Scintillator
size” determined ≈70 % total efficiency, which was determined only for
photons, hitting a flat scintillator perpendicular to the input face and is
therefore expected to be higher for this geometry.
A simulation of 1294 keV photons, being initialised in the CZT detectors,
showed that 17.7 % of these photons are interacting neither with
the CZT, nor with the CsI detectors.