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

5.3 Monte Carlo studies of the designed upgrade 75
gamma lines and a beta spectrum, overlapping with these lines to energies
of up to 2300 keV. Figure 5.11 shows the deposited Energy in CsI
(plotted at the abscissa) and CZT (plotted at the ordinate) for coincident
events.
number of absolute number
involved detectors of 134 Cs events ɛ (%)
1CZT&1CsI 5.20×10 4 47.66
2CZT&1CsI 1.38×10 4 6.89
1CZT&2CsI 1.29×10 5 17.01
2CZT&2CsI 2.31×10 4 1.91
Table 5.8: Absolute numbers of 134 Cs background events in the first
year and the detection efficiency for the 116 Cd decay into the first excited
state.
Table 5.8 lists the numbers of 134 Cs background events in the first year
and the detection efficiency for the 116 Cd decay into the first excited
state in dependence of the number of involved detectors. For coincident
events in CZT and CsI detectors, the combinations of 1 and 2 CZT
and CsI detectors were listed. The total detection efficiency for the regarded
double beta decay and coincident CZT and CsI events is highest
for 1CZT&1CsI. It has a 134 Cs background rate of 5.20×10 4 events in the
first year. The efficiency at 1CZT&2CsI is with 17 % the second highest,
but the background is nearly three times higher. Events in 2CZT&1CsI
have a 7 times smaller efficiency, but only a nearly 4 times smaller background
than 1CZT&1CsI events. 2CZT&2CsI events have lower efficiencies
and higher background than the previous 2CZT&1CsI events.
In order to decide whether the detection of this double beta decay
events is possible, only events in 1CZT&1CsI and 1CZT&2CsI configurations
are now regarded further, being together the majority of the
simulated double beta events.
Applying the cut from figure 5.9, being within the 5σ energy interval of
the 1294 keV line in the scintillator, 0.29 % of the 134 Cs events survive,
being nearly 35000 events per year. This background component is
therefore very harmful. An additional cut, demanding a sum energy of
the entries in the CsI and CZT detector of more than 2200 keV, reduces
this background to zero. This cut also reduces the detection efficiency
for the double beta decay significantly from former 33.13 % to 3.14 %,
making even a direct detection of the maximum expected signal of
around 20 events per year difficult.