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

5.3 Monte Carlo studies of the designed upgrade 69
ficially. A more detailed background study, as done by M. Heine [54]
and previous works was not performed. They require a more detailed
knowledge about the efficiency of the nitrogen flushing and the CZT
colourless paint and scintillator read out contamination level. These
examinations or not necessary at the current stage, where primary the
suitability of the scintillator is to be determined.
Detector processes, leading to an energy resolution or even position
dependent energy resolution are not implemented in VENOM. The energy
spectra, produced by VENOM, were convoluted by a Gaussian
with an energy dependent standard deviation for each detector type.
The used detector resolutions for the CZT and CsI:Tl detectors are:
CZT : σ
E
CsI : σ
E
= 3.787286
E
+ 0.013377 [54] (5.1)
= 0.58
√ E + 0.019 [43] (5.2)
The CsI resolution function was published by Kim et al. [43] for similar
crystal sizes and PMT readout. It was used for both sizes of scintillation
detectors. Although the upgrade is planned with semiconductor readout,
this resolution was used to estimate the expectable efficiency. The
used resolution function of the CZT crystals is expected to be worse
than the resolutions of the soon installed 64 CZT crystals.
For this study, the raw VENOM data is convoluted with a Root routine.
Afterwards the convoluted data is analysed with regard to the amount
of deposited energy in a single or all detectors and the number of involved
detectors. It is analysed in order to find cuts with high detection
efficiency for the decay signal and a low background count rate. The
emphasis lies on coincident events in the CZT and CsI detector, which
are unlikely to be imitated by the majority of background sources.
5.3.2 2νββ signal
At first the signal, the neutrino accompanied double beta decay of
116 Cd into the first excited state, was studied.
116 Cd → 116 Sn(2 + ) + 2e − + 2¯ν, E ∗ = 1294 keV. (5.3)
This transition was not measured for 116 Cd yet. The current upper limit
of the half life is 2.4·10 21 years (90 %C.L.) [12]. Accordingly, the amount