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

Chapter 6
Summary and future work
In this thesis possible scintillators for a COBRA upgrade were investigated.
Emphasis was laid on inorganic crystal scintillators with high
light yield, high stopping power, moderate price and low intrinsic
background contamination. The scintillator materials NaI:Tl, CsI:Tl,
BGO and CWO were short-listed. A Monte Carlo simulation and a
price research helped to choose the optimal crystal size. CsI:Tl seemed
to be the most promising choice and was studied further. A strong
dependence of the scintillator energy resolution from the crystal dimensions,
the readout area and the spectral matching of the scintillator
emission spectrum and readout quantum efficiency required further
studies. A second Monte Carlo simulation compared different readout
devices and indicates, that a readout of CsI:Tl with APDs is possible
for this crystal size. A COBRA upgrade with CsI:Tl was designed and
implemented into the COBRA simulation framework VENOM. Monte
Carlo studies of this upgrade demonstrated that CsI:Tl is not as suitable
as anticipated for the COBRA experiment. The cosmogenic 134 Cs
is expected to be the main background of CsI:Tl and reduces the detection
efficiency for the neutrino accompanied transitions of 116 Cd into
the first excited state by one order of magnitude. This prevents a direct
detection of the 2νββ-decay of 116 Cd into the first excited state even
for half-lives, being equal to the lower half-life limit. The background
contribution was reduced by considering the position of involved CsI
crystals, for events with one involved CZT and two involved CsI detector.
A further background reduction for all events with more than two
involved detectors, by considering the angle correlations between the
detectors is expected. With the current simulation, this is only promising
for events with multiple CZT hits, owing to the lager CsI crystal
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