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

5.1 Monte Carlo studies for the upgrade design 57
cage is also limited. It is therefore favourable to build a compact scintillation
detector.
The advantages and disadvantages of these four different scintillators
for the planned COBRA upgrade are listed in table 5.4. NaI:Tl, although
being a common used scintillator with a very high light yield
and low radioactive contamination, has for this project a to low density
and atomic number. Additionally, the required PMT readout and
the difficult handling of this hygroscopic scintillator is not suitable. An
upgrade with BGO can be used with diode readout and is compact in
size, but BGO has a low light yield, leading to a low energy resolution
and the contained radioisotope 207 Bi (Q-value = 2.3 MeV) can be a serious
background component for the searched signal. CWO is a suitable
scintillator for this upgrade, but is nearly twice as expensive as BGO.
CsI:Tl has the highest light yield, a readout with diodes is possible and
the radioactive 137 Cs is not expected to be a serious background for the
2νββ signal of transitions into the excited state. Although it is not as
compact as BGO or CWO, this upgrade is also less expensive.
scintillator light yield size handling diode readout radioactivity
CsI:Tl �� � � �� �
CWO � �� �� � �
BGO - �� �� � -
NaI:Tl �� - - - ��
Table 5.4: Advantages ( �) and disadvantages (-) of different scintillators.
Accordingly, 5 cm of CsI:Tl were considered as a compromise between
cost and detection efficiency. Due to its much higher light yield than
BGO and CWO and, in comparison to NaI:Tl, more easy handling, it
was chosen as the most promising scintillator material for this upgrade.
5.1.3 Electronic readout
The Cobra Nest will be surrounded by a layer of 5 cm CsI crystals. For
a high detection efficiency and smaller amounts of needed scintillator
material, the scintillator should be placed as close as possible to the
Nest with as little material as possible between the CZT and the scintillators.
The scintillator readout device is therefore placed at the outer
sides of the scintillator layer. Additionally, electronic devices nearby
the CZT detector increase the background level. In general, all six outer
sides of the layer could be used for placing the readout. The front and