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

5.2 The design 65
5.2 The design
The considerations of the previous section 5.1—“Monte Carlo studies
for the upgrade design” lead to an design with twelve 5×5×20 cm 3 and
eight 5×5×5 cm 3 crystals. These are arranged as shown in figure 5.4.
Four scintillator blocks are placed on a ground plate. A newly designed
Nest is placed on the inner two scintillators. The Nest is surrounded by
two crystal blocks on the left and right side and four crystal cubes on
the front and back side. The crystals are place on top of one another.
The upper four scintillators are placed on a delrin plate, being fixed
with pins at the outer side of the crystals. The four cubic crystals on
the front side will have a reduced height, to allow the high voltage and
signal cables of the CZT detectors and Teflon tubes to lead through.
The necessary size for the cable feed is not yet known, owing to still
running studies to decide the cable types.
The new Nest was designed by Ebert [51]. It consist of 2 mm thick delrin
plates and delrin holders of 10 mm edge length. Five Teflon tubes
will feed into the side walls of the delrin Nest, being used for calibrations
and nitrogen flushing. The tubes are bond to the Nest and lead
between the layer, horizontally from the front left to the back right edge
of the Nest, passing the middle over or under a CZT layer. This way,
wire-moved calibration sources can be inserted into the setup without
opening the apparatus. The tubes will also be used to flush the Nest
with nitrogen in order to reduce the radon level and to cool down the
inner Nest. The proposed Nest is shown in fig. 5.4.
The Nest will be housed in a layer of copper, which again will be surrounded
by lead. Beyond this the Faraday cage and neutron shield will
not change from those described in [52].
5.3 Monte Carlo studies of the designed upgrade
In this step the efficiency for detecting the expected double beta decay
signal is estimated. Therefore the COBRA Geant4 application VENOM
was expanded. Afterwards the neutrino accompanied double beta decay
of 116 Cd and expected background was simulated. The CsI crystal
background will be discussed in more detail, whereas the background
from 222 Rn in the Nest air, and contamination of the CZT passivation
and the glass of possible readout electronics was analysed only quali-