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

18 3 The COBRA experiment
They are commercially supplied with a red passivation, which
prevents the degradation of the crystal material due to oxidation
processes or moisture. This passivation has been identified as the
main background source in a former test set-up with 16 crystals.
Crystals with a low background optimised colourless passivation
had a reduced background level of about a factor 10 [24] and are
investigated in the current set-up with eight crystals of two different
passivations. The left side of figure 3.2 shows one of 64
CZT detectors with colourless passivation, which will be installed
soon.
CZT detectors operate with a high voltage of about one kV/cm,
being applied between the cathode (bottom side of crystal in figure
3.2) and a coplanar grid anode (upper side of crystal in figure
3.2). CZT detectors with a common anode have, due to different
charge carrier mobilities and trapping times, a signal, which
depends on the collection time and point of interaction. Thus,
a suitable energy resolution could not be obtained. The coplanar
grid anode has been developed as electron-only readout (see
[25]) and therewith energy resolutions of under 1 % (FWHM at
662 keV) can be achieved. Typical crystals used for the COBRA
prototype set-up have resolutions of around 3-5 % [8].
3.3 Motivation for scintillator upgrade
Besides the 0νββ decays, the 2νββ transitions are of physical interest as
well. The 2νββ decay of 116 Cd into the first excited state is of interest
for this thesis. Figure 3.3 shows the transitions of 116 Cd into the ground
and first excited state of 116 Sn. The decay of 116 Cd into the first excited
state is accompanied by the emission of a 1294 keV photon.
The current Cobra experiment has a low efficiency for gamma ray detection.
Only about every tenth of the in this decay emitted 1294 keV
photons interacts with the small CZT detectors and the full energy of
these photons is detected for only 1% of the decays. This leads to a
small detection efficiency for this double beta decay. Accordingly, an
upgrade of the current COBRA setup to enable a better gamma ray detection
is desirable.
The aim of this thesis is therefore to develop a design of a COBRA upgrade
with a high detection efficiency for the decay of 116 Cd into the
first excited state. A desired upgrade should achieve high detection ef-