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

5.3 Monte Carlo studies of the designed upgrade 77
Figure 5.12: Distribution of the number of involved 134 Cs events
tion of events is listed in table 5.9. The restriction to neighbouring CsI
crystal reduces the detection efficiency for the regarded double beta decay
from 17.01 % to 13.31 %, while reducing the background rate by a
factor 2 to 6×10 4 counts/year for this configuration and without further
cuts. Regarding also the source CZT crystal position improves the
background reduction further, but not significantly with the bad position
information for events in CsI crystals.
The background reduction, using the information of angle correlated
gamma rays, is more efficient for events with better position resolution,
like multiple CZT detector hits. Kim et al. [43] cited position resolutions
of less than 1 cm for the crystal length. For the scintillator
blocks, this reduces the position information from an former volume
of 20 × 5 × 5 cm 3 to a volume of 1 × 5 × 5 cm 3 . It should be studied
whether this can reduce the background rate sufficiently.
Kim et al. [56] reported their studies about contamination and inhibition
of 134 Cs in CsI powder. In contrast to 137 Cs, the inhibition of 134 Cs
does not occur due to the water used during the CsI powder processing,
which could contribute only a few µBq/kg. The contamination
were already present in the pollucite before the Cs extraction and can
also be produced in the CsI crystals and powder by the neutron cap-