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

80 5 Scintillator upgrade
and gamma ray which have each a much lower energy than the signature.
The direct transition of 137 Cs to the ground state of 137 Ba can have
only a signature similar to the double beta decay event with very low
electron energy.
Simulations of a background level of 7.5 mBq 137 Cs, corresponding to
7.5 × 10 6 137 Cs decays per year in the planned scintillators, producing
14.5×10 6 noncoincident events per year. This number of events was
simulated and all events deposit energy in one or more CsI crystals.
The majority of these events deposit energy in one (28.5 %), two (10.8 %)
or three (1.6 %) scintillators. Only 1.23 % of all 137 Cs events have also
coincident entries in the CZT detectors. 0.39 % have entries in one CZT
and one CsI and 0.53 % in one CZT and two CsI.
The coincident deposited energy in CZT and CsI detectors can in figure
5.14. The energy deposition in the CsI detectors is except for small
energy depositions in the CZT detectors, below the 3σ interval around
1294 keV. The maximum deposited energy in the CZT detectors around
662 keV, corresponding to the full energy deposition of the 662 keV
gamma ray. The pure 137 Cs background contribution can already be
excluded with the general CZT energy threshold of a few hundred keV.
The 137 Cs contamination is therefore of minor importance, as long as
the background level of the hole experiment is small enough to avoid a
pile up with other background sources.
87 RB is a primordial nucleus, decaying with a half life of 4.75 ·10 10 years
through beta decay with a Q-value of 282 keV. Natural Rb includes
27.8 % 87 Rb and Kim et al. [56] located contamination of 1 to 1000 ppb in
different CsI powders. A reduction to 1 ppb was possible with recrystallisation
of the powder with pure water. Without further reduction a
contamination level of 11.8 ppb (corresponding to 12.14 mBq/kg) was
reported. Accordingly 12 × 10 6 decays per year are expected in the
extended COBRA detector. The single decays of 87 Rb are no serious
background, but might contribute as background by pile up with other
background sources.
RESULTS
These estimations show, that CsI crystal, which have not been stored
underground for a few decades, cannot be easily used for searches of
the neutrino accompanied double beta decay of 116 Cd into the first excited
state of 116 Sn. These estimations were made for already background
reduced crystals. For crystals that are not background reduced,