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

78 5 Scintillator upgrade
crystal positions 134 Cs events 116 Cd events
N 48.32 % 78.31 %
NN 17.01 % 15.26 %
O 8.69 % 1.75 %
R 25.98 % 4.68 %
Table 5.9: Contribution of 2νββ decay of 116 Cd into the first excited
state and background contribution of 134 Cs, for events with one involved
CZT and two CsI detectors, categorised by positions of involved
CsI crystals. Neighbouring crystals, which have a joint surface (N), next
neighbouring crystals with a joint edge (NN), crystals at opposite sides
of the Nest, having joint surfaces with the Nest (O) and crystals, being
not in the former categorisation (R) are distinguished.
ture process on 133 Cs. The measured 134 Cs contamination in the pollucite
powder of around 12 mBq/kg corresponds to an exposure time
of one year of neutron flux at see level. The saturated contamination
after infinite exposure with a typical thermal neutron flux at see level
was reported to be about 50 mBq/kg for pollucite powder and around
80 mBq for CsI powder. Accordingly, the 134 Cs contamination can only
be reduced by limiting the exposure to thermal neutrons. This can be
achieved by storing the crystals as soon as possible underground, having
short crystal production times and by storing and transporting the
powder or crystal protected from thermal neutrons.
238 U: With a reported contamination level of less than 0.75 ppt [55],
there will be only around 10000 decays from the 238 U chain in the extended
COBRA detector (assuming secular equilibrium). One million
of the events from the nat. 238 U decay chain were simulated and they
produce only 0.13 % coincident entries in single CZT and one or two
CsI detectors (1CZT-1&2CsI). This lead to about 13 expected coincident
events in the detector and to about 1 event in three years in the 3σ
region around the 1294 keV line in the scintillator. The 238 U contamination
in the scintillator are therefore negligible, compared to the other
background sources.
232 TH: An even lower 232 Th contamination level of 0.38 ppt was reported
by Lee et al. [55], leading to around 1600 decays per year in the
scintillation detectors. Of these only 0.2 % produce coincident entries
in one CZT and one or two CsI detectors. These are 3 coincident events
per year, and about one in 5 years in a 3σ region.