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

5.3 Monte Carlo studies of the designed upgrade 83
decay chains produce 0.03 % and 0.04 % coincident (1CZT-1&2CsI) entries,
being about 15000 and 7000 coincident events.
Accordingly, the PMTs could become a main source of background and
their use should be avoided.
PAINT: The upper limit for contamination in the colourless paint for
the CZT crystal was measured to be lower than 1.5 mBq/kg for 238 U,
0.6 mBq/kg for 232 Th and 50 mBq/kg for 40 K [58], resulting in less
than about 220000, 11000 and 55000 decays per year (paint mass of
0.027g/CZT crystal). Less than 5 % of these decays produce coincident
entries, whereas about 100 are in the cut C-3σ region.
222 RN decays with a half-life of 3.82 days, but feeds into 210 Pb with a
half-life of 22 years. The last gamma active nuclei in the 222 Rn decay
chain is 214 Bi. As Rn is a gas, it diffuses into every area with lower partial
pressure. The decay product will be charged and accumulate on all
surfaces. The air in the Gran Sasso laboratory can have radon activities
of 35±2 Bq/m 3 [20]. The cavity in the Nest contains roughly 4·10 −4 m 3 ,
yielding about 15 mBq or half a million 222 Rn decays in one year. These
is a maximal estimation, without nitrogen flushing. Simulation of one
million events of the 222 Rn decay chain in the Nest air were produced.
3 % of these decays produce coincident (1CZT-1&2CsI) entries. These
decays will produce about 520 events in the C-3σ region, being less than
7 % of the minimal expected 134 Cs background.
RESULTS
Background PMT (K/U/Th) Paint (K/U/Th) 222 Rn
activity 504/78.2/25.5 mBq/PMT 4/0.1/1 Bq/kg 35 mBq/m 3
events/year 318/50/17×10 6 0.2/0.1/0.06×10 6 0.5×10 6
1CZT-1&2CsI 0.04/0.03/0.04 % ?/3.2/4.6 % 2.95 %
Table 5.11: List of different background sources
The detection of the neutrinoless double beta decay into the first excited
state is not possible or at least very difficult with CsI crystals. The
background of 134 Cs is more than one order of magnitude higher than
the upper limits of the paint and 222 Rn background.
As alternative scintillator CWO or BGO should be studied. NaI:Tl is
due to is small density and the preferred read out with PMTs no solution,
as even background optimised PMTs are expected to be a main