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

72 5 Scintillator upgrade
In 3.1 % of the simulated double beta decay events, two CZT and no CsI
crystals were involved. 1 % of all double beta events deposit energy in
a 3σ interval around 1294 keV in one CZT crystal and an energy of less
than 1511 keV in another CZT crystal.
76.77 % of all 116 Cd decays into the first excited state produce coincident
hits in the CZT and scintillation detectors. 47.60 % and 17.00 % of
all events deposit energy in one CZT and one or two CsI crystals. Further
studies will consider coincident hits with one CZT and one or two
CsI detector hits, being 64.6 % of the total amount of events. Coincident
hits are defined in the simulation as hits in the CZT and CsI detectors,
that belong to one event, whereas in the experiment these will be hits
(belonging to a single decay or not) in multiple detectors within a time
window of several tens of µs. For low count rates and small time windows,
random coincidences are strongly suppressed.
Events with deposited energy of up to the electron endpoint energy
of 1511 keV in one CZT detector and an energy deposition of around
1294 keV in the CsI detectors are the majority of all coincident events.
By considering an interval of 3σ (5σ) around the 1294 keV gamma line
in the CsI detectors 43.4 % (50.8 %) of all coincident (1CZT and 1&2CsI)
events and 28.0 % (32.9 %) of all signal events are included.
In figure 5.9 the deposited energy of the CZT is plotted against the energy
entry of the scintillation detectors, if only one CZT and one or two
CsI crystals were involved.
The red lines mark a distance of 3σ to the expected full energy deposition
of the 1294 keV photon in the CsI crystal. The standard derivation
from equation (5.2) at 1294 keV was used. The interval between the red
limes contains 99.7 % of all energy depositions with an expected value
of 1294 keV.
Cut ECZT (keV) ECsI(keV ) ECZT + ECsI (keV) ɛ (%)
A-3σ - 1231-1357 - 28.0
A-5σ - 1190-1398 - 32.9
B-3σ - 1231-1357 >2200 2.9
B-5σ - 1190-1398 >2200 3.4
C-3σ 300-900 1231-1357 - 26.2
C-5σ 300-900 1190-1398 - 27.6
Table 5.6: Efficiencies for different cuts for events in one CZT and one
or two CsI detector.