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

36 4 Scintillation detectors
Figure 4.8: Discrimination of γ/α events with double charge method. The partial
charge is plotted versus the total charge for high (a) and low (b) energy α
and γ events in CsI:Tl [32]
.
The fast and slow decay components arise from the de-excitation of
different excited states of the luminescence centres. These states are finally
populated in relative intensities, depending on the density and
kind of excitations. Here, high ionisation loss leads to a high density
for bound excitons [33], which, when captured by an impurity centre,
excite the latter to states that causes a fast component. Successively
captured single free electrons and holes give rise to excitations
of metastable states of the luminescence centres, causing a slow decay
component [27, 28, 29].
The TEXONO collaboration achieved with CsI:Tl crystals and PMT
readout a efficiency for γ/α discrimination of more than 99 % [32]
(figure 4.8). They used the double charge method [34] for the pulse
shape discrimination and could maintain this efficiency for events
of down to ≈ 100 keV electron-equivalent light output. The different
decay time constants of γ (τfast = 0.87 µs, τslow = 5.20 µs) and α
(τfast = 0.54 µs, τslow = 2.02 µs) events enable the discrimination by
comparing the ”total charge” (integrated charge over 4 µs) and the
”partial charge” (charge integration also over 4 µs, with a delay time
of 0.5 µs).
Dinca et al. [33] quoted to obtain the optimum for α/γ discrimination
for a short (1 µs) and a long (6 µs) time gate.