5.2 Determination of the Fast/Total Ratio
One of the characteristics of pure CsI is the scintillation emission at
wavelengths of 305nm. The time response of this fast component can be
characterized by two decay times of 10 ns and 36 ns respectively
[Kub 87]. In addition to the fast component, there is a slow component
with a decay time of about one microsecond, which forms a continuous emission
spectrum in the wavelength region of 380-600nm (Fig 5.1).
The ratio of the intensity of the fast component to that of the slow component
depends upon the quality of the raw material used and the conditions during the
growth of the crystals. It is very important to know the portion of the slow
component for each individual crystal, since it affects the rate capability and
the energy resolution of the entire calorimeter.
Figure
5.1 [Ham 95] Emission spectrum of a pure CsI crystal.
For the PIBETA experiment, the "fast to total ratio" (F/T) is defined as the
ratio of the fast integral (100 ns) to the total integral
(1µs) of the raw CsI-pulse. A crystal is accepted, if the following
condition is fulfilled:
F/T>70%. (5.1)
A typical CsI-pulse and the definition of the integration limits are shown in
Fig. 5.2. The F/T-value is measured for each CsI-crystal when it is delivered
at PSI.
Figure
5.2:
CsI-pulse of a cosmic muon penetrating a pure CsI-crystal. The PMT signal has
been digitized with a digital scope TDS740. The software gates corresponding to
the fast and the total integral are indicated.
Since the F/T measurements are very sensitive to various parameters like
electronic noise, integration starting points etc., care has been taken to
develop a measuring method which allows the comparison of F/T-values from
measurements performed over several years: The unwrapped crystal is coupled to
a dedicated PMT (EMI 9822QB) by air gap coupling. The PMT signals are recorded
with a digital scope TDS740, which is read out by a PC. Cosmic muons
penetrating the CsI-crystal produce CsI-pulses for each of which the fast and
the total integral is calculated as shown in Fig. 5.2. The software gates
start 15 ns before the peak, the gate lengths are 100 ns and 1
µs for the fast and the total gate, respectively. The fast and the total
integral are histogrammed for 1000 events. A typical result is shown in Fig 5.3
(top).
One source of background which could falsify the result for the F/T value are
cosmic muons penetrating the quartz glass of the PMT producing Cerenkov light.
These events can be seen by recording PMT signals without any scintillator
connected. Cerenkov signals and CsI-pulses can be identified by their decay
time, which is about 3 to 5 ns for the former.
Figure
5.3:
Result of a F/T measurement on a HEXC type CsI-crystal. Top left: Fast vs.
total integral with the slope corresponding to the F/T value. Top right: the
ratio F/T for each event; the indicated mean value of 80.1% is defined as the
F/T-value of the crystal. Bottom left (right): Rise-time (fall-time) of the
pulses as defined in Eq.5.2 (5.3).
In order to clean the CsI-signal from background, its rise-time
tr
(
5.2)
and fall-time tf
(
5.3)
are calculated, where t0 is the time at the maximal amplitude
A0 in Fig. 5.2. The values for tf and
tr are filled into the appropriate histogram (Fig 5.3
bottom); a cut is set on the fall time (tf>40 ns) for
CsI-pulses. The resulting F/T-value[12] of
each CsI-crystal is entered into the pibeta crystal database (see 5.6).
[12] The value obtained with the unwrapped
crystal are somewhat higher than the effective F/T-value for the wrapped
crystal, although the absolute intensity of both, fast and slow component, are
increased by wrapping the crystal.