326 Chapter 6. Scintillation Detectors and Photodetectors
Let us now see how the constantsτrandτdcan be defined. We start by writing
only the rising part of the above equation and substitutingt=τrin it. This gives
L = L 0(
1 −e−t/τr)
= L 0
(
1 −e−^1)
=0. 63 L 0.
This implies thatτris the time taken by the pulse to reach 63% of the maximum
height of the pulse. We will call this constant therise time constant.
In a similar fashion, for the other constantτdwe use only the decaying part of
the pulse profile 6.1.4 with the substitution oft=τd.
L = L 0 e−t/τd
= L 0 e−^1
=0. 37 L 0.This implies thatτdrepresents the time for the light pulse to decay to 37% of its
maximum height. τdis generally known as thedecay constantand is one of the
most sought after parameters for any scintillator. The rise time constant, on the
other hand, does not get much attention merely because the typical rise time of
scintillation pulses is extremely small.
Each material has its own characteristic decay time, which is determined ex-
perimentally. The difference between the decay profile of materials can be quite
significant and therefore the profile of one material can not be used to deduce any
meaningful conclusion about the behavior of another material. The dependence of
pulse decay time on material is demonstrated in Fig.6.1.5, which has been plotted
for three different kinds of scintillators.
The decay time of a pulse produced by a scintillator depends not only on the
scintillation material but also on the type and energy of the incident particle. The
dependence on the type of incident particle arises due to the difference in the stop-
ping powers of different types of radiation. for example the particles having high
dE/dx,suchasα-particles, fill more long lived states than particles having lower
stopping power. The consequence of trapping more electrons in the metastable states
is significant emission of delayed light. Thus the decay component of the pulse is
slower and is characterized by a longer decay constant.
In the preceding paragraph we argued that a scintillation material may have more
than one decay constant for different types of incident radiation. The main reason
behind this difference, as we discussed, is the availability of more than one metastable
energy states. Now since all scintillation materials have such energy states due to
intrinsic and added impurities therefore we can conclude that, in principle, every
scintillation material should have more than one decay constant. This, in fact,
has been found to be the case. However since the difference between these decay
constants is generally not significantly large therefore an average decay constant or
an average decay time can be used to characterize the material. The decay constants
found in literature are actually averages.
We will see shortly that all the decay constants of a material have temperature
dependence. Generally the decay constant decreases with increase in temperature.
Since lower decay constant would mean slower decay of the pulse therefore operating
a scintillator at low temperatures is more suitable.