3.8. Detector Efficiency 209
pulse counting systems we essentially mean that the system does not miss any real
pulse and does not count any false pulse. This is a very stringent requirement and is
rarely met even by most sophisticated systems. One can however safely say that a
well designed modern system has an efficiency that approaches unity. Let us assume
that the efficiency of the electronic system is about The total efficiency of the system
is then given by
η = ηwηgηpηaηe
=(
e−μwxw)(
1 −e−μgxg)
(1) (1)
= e−μwxw(
1 −e−μgxg)
. (3.8.6)
It should be stressed here that this is a somewhat simplified picture of the actual
situation. We assigned perfect efficiencies to two factors. This might not be the
case in a real system. But then the question is:how accurately we actually want to
know the overall efficiency?. There is no general answer to this question since it is
the application that dictates the answer.
The efficiency factor related to absorption in the detector window can also be,
for most practical, purposes be assumed to be very close to unity. The reason is
that the windows are generally made of very thin materials having low absorption
cross sections in the energy range of interest. In such a case, the overall efficiency is
simply given by the quantum efficiency of the system, that is
η≡ QE =ηg
=1−e−μgxg. (3.8.7)This is the reason why most experimenters concern themselves with the quantum
efficiency of the system. Quantum efficiency actually sets aphysical limiton the
efficiency of the system. A system’s efficiency can not be better than its quantum
efficiency no matter how well the system has been designed.
Quantum efficiency, though very useful, does not tell us how efficiently the detec-
tor detects the incident particles. The reason is that it is only concerned with the
efficiency of absorption of particles in the detection medium. A much more useful
quantity is the so calleddetective quantum efficiencyorDQE, which actually tells us
how well the system works in terms of detecting and measuring radiation. Earlier in
the chapter we looked at two types of gas filled detectors: integrating and quantum,
though we didn’t assign them these names. An ionization chamber is an integrating
detector since it integrates the charges on an external capacitor for a predefined
period of time and the resulting voltage is then measured by the readout circuitry.
The voltage measured is proportional to the charge accumulated on the capacitor,
which in turn is proportional to the energy deposited by the incident radiation. A
quantum detector, such as a GM tube, on the other hand, counts individual pulses
created by incident particles. Due to the difference in their modes of operation,
these two types of detectors have different detective quantum efficiency profiles. For
an integrating system,DQEis given by (43)
DQEint=QE
1+ σ(^2) m
(QE)(Nin)