208 Chapter 3. Gas Filled Detectors
ηp. The electrons and ions thus produced drift toward opposite electrodes under the
influence of the applied electric potential. If the signal is measured at anode then
we are mainly concerned with the drift of electrons (which, of course, also depends
on the distortion of field due to the slow movement of heavy ions). We will call the
efficiency of these electrons to reach the anode without being parasitically absorbed
or getting escaped from the active volume byηa. Lastly we will lump together the
efficiencies of the electronic circuitry into a single parameter,ηe. The total efficiency
will then be given by
η=ηwηgηpηaηe. (3.8.1)
The efficiency associated with absorption in the window can be easily calculated for
photons by recalling that a photon beam in matter follows exponential attenuation,
that is
I=I 0 e−μwxw, (3.8.2)
whereI 0 andIare the incident and transmitted photon intensities,μwis the atten-
uation coefficient of photons for the material of the window, andxwis the thickness
of the window. The efficiency is then given by
ηw =I
I 0
= e−μwxw. (3.8.3)To calculate the absorption efficiencyηg, we again use the exponential attenuation
term but since this time it is the absorption we are are interested in, the efficiency
is given by
ηg =I 0 −I
I 0
=1−e−μgxg, (3.8.4)whereμgis the attenuation coefficient of the filling gas andxgis the thickness of the
active volume of the detector. The value is also referred to as thequantum efficiency
of the detector. It depends on the attenuation coefficient, which is a function of the
energy of the incident radiation and the type of the material. Therefore quantum
efficiency has implicit energy and material dependence.
Let us now move on to the next efficiency factor, that is the efficiency of creating
electron ion pairs by absorbed radiation. This process, though not 100% efficient,
can still be considered so for the gases typically used in radiation detectors. The
reason is that in these gases the possibility of loss of energy in the form of phonons
(heat carrying particles) is fairly small. Also, the low levels of contaminants in
these gases ensure that energy does not get parasitically absorbed. Hence for most
practical purposes we can safely assume that
ηp≈ 1. (3.8.5)The efficiency of the electronic circuity highly depends on its type and therefore
can vary significantly for values very close to unity for well designed systems to
moderately low for systems with not so perfect designs. Therefore there is no general
function or value that could be assigned to every type of electronic signal analysis
and data acquisition chain. It should be mentioned that by electronic efficiency of