150 Chapter 3. Gas Filled Detectors
certainly true but only to a certain extent. In fact it has been found that the W-
value depends only weakly on these parameters and lies within 25-45eV per charge
pair for most of the gases and types of radiation (see Table 3.1.1). An interesting
point to note here is that the W-value is significantly higher than the first ionization
potential for gases, implying that not all the energy goes into creating electron-ion
pairs. Of course this is understandable since we know that radiation is not only
capable of ionizing the atoms but can also just excite them.
The charges created by the incident radiation are calledprimarycharges to dis-
tinguish them from the ones that are indirectly produced in the active volume. The
production mechanism of these additional charge pairs are similar to those of pri-
mary charges except that they are produced by ionization caused by primary charge
pairs and not the incident radiation. The W-value represents all such ionizations
that occur in the active volume. For a particle that deposits energy ∆Einside a
detector, the W-value can be used to determine the total number of electron-ion
pairs produced by
N=
∆E
W
. (3.1.1)
If the incident particle deposits all of its energy inside the detector gas, then of
course ∆Ewould simply be the energyEof the particle. However in case of partial
energy loss, we must use some other means to estimate ∆E. An obvious parameter
that can be used is the stopping powerdE/dx, which we discussed in chapter 2. In
terms of stopping power, the above relation can be written as
N=
1
W
dE
dx∆x, (3.1.2)where ∆xis the path covered by the particle. Sometimes it is more convenient, at
least for comparison purposes, to calculate the number of electron-ion pairs produced
per unit length of the particle track
n=1
W
dE
dx. (3.1.3)
As we saw in chapter 2, due to energy straggling, the stopping power fluctuates
around its mean value. Similarly the W-values for different gases as measured by
different experimenters suffer from significant uncertainties. Variations of as much
as 30% in the reported values have been observed. These factors must be taken into
consideration while estimating the total number of charges.
As mentioned above, the W-value represents all ionizations that occur inside the
active volume of the detector. Sometimes it is desired to know the primary charge-
pair yield as well. However, because of almost inevitable secondary ionizations that
occur at nominal applied voltages, it is not always possible to determine this number
experimentally. Nevertheless, a number of experiments have been performed and
primary as well as total ionization yields have been reported by several authors (see
Table 3.1.1).