188 Chapter 3. Gas Filled Detectors
3.5.B ChoiceofGas...........................
As with ionization chambers, virtually any gas can be used in proportional counters
as well. This follows from the fact that all gases and their mixtures allow the
process of gas multiplication, which is the basic requirement for a detector to work
as a proportional counter. However there are other factors that must be taken into
account while deciding on a filling gas, some of which are discussed below.
B.1 Threshold for Avalanche Multiplication
During the discussion on the multiplication factor we realized that every gas at a
certain pressure has a threshold electric potential below which the Avalanche does
not take place. This threshold depends on the type of gas as well as its pressure. In
Fig.3.5.4 we have plotted the first Townsend coefficient for three different types of
gas mixtures. It is evident from this figure that not only the threshold for Townsend
avalanche is different for each gas mixture but its voltage profile also differs some-
what. The reader should be warned that the published values of the first Townsend
coefficients vary significantly from one source to another due to the difficulty in the
associated measurements. Care should therefore be exercised while using the values
available in literature specially for the design purposes.
E (x10^3 V/cm)50 100 150 200 250
(x10^3 1/cm)
α0
0.5
1
1.5
2
2.5
3
3.5
4
Figure 3.5.4: Dependence
of the first Townsend coeffi-
cient on electric field inten-
sity for three gas mixtures:
80%Ne+20%CO 2 (solid line),
90%Ar+10%CH 4 (dashed line),
and 70%Ar+30%CO 2 (dotted
line).Since the threshold for gas multiplication in noble gases is much lower than in
polyatomic gases, therefore in general, the standard practice is to use a noble gas
as the main component of the filling gas mixture. Perhaps the most commonly