230 Chapter 4. Liquid Filled Detectors
Proportionality:In the Chapter on gas filled detectors we saw that one of
the strongest points of ionization chambers is their excellent proportionality to
the deposited energy. This is also true for liquid filled ion chambers.Dynamic Range:Liquid filled ionization chambers are not very well suited
for low radiation fields due to the reasons that will become clear later in this
section. However, from moderate to high fluxes, they do cover a wide dynamic
range.The basic principle of operation of a liquid filled ionization chamber is the same
as a gas filled ionization chamber. In its most simple form, a sealed container having
two electrodes is filled with a suitable liquid. The incident radiation produces charge
pairs, which move in opposite directions under the influence of the applied electric
field between the electrodes. The resulting current or the voltage pulse is measured
at the anode.
4.2.A Applications of Liquid Filled Ion Chambers
Dosimetry is one of the applications where liquid filled ionization chambers are now
beginning to be used. In particle accelerators used for radiation therapy, where the
flux of particles is quite, such detectors are being successfully used. Liquid filled
detectors have two characteristics that make them suitable for such applications:
more radiation tolerant as compared to semiconductor detectors and smaller in size
than gas filled detectors. For high radiation fields they offer good precision and can
even be designed to provide a good degree of spatial resolution (29).
4.3 LiquidProportionalCounters
Let us now see if it is possible to develop a liquid filled proportional counter. We
know from our discussions in the chapter on gas filled detectors that charge multi-
plication is a prerequisite for a detector to operate in the proportional region. This
process can be achieved fairly easily in gases but in liquids it is neither easy to
initiate nor to maintain for the reasons that will become clear shortly. Nevertheless
several researchers have shown that such detectors can in fact be built.
4.3.A ChargeMultiplication
As we saw earlier, in a liquid filled detector, the number of charge pairs produced
is generally higher than in a typical gas filled detector. The reason is mainly the
higher density of molecules and, in case of liquefied noble gases, the lowerW-value.
Although the larger number of charges produces, together with less diffusion of elec-
trons, is a desirable factor specially for position sensitive detectors, but in certain
situations it is also desired that the charge pair population is further increased. This
can be accomplished by allowing the process of charge multiplication in much the
same way as we saw in the gas filled proportional counters. That is, a very high
potential can be applied in a chamber having suitable geometry such that the elec-
trons could gain enough energy between collisions to create additional charge pairs.
The most suitable geometry, as in case of gaseous proportional counters, is of course