190 Chapter 3. Gas Filled Detectors
charge of positive ions gets accumulated near the cathode. These ions attract free
electrons from the surface of the metallic cathode and recombine with them to form
neutral atoms. Although the atoms are neutralized but they are left in excited states
due to the excess energy available to the ions and the electrons. The transition of
such an atom to the ground state is generally accomplished through the emission of
a photon, which may as well induce secondary electron emission from the cathode.
Furthermore since there is a sheath of ions attracting electrons from the cathode,
more electrons can be freed than required for the neutralization. All such electrons
are potential avalanche initiators. This process, if not controlled, may lead to sec-
ondary avalanches in proportional counters. This problem is also solved by adding
a polyatomic quenching gas in the main filling gas. The quencher molecules, having
large number of rotational and vibrational energy levels, neutralize the ions through
charge transfers. But since their own de-excitation processes are mostly non radia-
tive therefore they considerably decrease the probability of secondary avalanches.
Although the polyatomic quenchers have obvious advantages we just discussed,
however their use is not free from negative effects on the chamber. There are two
main problems associated with the polyatomic quenchers: the buildup of polymers
on anode and cathode surfaces and the decrease in the lifetime of the chamber.
Both problems have the same cause, that is, the dissociation or polymerization of
quenching molecules during the process of their de-excitation. The degradation of
the electrodes with time is a serious problem because it may change the properties of
the chamber with time. The decrease in lifetime of the chamber is a serious problem
for sealed detectors. This can be circumvented by allowing the gas to continuously
flow through the chamber.
B.3 GasGain
An important consideration while choosing filling gas for a proportional chamber is
the maximum attainable gain or multiplication factor. We noted earlier that most
proportional counters are operated with a multiplication factor on the order of 10^4.
However sometimes it is desired to achieve higher gain before the Geiger breakdown,
that is before the onset of multiple avalanches caused by a single primary avalanche.
The quenching mechanism we just discussed serves this purpose to some extent.
However, if the voltage is raised to high values to increase the gain, the free electrons
can get enough energy to cause multiple avalanches. Therefore one must ensure that
the active volume gets continuously depleted of these low energy free electrons. The
best method to achieve this is by adding an electronegative impurity in the main
filling gas. Freon is one such polyatomic gas. The good thing is that such gases act
both as electronegative impurities as well as quenchers. The bad thing about them is
their capability to parasitically capture thegoodelectrons as well, thus suppressing
even the primary avalanche processes. Certainly such an effect should be minimized
as it can lead to appreciable decrease in detection efficiency.
3.5.C SpecialTypesofProportionalCounters
C.1 BF 3 ProportionalCounter...................
BF 3 filled proportional counter is one of the most widely used neutron detectors.
When a slow neutron interacts with boron-10, it produces anα-particle with two