3.6. Geiger-Mueller Counters 191
possible energies (2.31MeVand 2.79MeV).
n+B^105 →Li^73 +αTheα-particle thus produced has a very short range and therefore quickly inter-
acts with gas molecules to produce electron-ion pairs. The electrons then under the
influence of high electric field, initiate the avalanche. This usual gas multiplication
process typical of proportional counters then ensures a large pulse at the readout
electrode.BF 3 counters have a very good neutron discrimination capability due to
good deposition of energy by the neutrons.
Fig.3.5.6 shows the energy spectra obtained from a very large and a typicalBF 3
proportional counter. In a large counter, the electrons produced by theα-particles
deposit their full energy in the active volume of the detector and thus only two well
defined peaks corresponding to the twoαenergies are obtained. A typical counter
is however not that large. In fact, the counter diameters are generally smaller than
the range of energetic electrons.
ABF 3 counter can not be directly used to detect fast neutrons because of the low
interaction cross section of boron-10 for fast neutrons. Therefore in order to detect
fast neutrons some kind of moderator, such as paraffin, is used to firstthermalizethe
neutrons. Thermalization ormoderationis the process through which the neutrons
quickly transfer their energy to the medium.
C.2 HeliumProportionalCounters
A proportional counter filled with helium gas can be used to detect thermal neutrons.
When an incoming neutron interacts with the helium nucleus, a proton is emitted.
This proton creates secondary ionizations in the counter volume, which form the
output pulse. The primary neutron interaction with a helium-3 nucleus can be
written as
n+He^32 →H 13 +p. (3.5.12)
C.3 Multi-WireProportionalCounters
A Multi-Wire Proportional Counter or simply MWPC consists of an array of closely
spaced wires in a gas filled container. The wires are roughly 1mmapart and act
as anodes of individual proportional counters. Since each wire is read out through
a separate electronic channel therefore the counter is used as a position sensitive
detector. We will therefore defer a discussion on it to the chapter on radiation
imaging.
3.6 Geiger-MuellerCounters.........................
We mentioned earlier that, if the voltage is increased to very high values, the process
of avalanche multiplication can spread throughout the detector to produce the so
called breakdown in the gas. The spread is mainly caused by the ultraviolet photons
emitted during the localized avalanches (see Fig.3.6.1). These photons have high
enough energy to produce secondary electrons in the gas as well as in the electrodes
and windows of the detector. The electrons thus produced drift towards the anode
under the influence of the effective field inside the chamber. Since this field is very