An instrument which uses a detector operated at a voltage in the region DE is called a proportional counter. The number of electrons collected by the anode will be much greater than, but proportional to, the number of primary electrons produced in the chamber. These electrons are collected in a short period of time to produce a pulse of current. Because of this, proportional counters can be used to detect each ionizing event separately. They measure the number of ionizing events, whereas ionisation chambers measure the amount of ionisation produced by these events.
The pulses produced by a proportional counter provide two useful pieces of information:
This means that an alpha particle will give a much larger pulse than either a beta particle , or the photoelectrons and Compton electrons from a gamma interaction. This property exploited by suitable electronics enables us to identify and measure radiations of high LET (alpha) in the presence of others with lower LET (beta and gamma). Some hand-and-foot monitors which are capable of detecting alpha contamination use proportional counters.
The main limitation in using proportional counters is that they require a very stable power supply and amplifier to ensure constant operating conditions. This is difficult to provide in portable instruments, which is why proportional counters are more frequently used in fixed or laboratory instruments.
One of the exceptions is the portable proportional counter used for neutron measurements described below.