2-4.1 Proportional Counters

A proportional counter is simply an ionization chamber designed to operate using gas multiplication, a form of current amplification caused when the drift velocity of electrons and ions is sufficiently energetic to increase the total ionization 1 9-22

Gas multiplication requires a strong voltage gradient, which is difficult to produce in a parallel plate chamber One electrode is a surface and the other electrode is a thin wire The thin wire creates a strong electric field gradient near its surface Figure 2 17 shows a proportional counter The percentage of the total gas volume in which multiplica­tion takes place is adjusted by changing the spacing between electrodes The gam per pulse is most nearly constant when the detected particle is ionizing the non multiplying gas zone The collected ionization then passes entirely through the multiplying gas zone

Gas amplification is electrically adjusted by changing the applied high voltage The sharp gradient about the wire electrode permits operation at moderate voltage Thus, the high voltage supply may be interchangeable with the high voltage supply used for ionization chambers However, unlike an ionization chamber, the proportional counter is

extremely sensitive to voltage variation. A well-regulated high-voltage supply is essential.

The operating voltage of the proportional counter, like that of the ionization chamber, has a plateau.23 In any particular design there is a range of amplification in which the pulses have a fixed range of amplitudes for a given type

of radiation. When all pulses over some minimum voltage are counted (pulse height), the plateau is manifested as in Fig. 2.18. Operation on the plateau reduces sensitivity to voltage variation. However, the plateau is normally never as good as that for an ionization chamber.

When detecting neutrons, a proportional counter does not have a gamma tolerance as good as is possible with an
ionization chamber; it is typically of the order of 103 R/hr. Proportional counters for reactor use are usually BF3-filled or boron-coated. If boron-coated, a number of gas fills may be used. Figure 2.18 also shows the effect of gamma background. The gammas may be observed to have a deleterious effect on the plateau.