A scintillation dose-rate meter using a tuned low-frequency amplifier.

Abstract

Scintillation dose-rate meters have been described by a number of workers, e.g. by Griffiths and Swindell (1951) and by Belcher (1953). All these instruments consist essentially of a photomultiplier which detects the light produced by the absorption of X or γ radiation in a luminescent crystal. The direct current from the collector electrode is used as a measure of the radiation doserate. Since this current is small some form of d.c. amplification has been used. The high specific sensitivity of this type of radiation detector may be used to advantage where a small detecting volume is required, for example in measuring the dose-rate near small radioactive sources. Under these conditions, the illumination of the photocathode may be so feeble that the direct effect of radiation on the photomultiplier and light guide becomes appreciable (Griffiths and Swindell, 1951; Ittner and Ter-Pogossian, 1952). Light emission from the guide may be eliminated by using internally polished metal tubes (Belcher, 1953; Cole, Moore and Shalek, 1953). The transmission efficiency is less than with Perspex rods and the direct effect of radiation on the photomultiplier becomes more important. With unfavourable arrangements this may account for 30 per cent of the current measured. When working near the limit of sensitivity the signal may be less than the photomultiplier dark current. This dark current is usually balanced out by a zero adjustment, but it is very sensitive to changes in temperature and E.H.T. (Engström, 1947; Belcher, 1953).