The design of portable gamma- and beta-radiation measuring instruments

Portable instruments capable of detecting or measuring gamma- or beta-radiation usually have as radiation-detecting element either an air ionization chamber or a Geiger-Mÿller counter. The ionization- chamber instrument is comparatively insensitive and is used extensively in health instrumentation where the radiation characteristics of the chamber can readily be made to simulate those of the human body. Geiger-Mÿller counter instruments are used to achieve extreme sensitivity, and find application in the field in the detection of uranium-bearing ores, and in the laboratory for finding slight radioactive contamination. This paper describes the principal considerations which enter into the design of both types of instrument, and discusses methods which have been used and are now being investigated to achieve optimum performance with high reliability. In ion chamber-electrometer valve instruments the conflicting requirements of sensitivity, fast response and low zero drift have to be balanced against one another, and a compromise achieved which is tolerable in instrument size and weight, and in battery life. In the Geiger-Mÿller counter instrument the chief design problem centres around the E.H.T. supply for the counter. Power supplies using cold-cathode valves and selenium rectifiers can be made to give stable output voltage over a wide range of H.T. battery voltage; nevertheless, the short shelf life of miniature H.T. batteries is a drawback, and their entire elimination is desirable. A spring-driven generator is being designed for this purpose. Cold-cathode valves are used throughout in preference to filament valves, in the interests of reliability and robustness, and the problems introduced by their use are discussed