The measurement of gamma-radiation by scintillation counting.

摘要

This thesis is concerned with the application of the technique of scintillation counting to the detection of gamma radiation. In counters of this type the scintillations produced in a luminescent medium are detected by means of a photo-multiplier tube and the electrical pulses at the output of the latter are amplified and counted electronically. An attempt is made to interpret the behaviour of such counters and to relate the observed counting rates to the physical processes occuring in the luminophor. Following a brief historical review of the scintillation counting technique, a theoretical analysis of the gamma scintillation counter is developed. This analysis describes how the number and energy distribution of the secondary electrons produced in a luminophor of known composition and dimensions subjected to a known flux of gamma radiation can be calculated. It shows that, providing the luminescent decay of the luminophor is rapid, each secondary electron will produce a single scintillation the intensity of which will be proportional to the energy of the electron. When the luminophor has a slow decay, on the other hand, this simple behaviour is not to be expected. The amplitude distribution of the resulting electrical pulses will be modified by statistical processes in the photo-multiplier tube, and by the resolving time of the amplifying and counting equipment. Experimental studies designed to test the validity of the theoretical analysis ore described. The first part of these studies is an investigation of the statistics of the type 1P21 photo-multiplier tube, stimulated by both continuous and pulsed light. The results of this investigation are in good accord with the theoretical predictions. The crystalline lumisophors investigated experimentally included calcium tungstate, thallium-activated potassium iodide, and thallium-activated sodium iodide. Following some preliminary studies on the effect of resolving time on counting rate, the absolute counting rates obtained when specimens of ench of these luminophors were irradiated under scatter-free conditions with the gamma radiation from sodium24, cobalt60, bromine82, iodine131 and gold198 were measured. These rates were then compared with the calculated rates of production of secondary electrons in the media. In the case of calcium tunstate under appropriate conditions, good agreement between counting rate end rate of secondary electron production was obtained. In the alkali halide luminophors, on the other hand, counting rates were abnormally high because of their relatively slow phosphorescent decay. These findings were confirmed by the study of the pulse amplitude distributions obtained with the luminophors under various experimental conditions. Studies on the variation in luminescent efficiency with temperature of eaoh of these luminophors, and on the spectral distributions of the emitted light are also described. Comparative measurements on a large number of liquid luminescent systems were made, with particular reference to the variation in luminescent efficiency with concentration of solute. These variations are interpreted in terms of a general theory due to Johnson and Williams. The system p-terpheny1-benzene was selected for more detailed study and the absolute counting rates obtained in this medium when irradiated by the gamma rays from ench of the five radio-isotopes already mentioned were measured. When a correction is applied for the electrons scattered into the medium from the walls of the containing vessel, the observed rates are found to be in good agreement with the calculated rates of production of secondary electrons. It was observed in the course of the above studies that both pure liquids such as water and transparent solids such as Perspex luminesce under gamma irradiation, and this luminescence is shown to be often explicable in terms of the Cerenkov effect. Detailed studies of this effect in Perspex were carried out, and the angular distributions and relative intensities of the emission due to each of the radio-isotopes: sodium24, cobalt60, bromine82 and iodine131 were measured. Good agreement is obtained between the observed results and the classical theory due to Frank and Tamm for the effect. Finally, experimental studies made on the luminescent decay of a copper-activated zinc sulphide phosphor of long afterglow following excitation by gamma radiation from each of the five radio-isotopes mentioned are described. The decay is shown to be non-exponential in form, but provided that certain precautions are observed, the counting rate at a known time after irradiation can be related to the gamma radiation flux through the phosphor during the exposure. The thesis ends with a critical discussion of the experimental results and an assessment of their implications, together with some suggestions for future research in this field. (Abstract shortened by ProQuest.).