Proefschrift--Utrecht. / Bibliography: p. 62-64.

Thesis--Harvard University. / Photocopy (positive) made by University Microfilms. Abstract (5 L., with special t.p.) bound in at end.

Proefschrift--Utrecht. / Summary in Dutch and English. Bibliographical footnotes.

Thesis (Ph. D.)--Syracuse University, 2005. / "Publication number AAT 3182634."

Thesis (Ph. D.)--Syracuse University, 2005. / "Publication number AAT 3194019."

Thesis (PH.D.) -- Syracuse University, 2006 / "Publication number AAT 3251795."

Thesis (PH.D.) -- Syracuse University, 2006 / "Publication number AAT 3242486."

The object of this work was first to investigate, by the ionization method the dependence on the wall material of the ionization produced within chambers by high voltage-radiations of wave lengths from 0.013 A.U. to 0.5 A.U. The ionization chambers were pressed from mixtures of bakelite powder and graphite powder either with vanadium or cerium oxide. Secondly to investigate the possibility of measuring the quality of radiation by means of the ratio of ionizations in two chambers one of them is air walled and the other of higher atomic number. A study was also made of the validity of Gray's theory for calculating the ratio of ionization in such small chambers. The results of these investigations have shown that: I- Chambers moulded from the above mentioned mixtures are satisfactory electrical conductors and behave consistently in their interactions with the radiations used. The method may be useful in examining the quality of the scattered radiations from harder beams of X-rays generated at say between 400 and 1000 KV. The ratio of ionisation calculated according to Gray's theory agrees with the experimental one up to a wave length of 0.08A.U. The effective wave lengths of the X-ray beams were obtained from the complete spectral distribution of the energy and from measurement of the half value layers of the beams. It was found that in most cases it is sufficiently accurate, when considering the calculation of the ratio of ionizations in two chambers, to regard the beam as having that effective wave length corresponding to its half value layer.

This experimental and theoretical study aims at further investigation, by means of an extrapolation type of ionization chamber, of the ionization measurement of energy absorption from high-voltage radiations within a medium. Wavelengths ranging between 0.08 and 0.5 A0 were used. The walls of the ionization chamber were made of simple elements (graphite, aluminium and copper) or pressed bakelite - graphite mixtures which were loaded with cerium oxide in order to control the effective atomic number of the mixtures. A distinct advantage of the experimental arrangement used is the possibility of measuring the ionization per unit spacing when the air space is vanishingly small which thus eliminates the variable effects of chamber size. The results of earlier workers with chambers of fixed finite dimensions have been difficult to interpret in terms of theoretical considerations because of these effects. Furthermore, by varying the thickness of the upper plane electrode of the chamber, correction could be made for absorption of radiation in this electrode, which, at long wavelengths, maybe considerable in the media of higher atomic number. The results obtained with a chamber of graphite walls show that graphite behaves approximately as air walled material, the ionization per unit volume is constant i.e. the ionization I0 being proportional to V where V is the air volume. With walls of atomic number greater than that of air the ionization per unit spacing increases slightly as the spacing decreases up to a certain threshold spacing blow which it increases very rapidly. The ionization per unit spacing and the threshold spacing both depends upon the material of the electrodes and the wavelength of the radiation. The ionization per unit spacing at zero dimensions may be measured in two ways. Firstly, by drawing at the origin a tangent to the ionization - spacing curve and secondly by extrapolation to zero dimensions of the ionization per unit spacing - spacing curve. It was thus possible to compare these experimental observations with expectations based upon the Bragg -Gray theory of ionization within a cavity. This comparison suggests that the Bragg - Gray theory may be regarded as a satisfactory description of the facts for the range of wavelengths studied, at least for elements of atomic number up to that of aluminium (Z = 13). For copper (Z = 29) and the mixtures (depending upon the electron emission from Ce of Z = 58) the experimental results disagree with the theory except for the shortest wavelengths, and the disagreement increases with increase of wavelength. Suggestions are advanced and a modification made to Gray's equation in an attempt to correct for this disagreement. These are based upon a consideration of the sources and the energy of the photoelectrons omitted from the wall materials.

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.