The lifetimes of positrons in matter

The writer has attempted to show in this thesis how the scintillation counter may be used for the measurement of short time intervals in nuclear physics and how this technique may be applied to the study of positron annihilation. In the first section, the significance and the occurrence of short lifetimes in the general field of nuclear physics is briefly surveyed, and the general methods of measuring short time intervals are discussed. A more detailed study is made of positron annihilation. The material of this section is drawn mainly from the literature. Section two consists of the analysis leading to the design of a delayed coincidence apparatus using scintillation counters. The design and construction of the author's apparatus are described and evidence of its performance is presented and discussed. The analysis and designs which are presented here are generally original though their basic principles are not. In section three are described the experimental investigations of the lifetimes of positrons in solids. The matter here is original, all the work having been carried out solely by the author who was also responsible for the interpretation of the results. Section four describes the experimental investigation of the spectrum if annihilation gamma-rays from positrons annihilating in freon and oxygen. This work was carried out in collaboration Mr. C. Lewis who had the major responsibility for the experimental work. The interpretation and discussion were shared equally by us. Work done the writer in the field of gamma-ray spectroscopy using scintillation counters and a pulse amplitude analyser is presented in Appendix I. The study of Lu[176] was initiated by Mr. D. Dixon who suggested the application of scintillation counter techniques. The scintillation counter measurements were made by the writer. In Appendix II the final circuits of the coincidence unites are shown by diagrams.