Proportional counter studies of electron capture to positron emission ratios in light isotopes

The Introductory chapter of this thesis comprises a brief review of the theory of nuclear beta decay, with emphasis on electron capture to positron emission ratios in allowed transitions. Reasons for interest in the ratio of K capture to positron emission are discussed, and the techniques which have been employed to measure this quantity are outlined. A summary is given of the available experimental data for Isotopes of low atomic number. There have been few measurements of K/β+ ratios In Fermi decays, and few in decays where recent calculations have suggested t.hat Inclusion of atomic matrix elements In the amount. The need for such measurements, and also for measurements at very low atomic number where, as yet, there is little accurate theoretical informatlon, is emphasised. The proportional counter techniques used by the author to Investigate gaseous electron capture sources of low atomic number are described in the second chapter. Different types of continuous flow counters designed to minimise the "wall effect" are discussed, and methods of overcoming the difficulties arising from the short half-lives and weak electron capture Intensities of the Isotopes of Interest are outlined. Two chapters are devoted to a study of the response of proportional counters at very low energy, undertaken to provide the Information necessary for the analysis of pulse height distributions produced by electron capture sources. The distribution produced by monochromatic x-rays of energies below 3 kev were Investigated and found to approximate closely to the Poisson function a further experiment determined the contribution to the width of such distributions from statistical variations In the gas multiplication process. A measurement of the K/β+ ratio In the 21 minute decay of carbon 11 is described; the special plastic anticoincidence counter used was designed earlier by Drever for carbon dating work, but was well suited to the present appllcation, where propane. irradiated by the x-ray beam of the Glasgow synchrotron was flowed through it. The result was consistent with the available theory. Finally, the development of a new technique for application to very short-lived sources Is described; this method,in which a proportional counter Is exposed directly to the activating x-ray beam to produce the isotope of interest, was suggested for use In conjunction with the high Intensity beam of the new Glasgow linear accelerator. A preliminary experiment, using the synchrotron to Irradiate a neon-filled counter, gave a result for the K/β+ ratio of neon 19 in agreement with theory, and indicated that the technique would be well suited to half-lives of the order of a second. The results on K/β+ ratios obtained here do not indicate any serious failure or omission in the available theory, while the study of proportional counter response helps to consolidate the role of the instrument as a reliable detector of soft x-rays and ultra-violet radiation