A triggered cloud chamber investigation of high energy photonuclear reactions

This thesis is an account of the investigation performed at Glasgow University, between April, 1956 and July, 1958, under the supervision of Mr. J.R. Atkinson. The experiments which it describes were done by using, as a photon source, the bremsstrahlung spectrum of the 330 MeV. Glasgow University synchrotron. A triggered cloud chamber was developed and employed for the investigation of high-energy photonuclear reactions. The triggering pulses were provided by scintillation counter telescopes, which were set to detect high-energy protons produced by the X-ray beam in the cloud chamber gas. In the introductory chapter, a survey of the work in the field of high-energy photonuclear reactions is given. Experimental and theoretical evidence, supporting the so-called quasi-deuteron model of photonuclear reactions, is presented. It is shown that the role of other possible mechanisms of gamma-ray absorption at high energies has not been ascertained. Furthermore, in the case of the quasi-deuteron disintegration, the experiments performed so far have not provided data about the angular distributions of protons and neutrons in the centre-of-mass of the quasi-deuteron. Data of this nature is needed to provide information about the nature of the short range pair correlations in complex nuclei. The triggered cloud chamber is shown to be able so supply some new information on these questions. The author's idea of using the recoil energy distribution for the measurement of internal momentum distribution is also explained. In Chapter 3, "the experimental apparatus is described. The cloud chamber used in this experiment was designed by Messrs. J.R. Atkinson and J.M. Reid. The author contributed to the development of some details such as the expansion system, spark valve, clearing field, illumination system, etc. The chamber was triggered by means of three scintillation counter telescopes using plastic scintillators. The author was responsible for the design and construction of the telescopes and accompanying electronics, and these are fully described. Various problems - the calibration of pulse height infinity particle energy, non-linearity of the scintillator response, sources of background, method of analysis of photographs, etc., are discussed. The experimental results are presented and discussed in Chapter 3. The experiments were performed in collaboration with Mr. J.M. Reid. The analysis of all photographs was done entirely by the author. The interpretation and discussion of results are also the author's independent work. Some of the calculations of the particle kinematics were done by Mr. J.M. Reid, with whom many discussions relevant to the work were held. The experiments were done with helium, nitrogen and neon, as irradiated gases. Reactions yielding from one up to six charged fragments were found to be associated with the high-energy proton emission. In most cases a high-energy neutron was also emitted, in agreement with the quasi-deuteron model. The proportion of events in which the residual nucleus is left intact is much smaller than expected for light nuclei. A small number of events in which only a single proton was ejected as well as a few other events not explainable on the basis of the quasi-deuteron model, were also found. Recoil energy distributions in the case of (?, pn) events were obtained, but the statistics attained were not sufficient to derive accurate values of the average internal momenta of nucleons