title: Some studies of East neutron reactions in medium weight elements
Creators: name: Stones, Patrick J.
Publisher: name: University of Glasgow
Year: 1966

description: The theory, experimental method, and results of studies of (n, p) and (n, α) reactions induced in medium weight nuclei in the neutron energy regions 3 Mev and 15 Mev, respectively, are presented.

The part of the thesis is concerned with the determination of the absolute cross sections for the (n, p) reaction in the target nuclei Ni58, Fe54, and Cu63 at several neutron energies between 2.4, Mev and 3.4 Lev. Protons emitted into a large solid angle in the backward direction (with respect to the incident neutron beam) were detected. The experimental apparatus consisted of a) a Cockroft-Walton set for the production of a neutron flux by means of the reaction H2 (H2, n) He3, b) a target material in the form of a thin foil and c) a detection system consisting of a silicon semiconductor counter; the neutron flux was monitored absolutely by the detection of recoil protons from a hydrogenous foil. With the same detection system, a measurement of the cross section for the emission of protons into a large solid angle in the forward direction was also carried out, at one neutron energy. The energy spectra of the emitted protons from the target nuclei Fe54 and Ni58 were determined with a detection system of improved resolution, incorporating a proportion counter and semi-conductor counter.

The variation of the cross sections with energy of incident neutrons is monotonically increasing in the case of the target nuclei Fe54 and Ni58, while the cross section for the case of the target nucleus Cu63 is relatively independent of neutron energy in the energy range considered. The results are in fair agreement with the results of activation experiments carried out by other workers in the field. The ratio of the cross sections for the emission of protons in the forward and backward directions show the reaction mechanism to be compound nuclear, with no contribution from the direct interaction mechenism. The energy spectra of the emitted protons suggest the presence of previously unknown levels in the residual nuclei Mn54 and Co58. Also presented are the results of theoretical Hauser-Feshbach calculations on the cross section for the Ni58 (n, p) Co58 reaction, with different assumptions for the possible nodes of neutron and proton decay channels.

The account of the experimental method and results of a study of the angular distribution of alpha particles produced by the (n, α) reaction in various target materials for an incident neutron energy of 14.8 Mev is presented in less detail.

The detection system used in this series of experiments consisted of three dE/dx proportional counters. Total numbers of alpha particles emitted into a narrow solid angle, at several mean angles of emission, were obtained, Absolute total cross sections for the (n, α) reactions were determined by integration over the angular distributions. From the resultant angular distributions an estimate was obtained of a) the relative importance of direct interaction and compound nuclear mechanism, and b) the magnitude of the 'spin cut-off' parameter present in the expression for the density of levels in the residual nuclei.

A comparison of the experinental results with those of other workers is given, and the position of the results in the general scheme of nuclear reactions is considered.