Quasifree knockout of charged particles from ⁴He with 100 MeV protons

Doctoral Thesis


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University of Cape Town

Exclusive measurements have been made of the reactions ⁴He(p,2p ), (p,pd), (p,pt) and (p,ph) at 100 MeV. The primary protons were measured at two angles, 45° and 60°, in coincidence with secondary protons, deuterons, tritons or helions covering a wide range of angles on the opposite side of the beam, from -15° to -90° in plane, and from 0° to 30° out-of-plane. The purpose of the experiment was to explore the validity of the Distorted Wave Impulse Approximation (DWIA) as a description of these quasifree knockout reactions, at all geometries where the knockout cross section is significant. Six particle telescopes, each consisting of two silicon detectors and a NaI detector, were used to measure the coincident particles. The experimental data are presented in the form of energy-sharing distributions, which were generated by projecting the two-dimensional quasifree knockout loci onto the energy axis of the primary proton. The comparison between these energy-sharing distributions and the curves calculated by means of the DWIA is satisfactory for all geometries, except at the most forward secondary angles for the (p,pt) and (p,ph) reactions. It is concluded that the DWIA is an adequate formalism for modelling the quasifree knockout reactions induced by 100 MeV protons on ⁴He. Based on this conclusion, the total contribution of quasifree knockout to the existing inclusive (p,p') spectra has been calculated by integrating the DWIA cross sections over the solid angles of the secondary particles. The spectroscopic factors used were those extracted from the coincidence data. For the (p,pn) reaction, it was assumed that the spectroscopic factors were the same as for (p,2p). It is found that 70 ± 10% of the inclusive continuum yields is attributable to quasifree scattering, with ~30% arising from multiple scattering. At low energies of the scattered proton, the knockout of nucleons constitutes over 90% of the quasifree component, whereas at high energies the knockout of deuterons, tritons and helions dominates the yield. Thus the absence of discernable quasifree peaks in the inclusive spectra is probably due to the cluster knockout contributions filling in the yield at higher energies.

Bibliography p. 214-226.