The interaction of intermediate energy nucleons with nuclei

Measurements have been made of the differential cross-sections for (p,n) reactions at 98.5 MeV on the 1p-shell nuclei 7Li, 9Be, B, 12C and 160 over an angular range of 0°-45° (lab) . Angular distributions for transitions to excited states of the residual nuclei as well as their ground states were obtained. The energy resolution was ~1.2 MeV (f.w.h.m.) and the angular resolution 3.2° (f.w.h.m.). The angular distributions are subject to an error which varies systematically with angle and could be up to ± 10% at 12.5°, though it is thought likely to be less than this. The absolute normalization error is ± 14%. The experiments were done using the neutron time of flight facility on the Harwell 2.79 m synchrocyclotron,, Thin targets were bombarded with the internal proton beam of the synchrocyclotron and the neutron time of flight spectra determined using a 100 m flight path. The experimental systems were considerably modified prior to. the experiment and an investigation was carried out to determine the optimum energy resolution obtainable. It proved impossible to achieve the hoped for 0.5 MeV (f.w.h.m.). An improved value was obtained for the degree of excitation of the 0.431 level in 7Be in the 7Li(p,n) reaction and this is compared with an impulse approximation prediction (Mahalanabis, 1964). The two values agree within experimental error, but the prediction is somewhat higher than the experimental value. All the angular distributions established were discussed in terms of the structures of the states involved. The results are compared with previous (p,n) work at 100 MeV (Saltmarsh, 1965), 30 MeV and 50 MeV (Clough, 1969; Clough, 1970). Comparison is also made with results from 12C(p,p') experiments (Hasselgren, 1965; Jacmart, 1964). Shape fits to the angular distributions were made using a DWBA computer code (Friedman, 1966), The DWBA formulation is discussed, and the methods by which the code could be used to further analyse the results in order to determine the effective nucleon-nucleon interaction is explained. It is thought that future developments in the study of (p,n) reactions at this energy are likely to be towards achieving considerably improved energy resolution, and a precise absolute normalization of the cross-sections.