Radiative Capture and Neutron Emission inLa139+αandCe142+p

The excitation functions for the reactions ${\mathrm{La}}^{139}(\alpha ,\gamma ){\mathrm{Pr}}^{143}$, ${\mathrm{Ce}}^{142}(p,\gamma ){\mathrm{Pr}}^{143}$, ${\mathrm{La}}^{139}(\alpha ,n){\mathrm{Pr}}^{142}$, and ${\mathrm{Ce}}^{142}\mathrm{}(p,n)\mathrm{}{\mathrm{Pr}}^{142}$ at excitation energies of 8-38 MeV have been measured. The excitation function of the ($\alpha ,\gamma$) reaction rises to about 75 μb at 17-MeV excitation energy and falls to about 10 μb at 28-MeV excitation energy. The excitation function of the ($p,\gamma$) reaction rises to about 650 μb at 19-MeV and falls to about 80 μb at 36-MeV excitation energy. The large difference both in the shapes and in the magnitudes at the maxima of the two excitation functions, despite the fact that both reactions proceed through the same compound system ${\mathrm{Pr}}^{143}$, is evidence for a direct contribution to the radiative capture of protons. A calculation of the ($\alpha ,\gamma$) excitation function based upon the compound-nucleus model agrees reasonably well with the experimental results, while a direct-interaction calculation of the ($p,\gamma$) reaction, although not in good agreement with the experimental results, gives a substantially better agreement than one within the compound-nucleus formalism. The ($\alpha ,n$) and ($p,n$) excitation functions both rise to a maximum of about 130 mb at 15 MeV and fall to around 10 mb at 38 MeV. The calculated cross sections according to the compound-nucleus mechanism are in reasonable agreement with the experimental results except at the high-energy end—where, nevertheless, both the shapes and the magnitudes of the two excitation functions are the same.