Proton total reaction cross sections in the 10-20-MeV range: Calcium-40 and carbon-12

Total reaction cross sections for protons on Ca40 have been measured at 15 energies between 10.3 and 21.6 MeV by the beam-attenuation method. The total reaction cross section for Ca40 rises sharply at low energies, reaches a maximum value around 13 MeV, and reaches a minimum value around 16 MeV. The rise at the lower energies is a result of the Coulomb barrier. The dip at 16 MeV is probably associated with the (p,n) threshold for Ca40. A comparison is made between the present experimental values and preliminary optical-model predictions based on available elastic-scattering data and polarization data. The variation in the reaction cross section is also compared with the integrated partial cross sections for elastic scattering. Total reaction cross sections for protons on carbon have been measured at ten energies between 9.88 and 19.48 MeV. Resonances in the total reaction cross section are observed in the neighborhood of 10.4 and 13.8 MeV. Variations of 200 mb are seen in the cross sections with changes in energy of the incident protons of about 200 keV. A comparison is made of the present total reaction cross sections and the integrated partial cross sections for elastic scattering and inelastic scattering to the first excited state of C12.