Photonuclear reactions inCa40

Abstract

Natural calcium targets were bombarded with $\ensuremath{\gamma}$ rays from a 31-MeV betatron at several bremsstrahlung end-point energies. Angular and energy distributions of photoprotons were measured. In addition, the $\ensuremath{\gamma}$-ray spectra emerging from the thick target were recorded at several angles. They were used to study the $\ensuremath{\gamma}$ rays accompanying the decay of excited residual nuclei formed in the reactions $^{40}\mathrm{Ca}(\ensuremath{\gamma},p)$ and $^{40}\mathrm{Ca}(\ensuremath{\gamma},n)$. The analysis yielded the following results for the main reaction channels: the energy dependence of the cross sections, the energy dependence of photoproton angular distributions, and the bremsstrahlung-weighted angular distributions of deexcitation $\ensuremath{\gamma}$ rays. According to the experimental results the following amplitudes seem to dominate in the particle channels: ${f}_{\frac{5}{2}}$ waves for the ($\ensuremath{\gamma},{p}_{0}$) reaction, ${p}_{\frac{3}{2}}$ waves for ($\ensuremath{\gamma},{p}_{1}$), and $p$ waves for reaction channels in which the residual nuclei are left in states above 4.93 MeV. This is consistent with the giant dipole state configuration, predicted on the basis of the shell-model bound-state calculation by Gillet and Sanderson. The agreement with continuum theories is less satisfactory. It is also found that the shapes of cross sections are more uniform and that the angular distributions are less energy-dependent than expected from theory. The ($\ensuremath{\gamma},{p}_{1}$) cross section was separated into $S=0$ and $S=1$ channel spin contributions of which only the dominant $S=0$ part shows a resonant structure. Negative-parity hole channels bear evidence for two-step reaction processes and impurities in the ground state of $^{40}\mathrm{Ca}$. Ratios of ($\ensuremath{\gamma},p$) and ($\ensuremath{\gamma},n$) cross sections imply an admixture of less than 3% of $T=0$ strength in the $^{40}\mathrm{Ca}$ giant dipole resonance.[NUCLEAR REACTIONS $^{40}\mathrm{Ca}(\ensuremath{\gamma},p)$, ($\ensuremath{\gamma},p{\ensuremath{\gamma}}^{\ensuremath{'}}$), and ($\ensuremath{\gamma},n{\ensuremath{\gamma}}^{\ensuremath{'}}$), ${E}_{\ensuremath{\gamma}}=11\ensuremath{-}30$ MeV; measured $\ensuremath{\sigma}(E,\ensuremath{\vartheta})$ to states of residual nuclei; deduced reaction-channel configurations.]