Photoneutron cross sections for the silicon isotopes

The photoneutron cross sections for $^{28}\mathrm{Si}$, $^{29}\mathrm{Si}$, and $^{30}\mathrm{Si}$ have been measured up to 33 MeV with monoenergetic photons from the annihilation in flight of fast positrons, using neutron multiplicity counting. Average neutron energies were obtained simultaneously with the cross-section data by the ring-ratio technique. The giant dipole resonances for $^{28}\mathrm{Si}$ and $^{30}\mathrm{Si}$ exhibit appreciable fragmentation; that for $^{29}\mathrm{Si}$ does not. The ($\ensuremath{\gamma}, 2n$) cross section for $^{30}\mathrm{Si}$ is large; that for $^{29}\mathrm{Si}$ is consistent with zero. The ($\ensuremath{\gamma},1n$) cross section for $^{30}\mathrm{Si}$ decreases sharply with energy to values near zero as the ($\ensuremath{\gamma}, 2n$) cross section grows, then increases to appreciable values as the ($\ensuremath{\gamma}, 2n$) cross section diminishes; this extreme behavior, although never seen before, is attributable to the competition between the ($\ensuremath{\gamma}, n$), ($\ensuremath{\gamma}, 2n$), and ($\ensuremath{\gamma}, \mathrm{pn}$) decay channels. Some properties of the isospin components of the giant resonance are inferred. Other features of the data, including the integrated cross sections, are found to be similar in many respects to corresponding results for the oxygen and magnesium isotopes. The $^{28}\mathrm{Si}$ nucleus is found to be a better "core" for $^{29}\mathrm{Si}$ and $^{30}\mathrm{Si}$ than might have been expected from previous descriptions of its open-shell character.