Photoneutron Cross Sections forMg24,Mg26, and Natural Magnesium

Abstract

Photoneutron cross sections for $^{24}\mathrm{Mg}$, $^{26}\mathrm{Mg}$, and natural magnesium have been measured with the use of monochromatic photons obtained from the annihilation in flight of fast positrons. The resolution was less than 200 keV at 10-MeV photon energy, and 300 keV at 30 MeV. About 14 resonant structures in $^{24}\mathrm{Mg}$ and 32 in $^{26}\mathrm{Mg}$ were distinguished and parametrized. A Porter-Thomas treatment of the distributions of transition widths throughout the giant resonances of $^{24}\mathrm{Mg}$ and $^{26}\mathrm{Mg}$ indicates that the resonances belong to single-spin states, or have one degree of freedom in each case. The $^{26}\mathrm{Mg}(\ensuremath{\gamma},2n)$ cross section contains considerable structure, most of it coinciding with the structure in the total photoneutron cross section. It is confirmed that the giant resonance of $^{26}\mathrm{Mg}$ is split into two isospin components. Integrated total cross sections up to 28 MeV are 50 MeV mb for $^{24}\mathrm{Mg}$, 226 MeV mb for $^{26}\mathrm{Mg}$, and 76 MeV mb for natural magnesium.