Photonuclear Studies with Monoenergetic Gamma Rays from Thermal Neutron Capture

Monoenergetic $\gamma$ rays produced when neutrons are captured in various materials have been used to study the following reactions: ${\mathrm{Ta}}^{181}(\gamma , n){\mathrm{Ta}}^{180m}\left(8.15\mathrm{} \mathrm{hr}\right)$, ${\mathrm{Au}}^{197}(\gamma , n){\mathrm{Au}}^{196}\left(5.6\mathrm{} \mathrm{days}\right)$, ${\mathrm{Ho}}^{165}(\gamma , n){\mathrm{Ho}}^{164}\left(34\mathrm{} min\right)$, ${\mathrm{Ag}}^{107}(\gamma , n){\mathrm{Ag}}^{106}\left(24\mathrm{} min\right)$, and ${\mathrm{Nb}}^{93}(\gamma , n){\mathrm{Nb}}^{92}\left(10\mathrm{} \mathrm{days}\right)$. Cross sections for these reactions have been obtained at several discrete energies between 7.5 and 10.8 Mev. Estimates were also made of the thresholds of these reactions. Two of the thresholds so obtained, ${\mathrm{Ta}}^{181}(\gamma , n){\mathrm{Ta}}^{180m}$, $E_t=7.60\mathrm{}\pm 0.08$, and ${\mathrm{Nb}}^{93}(\gamma , n){\mathrm{Nb}}^{92}$, $E_t=8.99\mathrm{}\pm 0.04$ have precisions comparable with those of previous measurements of the same quantities.