Photoneutron Cross Sections With Monoenergetic Neutron-Capture Gamma Rays

Monoenergetic gamma rays produced by thermal neutron capture have been used to study photoneutron cross sections of the isotopes ${\mathrm{Ta}}^{181}$, ${\mathrm{Li}}^7$, ${\mathrm{Li}}^6$, ${\mathrm{C}}^{13}$, and $B^{10}$ for $\gamma$-ray energies between threshold and 10.8 MeV. Some cross sections were obtained with uncertainties as low as 10%. Wherever possible, these cross sections are compared with those obtained by other methods. The threshold for the ${\mathrm{Ta}}^{181}(\gamma , n)$ reaction has been determined to be 7.64±0.04 MeV. Information about the radiative widths of the 7.47-MeV level in ${\mathrm{Li}}^7$ and the 8.89-MeV level in ${\mathrm{B}}^{10}$ is presented.