The photoneutron cross sections of ${\mathrm{O}}^{16}$ and ${\mathrm{Tb}}^{159}$ have been measured up to 30 MeV using photons of 3% resolution resulting from positron annihilation-in-flight. Thirteen resonances corresponding to levels in ${\mathrm{O}}^{16}$ were observed in the ${\mathrm{O}}^{16}(\gamma ,n)$ reaction. One, at 29±1 MeV, is previously unreported. The ${\mathrm{O}}^{16}(\gamma ,n)$ cross section integrated to 30 MeV is 46±5 MeV-mb. Both the ($\gamma ,n$) and ($\gamma ,2n$) cross sections of Tb were measured using a statistical counting technique. The ratio of the two cross sections determines that the level density parameter, $a=17\mathrm{}\pm 5$ ${\mathrm{}\left(\mathrm{MeV}\right)\mathrm{}}^{-1\mathrm{}}$. The sum of the ($\gamma ,n$) and ($\gamma ,2n$) cross sections is a double peaked giant $E1\mathrm{}$ resonance, from which the intrinsic quadrupole moment was determined to be 7.0±1.1 b. Additional structure in the ${\mathrm{Tb}}^{159}(\gamma ,n)$ cross section was observed at 10.1 MeV. The sum of the ($\gamma ,n$) and ($\gamma ,2n$) cross sections of ${\mathrm{Tb}}^{159}$, integrated to 28 MeV, is 2.3±0.2 MeV-b.

• Received 7 October 1963

DOI:https://doi.org/10.1103/PhysRev.133.B869