$\gamma -\gamma$ cascades following thermal neutron capture in ${\mathrm{Hg}}^{109}$ have been measured. A three-crystal pair spectrometer was used to measure the high-energy gamma rays and a single NaI(Tl) crystal spectrometer detected the lower energy radiation. A detailed comparison is made between the experiment reported here, the previously known high-resolution mercury capture gamma-ray spectrum, and the previously measured decay scheme of ${\mathrm{Tl}}^{200}$. Several discrepancies between the capture gamma ray and $\beta$-decay work are resolved, though others remain. By combining the results of the three experiments, probable spin and parity assignments are made for several of the low-lying levels in ${\mathrm{Hg}}^{200}$. With these spin assignments, some conclusions are drawn as to the relative transition probabilities for the various multipolarity radiations present.

• Received 28 May 1958

DOI:https://doi.org/10.1103/PhysRev.111.1620