The $\gamma$-ray spectrum of 14.1-h ${}_{}{}^{240}{}_{}{}^{}\mathrm{U}$ was obtained almost free of interference by the much more intense $\gamma$ radiation from its ${}_{}{}^{240}{}_{}{}^{}\mathrm{Np}_{}^m{}_{}{}^{}$ daughter by means of continuous radiochemical separations. Thus 16 new transitions (49.1-299.8 keV) were revealed in addition to the single one at 44.1 keV previously identified, and a tentative decay scheme for ${}_{}{}^{240}{}_{}{}^{}\mathrm{U}$ is proposed. A detailed study of separated ${}_{}{}^{240}{}_{}{}^{}\mathrm{Np}_{}^m{}_{}{}^{}$ showed $70\gamma$ transitions whose $E_{\gamma }$ and $I_{\gamma }$ values agree well with previous data, and in addition 29 new transitions were found. The absolute intensity of the 554.6-keV $\gamma$-ray transition is (20.6±0.5)%, the ${}_{}{}^{240}{}_{}{}^{}\mathrm{Np}_{}^m{}_{}{}^{}$ half-life is 7.22±0.02 min, and the fraction decaying by isomeric transition is (0.12±0.01)%. A decay scheme of ${}_{}{}^{240}{}_{}{}^{}\mathrm{Np}_{}^m{}_{}{}^{}$ is proposed which shows several new levels in ${}_{}{}^{240}{}_{}{}^{}\mathrm{Pu}$ and some changes in previous $I^{\pi }$ and $K^{\pi }$ assignments. The isomeric state is assigned $I^{\pi }=1^{\mathrm{}(+)\mathrm{}}$, $K^{\pi }=0^{\mathrm{}(+)\mathrm{}}$ with the probable configuration ${\frac{7}{2}}^{+}{\mathrm{}\left[633\right]\mathrm{}}_p$, ${\frac{7}{2}}^{+}{\mathrm{}\left[624\right]\mathrm{}}_n$.

RADIOACTIVITY ${}_{}{}^{240}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{240}{}_{}{}^{}\mathrm{Np}_{}^m{}_{}{}^{}$ from ${}_{}{}^{244}{}_{}{}^{}\mathrm{Pu}\stackrel{\alpha }{\to }{}_{}{}^{240}{}_{}{}^{}\mathrm{U}\stackrel{{\beta }^{-}}{\to }{}_{}{}^{240}{}_{}{}^{}\mathrm{Np}_{}^m{}_{}{}^{}$; radiochemistry, Ge(Li) and $4\pi$ gas proportional detectors. Measured $T_{\frac{1}{2}}$, $E_{\gamma }$, $I_{\gamma }$; deduced $I_{\beta }$, $log\mathrm{ft}$, decay schemes, $I$, $\pi$, Nilsson assignments.

• Received 25 July 1980

DOI:https://doi.org/10.1103/PhysRevC.23.1217