Decay of 14.1-hU240and 7.22-minNpm240

Abstract

The $\ensuremath{\gamma}$-ray spectrum of 14.1-h $^{240}\mathrm{U}$ was obtained almost free of interference by the much more intense $\ensuremath{\gamma}$ 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 $70\ensuremath{\gamma}$ transitions whose ${E}_{\ensuremath{\gamma}}$ and ${I}_{\ensuremath{\gamma}}$ values agree well with previous data, and in addition 29 new transitions were found. The absolute intensity of the 554.6-keV $\ensuremath{\gamma}$-ray transition is (20.6\ifmmode\pm\else\textpm\fi{}0.5)%, the $^{240}\mathrm{Np}^{m}$ half-life is 7.22\ifmmode\pm\else\textpm\fi{}0.02 min, and the fraction decaying by isomeric transition is (0.12\ifmmode\pm\else\textpm\fi{}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 ${I}^{\ensuremath{\pi}}$ and ${K}^{\ensuremath{\pi}}$ assignments. The isomeric state is assigned ${I}^{\ensuremath{\pi}}={1}^{(+)}$, ${K}^{\ensuremath{\pi}}={0}^{(+)}$ with the probable configuration ${\frac{7}{2}}^{+}{[633]}_{p}$, ${\frac{7}{2}}^{+}{[624]}_{n}$.RADIOACTIVITY $^{240}\mathrm{U}$, $^{240}\mathrm{Np}^{m}$ from $^{244}\mathrm{Pu}\stackrel{\ensuremath{\alpha}}{\ensuremath{\rightarrow}}^{240}\mathrm{U}\stackrel{{\ensuremath{\beta}}^{\ensuremath{-}}}{\ensuremath{\rightarrow}}^{240}\mathrm{Np}^{m}$; radiochemistry, Ge(Li) and $4\ensuremath{\pi}$ gas proportional detectors. Measured ${T}_{\frac{1}{2}}$, ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$; deduced ${I}_{\ensuremath{\beta}}$, $log\mathrm{ft}$, decay schemes, $I$, $\ensuremath{\pi}$, Nilsson assignments.