A nuclear spectroscopic study of the high-spin ground state of ${}_{}{}^{240}{}_{}{}^{}\mathrm{Np}$ ($I^{\pi }=5^{+}$) showed substantial differences from earlier work. The sources, prepared by the ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}(\alpha , pn)$ and ${}_{}{}^{244}{}_{}{}^{}\mathrm{Pu}(n, p4n)$ reactions, were chemically separated and purified. A total of 34 $\gamma$ rays (139.9-1167.1 keV) were observed of which 13 are new; nine others which were reported previously are not confirmed. Absolute intensities of the $\gamma$ rays were determined with the aid of $4\pi$ beta measurements. A modified decay scheme is proposed and discussed; six levels in ${}_{}{}^{240}{}_{}{}^{}\mathrm{Pu}$ with $I^{\pi }=1^{-}, 2^{-}, 3^{-}, \dots , 6^{-}$ are assigned to a $K^{\pi }=1^{-}$ octupole vibrational band. The half-life measured with high purity sources is 61.9±0.2 min.

RADIOACTIVITY 61.9-min ${}_{}{}^{240}{}_{}{}^{}\mathrm{Np}_{}^g{}_{}{}^{}$ from ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}(\alpha , pn)$ and ${}_{}{}^{244}{}_{}{}^{}\mathrm{Pu}(n, p4n)$; radiochemistry, Ge(Li), and $4\pi \beta$ detectors. Measured $E_{\gamma }$, $I_{\gamma }$, $T_{\frac{1}{2}}$; deduced $E_{{\beta }^{-}}$, $log\mathrm{ft}$, ${}_{}{}^{240}{}_{}{}^{}\mathrm{Pu}$ level assignments and decay scheme.

• Received 7 July 1982

DOI:https://doi.org/10.1103/PhysRevC.26.2178