\alpha decay of ²⁴⁹₉₇Bk and levels in ²⁴⁵₉₅Am

I. Ahmad, J. P. Greene, F. G. Kondev, S. Zhu, M. P. Carpenter, R. V. F. Janssens, R. A. Boll, J. G. Ezold, S. M. Van Cleve, and E. Browne

Accepted Tuesday Apr 30, 2013

Alpha decay of $^{249}$Bk has been investigated by measuring its $\alpha$ and $\gamma$-ray spectra, both in singles and in coincidence modes. The $\alpha$ spectrum of a freshly purified $^{249}$Bk sample was measured with a high-resolution, double-focussing magnetic spectrometer. Gamma singles, $\gamma$-$\gamma$ coincidence, and $\gamma$-$\alpha$ coincidence spectra were also recorded. The absolute intensity of the 327.45-keV $\gamma$ ray has been determined to be (1.44$\pm$0.08)$\times$10$^{-5}$\% per $^{249}$Bk decay. Assignments of previously known single-particle states were confirmed. A new rotational band was identified in the $\alpha$ singles spectrum and Am K x rays have been observed in its decay. This single-particle state, with an energy of 154 keV, has been assigned to the 3/2$^{-}$[521] Nilsson state. This is the lowest excitation energy for this orbital in any Am nucleus. More precise energies and intensities of the $^{249}$Bk $\alpha$ groups and $\gamma$-ray transitions are provided.