Energy levels of ²⁴⁹Bk populated in the α decay of ₉₉²⁵³Es and β^- decay of ₉₆²⁴⁹Cm

The level structure of ²⁴⁹Bk has been investigated by measuring the γ-ray spectra of an extremely pure ²⁵³Es sample obtained by milking this nuclide from ²⁵³Cf source material produced in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Additional information on the ²⁴⁹Bk levels was obtained from the β^--decay study of ²⁴⁹Cm, produced by neutron irradiation of ²⁴⁸Cm. Using the results of the present study together with the data from previous ²⁴⁸Cm(α,t) and ²⁴⁸Cm(³He,d) reactions, the following single-particle states have been identified in ²⁴⁹Bk: 7/2⁺[633], 0.0 keV; 3/2^-[521], 8.78 keV; 1/2⁺[400], 377.55 keV; 5/2⁺[642], 389.17 keV; 1/2^-[530], 569.20 keV; 1/2^-[521], 643.0 keV; 5/2^-[523], 672.9 keV; and 9/2⁺[624], 1075.1 keV. Four vibrational bands were identified at 767.9, 932.2, 1150.7, and 1223.0 keV with tentative assignments of {7/2⁺ [633] ⊗1^-}9/2^-, {7/2⁺ [633] ⊗ 0^-}7/2^-, {7/2⁺ [633] ⊗ 1^-}5/2^-, and {7/2⁺ [633] ⊗ 0⁺}7/2⁺, respectively. A band at 899.9 keV was observed in γ-γ coincidence measurements and given a tentative spin assignment of 3/2. It is possibly associated with a 2^- phonon coupled to the ground state, with configuration {7/2⁺ [633] ⊗ 2^-}3/2^-. Three levels at 624.3, 703.5, and 769.1 keV were assigned spins of 5/2, 7/2, and 9/2, respectively. These could be the members of the 3/2⁺ [651] band, expected in this energy region.