Energy levels of 249Bk populated in the α decay of 99253Es and β- decay of 96249Cm -: art. no. 054305

The level structure of Bk-249 has been investigated by measuring the gamma-ray spectra of an extremely pure Es-253 sample obtained by milking this nuclide from Cf-253 source material produced in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Additional information on the Bk-249 levels was obtained from the beta(-)-decay study of Cm-249, produced by neutron irradiation of Cm-248. Using the results of the present study together with the data from previous Cm-248(alpha,t) and Cm-248(He-3,d) reactions, the following single-particle states have been identified in Bk-249: 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] circle times 1(-)}9/2(-), {7/2(+) [633] circle times 0(-)}7/2(-), {7/2(+) [633] circle times 1(-)}5/2(-), and {7/2(+) [633] circle times 0(+)}7/2(+), respectively. A band at 899.9 keV was observed in gamma-gamma 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] circle times 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.