Energy levels of 251Cf populated in the α decay of 255100Fm and EC decay of 25199Es -: art. no. 054308

Gamma-ray singles spectra of extremely pure (chemically and isotopically) samples of Fm-255, with strengths of similar to 1 mCi, have been measured with a high-resolution 2-cm(2)x10-mm germanium LEPS detector and with a 25% Ge spectrometer. Gamma rays with intensities as low as 1.0x10(-6)% per Fm-255 alpha decay have been identified. The electron spectrum of a mass-separated Es-251 source was measured with a cooled Si(Li) electron spectrometer. The spectrum provided the conversion coefficients of low-energy transitions in Cf-251 and thereby their multipolarities. The present measurements confirm the previous assignments of single-particle states in Cf-251. These include 1/2(+)[620], 0.0 keV; 7/2(+)[613], 106.30 keV; 3/2(+)[622], 177.59 keV; 11/2(-)[725], 370.47 keV; 9/2(-)[734], 433.91 keV; 5/2(+)[622], 543.98 keV; 1/2(-)[750], 632.0 keV; 9/2(+)[615], 683 keV; and 9/2(+)[604], 974.0 keV. A vibrational band was identified in previous studies at 981.4 keV and given an assignment of {7/2(+)[613]circle times 2(-)}3/2(-). Three new vibrational bands are identified in the present work at 942.5, 1086.5, and 1250.0 keV with tentative assignments {7/2(+)[613]circle times 1(-)}5/2(-),{7/2(+)[613]circle times 1(-)}9/2(-), and {7/2(+)[613]circle times 0(+)}7/2(+), respectively. A level was identified at 1185.5 keV with spin of 5/2 or 7/2 but it was not given any configuration assignment. Another level was identified at 1077.5 keV and given a spin of 9/2. Again, no configuration could be assigned to this level.