Energy levels of Cf251 populated in the α decay of 100255Fm and EC decay of 99251Es

Gamma-ray singles spectra of extremely pure (chemically and isotopically) samples of Fm255, with strengths of ~1 mCi, have been measured with a high-resolution 2-cm2×10-mm germanium LEPS detector and with a 25% Ge spectrometer. Gamma rays with intensities as low as 1.0×10-6% per Fm255 α decay have been identified. The electron spectrum of a mass-separated Es251 source was measured with a cooled Si(Li) electron spectrometer. The spectrum provided the conversion coefficients of low-energy transitions in Cf251 and thereby their multipolarities. The present measurements confirm the previous assignments of single-particle states in Cf251. 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]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]1-}5/2-,{7/2+[613]1-}9/2-, and {7/2+[613]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.