Energy levels of ²⁵¹Cf populated in the α decay of ₁₀₀²⁵⁵Fm and EC decay of ₉₉²⁵¹Es

Gamma-ray singles spectra of extremely pure (chemically and isotopically) samples of ²⁵⁵Fm, 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 ²⁵⁵Fm α decay have been identified. The electron spectrum of a mass-separated ²⁵¹Es source was measured with a cooled Si(Li) electron spectrometer. The spectrum provided the conversion coefficients of low-energy transitions in ²⁵¹Cf and thereby their multipolarities. The present measurements confirm the previous assignments of single-particle states in ²⁵¹Cf. 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.