Energy levels ofCf251populated in the α decay of100255Fm and EC decay of99251Es

Abstract

Gamma-ray singles spectra of extremely pure (chemically and isotopically) samples of $^{255}\mathrm{Fm}$, with strengths of $~1$ mCi, have been measured with a high-resolution 2-cm${}^{2}\ifmmode\times\else\texttimes\fi{}$10-mm germanium LEPS detector and with a 25% Ge spectrometer. Gamma rays with intensities as low as $1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$% per $^{255}\mathrm{Fm}$ \ensuremath{\alpha} decay have been identified. The electron spectrum of a mass-separated $^{251}\mathrm{Es}$ source was measured with a cooled Si(Li) electron spectrometer. The spectrum provided the conversion coefficients of low-energy transitions in $^{251}\mathrm{Cf}$ and thereby their multipolarities. The present measurements confirm the previous assignments of single-particle states in $^{251}\mathrm{Cf}$. These include $1/{2}^{+}$[620], 0.0 keV; $7/{2}^{+}$[613], 106.30 keV; $3/{2}^{+}$[622], 177.59 keV; $11/{2}^{\ensuremath{-}}$[725], 370.47 keV; $9/{2}^{\ensuremath{-}}$[734], 433.91 keV; $5/{2}^{+}$[622], 543.98 keV; $1/{2}^{\ensuremath{-}}$[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 ${7/{2}^{+}[613]\ensuremath{\bigotimes}{2}^{\ensuremath{-}}}3/{2}^{\ensuremath{-}}$. Three new vibrational bands are identified in the present work at 942.5, 1086.5, and 1250.0 keV with tentative assignments ${7/{2}^{+}[613]\ensuremath{\bigotimes}{1}^{\ensuremath{-}}}5/{2}^{\ensuremath{-}},{7/{2}^{+}[613]\ensuremath{\bigotimes}{1}^{\ensuremath{-}}}9/{2}^{\ensuremath{-}}$, and ${7/{2}^{+}[613]\ensuremath{\bigotimes}{0}^{+}}7/{2}^{+}$, 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.