Alpha-Decay Of Fm-251

The $\ensuremath{\alpha}$-decay scheme of $^{251}\mathrm{Fm}$ has been investigated by measuring the $\ensuremath{\alpha}$-particle, $\ensuremath{\gamma}$-ray, and conversion-electron spectra of mass-separated $^{251}\mathrm{Fm}$ samples. The $\ensuremath{\alpha}$ spectrum was measured with the Argonne double-focusing magnetic spectrometer and also with a semiconductor detector. 13 $\ensuremath{\alpha}$ groups were observed with energies and intensities 7.305 (1.5%), 7.251 (0.93%), 7.184 (0.29%), 7.106 (\ensuremath{\sim}0.05%), 6.928 (1.8%), 6.885 (1.7%), 6.833 (87.0%), 6.782 (4.8%), 6.762 (0.38%), 6.720 (0.44%), 6.681 (0.07%), 6.638 (0.56%), and 6.579 (0.26%). The $\ensuremath{\gamma}$ rays associated with the $\ensuremath{\alpha}$ decay of $^{251}\mathrm{Fm}$ were identified in an $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\gamma}$ coincidence experiment. The multipolarities of prominent transitions were deduced from an $\ensuremath{\alpha}\ensuremath{-}{e}^{\ensuremath{-}}$ coincidence experiment. The half-life of $^{251}\mathrm{Fm}$ was measured from the decay of $\ensuremath{\gamma}$ rays and found to be 5.30 \ifmmode\pm\else\textpm\fi{} 0.08 h. The $\ensuremath{\alpha}$ branching was determined from the measured (Es $K$ x rays)/$\ensuremath{\alpha}$ ratio and was found to be 1.9 \ifmmode\pm\else\textpm\fi{} 0.2%. From these experiments four rotational bands in $^{247}\mathrm{Cf}$ were identified. The ground state and the levels at 383.2, 480.4, and 678.0 keV in $^{247}\mathrm{Cf}$ were given Nilsson-state assignments of ${\frac{7}{2}}^{+}[624\ensuremath{\downarrow}]$, ${\frac{5}{2}}^{+}[622\ensuremath{\uparrow}]$, ${\frac{9}{2}}^{\ensuremath{-}}[734\ensuremath{\uparrow}]$, and ${\frac{7}{2}}^{\ensuremath{-}}[743\ensuremath{\uparrow}]$, respectively. On the basis of the favored $\ensuremath{\alpha}$ transition the ground state of $^{251}\mathrm{Fm}$ has been assigned to the ${\frac{9}{2}}^{\ensuremath{-}}[734\ensuremath{\uparrow}]$ single-particle state.