The $\alpha$-decay scheme of ${}_{}{}^{251}{}_{}{}^{}\mathrm{Fm}$ has been investigated by measuring the $\alpha$-particle, $\gamma$-ray, and conversion-electron spectra of mass-separated ${}_{}{}^{251}{}_{}{}^{}\mathrm{Fm}$ samples. The $\alpha$ spectrum was measured with the Argonne double-focusing magnetic spectrometer and also with a semiconductor detector. 13 $\alpha$ groups were observed with energies and intensities 7.305 (1.5%), 7.251 (0.93%), 7.184 (0.29%), 7.106 (∼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 $\gamma$ rays associated with the $\alpha$ decay of ${}_{}{}^{251}{}_{}{}^{}\mathrm{Fm}$ were identified in an $\alpha -\gamma$ coincidence experiment. The multipolarities of prominent transitions were deduced from an $\alpha -e^{-}$ coincidence experiment. The half-life of ${}_{}{}^{251}{}_{}{}^{}\mathrm{Fm}$ was measured from the decay of $\gamma$ rays and found to be 5.30 ± 0.08 h. The $\alpha$ branching was determined from the measured (Es $K$ x rays)/$\alpha$ ratio and was found to be 1.9 ± 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 ${\frac{7}{2}}^{+}\left[624\mathrm{}\downarrow \right]$, ${\frac{5}{2}}^{+}\left[622\mathrm{}\uparrow \right]$, ${\frac{9}{2}}^{-}\left[734\mathrm{}\uparrow \right]$, and ${\frac{7}{2}}^{-}\left[743\mathrm{}\uparrow \right]$, respectively. On the basis of the favored $\alpha$ transition the ground state of ${}_{}{}^{251}{}_{}{}^{}\mathrm{Fm}$ has been assigned to the ${\frac{9}{2}}^{-}\left[734\mathrm{}\uparrow \right]$ single-particle state.

• Received 16 February 1973

DOI:https://doi.org/10.1103/PhysRevC.8.737