The $\gamma$-singles and conversion-electron spectra of ${\mathrm{Fm}}^{255}$ were measured with a Ge(Li) diode and the Argonne double toroidal $\beta$-ray spectrometer, respectively. In the $\gamma$-ray spectra, in addition to the transitions expected from previously known decay schemes, $\gamma$ rays of energies 63.8, 131.8, 204.1, and 264.1 keV were observed. Two new $\alpha$ groups, ${\alpha }_{368}$ (6.765 MeV) and ${\alpha }_{433}$ (6.701 MeV), were identified in coincidence with 131.0- and 204.1-keV $\gamma$ rays. A two-parameter $\gamma -\gamma$ coincidence experiment showed that the 204.1- and 131.0-keV transitions populate the $\frac{9}{2}$ and $\frac{11}{2}$ members of the favored band. Conversion-electron studies and $\alpha -\mathrm{v}\mathrm{s}-\gamma$ intensity comparisons demonstrate their $E1\mathrm{}$ character. The half-life of the 370.4-keV level was measured by a delayed $\alpha -\gamma$ coincidence method and found to be 1.3 ± 0.1 μsec. On the basis of these observations and the $\alpha$ intensities to these levels, the 370.4- and 434.2-keV levels have been assigned to the $\frac{11}{2}-\left(725\mathrm{}\uparrow \right)$ and $\frac{9}{2}-\left(734\mathrm{}\uparrow \right)$ Nilsson states, respectively. A three-parameter $\alpha -\gamma$-time coincidence experiment indicates that the $\alpha$ intensity to the 105.73-keV ($I=\frac{7}{2},K=\frac{1}{2}$) level is less than 1%, implying that its 18% population comes primarily via a 0.57-keV transition from the 106.30-keV, $I=\frac{7}{2}$, $K=\frac{7}{2}$ level. The $\alpha$ transition probabilities to various bands are in agreement with the values expected from $\alpha$-decay systematics and theoretical calculations. The $K$-, $L$-, $M$-, and $N$-sunshell atomic electron binding energies in Cf ($Z=98\mathrm{}$), obtained experimentally by least-squares adjustment from the conversion-line data, show significant deviations below recent (Bearden and Burr) tabulated values.

• Received 18 August 1970

DOI:https://doi.org/10.1103/PhysRevC.3.390