The level structure of ${}_{}{}^{249}{}_{}{}^{}\mathrm{Cf}$ has been investigated by measuring the $\gamma$ rays and conversion electrons associated with the electron capture decay of 1.70-h ${}_{}{}^{249}{}_{}{}^{}\mathrm{Es}$. The $\gamma$-ray spectra were measured with a 2-${\mathrm{cm}}^2$×5-mm planar and a 25-${\mathrm{cm}}^3$ coaxial Ge(Li) detector and the conversion-electron spectra were measured with a cooled Si(Li) detector. Forty-six $\gamma$ rays were identified and multipolarities of most of the transitions were deduced. The half-life of ${}_{}{}^{249}{}_{}{}^{}\mathrm{Es}$ was determined from the decay rate of the 379.5-keV photopeak and was found to be 102.2 ± 0.6 min. The $log\mathrm{ft}$ values for the observed electron capture transitions were derived from the measured electron capture intensities. On the basis of the present work the following intrinsic states were identified: $\frac{9}{2}-\left[734\right]\mathrm{}$, 0; $\frac{5}{2}+\left[622\right]\mathrm{}$, 145.0; $\frac{7}{2}+\left[624\right]\mathrm{}$, 379.5; $\frac{7}{2}+\left[613\right]\mathrm{}$, 443.0; $K\pi =\frac{5}{2}-$, 813.2; $\frac{9}{2}+\left[615\right]\mathrm{}$, 1007.9; and $K\pi =\frac{7}{2}-$, 1218.5. The $\frac{5}{2}-$ band at 813.2 keV has been interpreted as the $\gamma$-vibrational band coupled to the $\frac{9}{2}-\left[734\right]\mathrm{}$ ground state. The $\frac{7}{2}-$ state at 1218.5 keV has been interpreted as a three-quasiparticle state with configuration ${\left\{p\mathrm{}\right[633]\mathrm{}\frac{7}{2} + ; p\mathrm{}[521]\mathrm{} \frac{3}{2} -; n\mathrm{}[622]\mathrm{} \frac{3}{2} +\}}_{\frac{7}{2}-}$. This is the first time a three-quasiparticle state in an actinide nucleus has been identified. An enhancement in the $B\left(E3\right)$ value of the 145.0→ 0 transition has been observed which suggests collective admixture in the 145.0-keV state. From the experimental $B\left(E3\right)$ value the admixture (amplitude squared) of the ${\{\frac{9}{2}-[734]\mathrm{}\otimes 2^{-}\}}_{\frac{5}{2}+}$ configuration in the 145.0-keV state has been derived to be ∼ 29%, in excellent agreement with the results of ($d,d^{\prime }$) reaction studies.

RADIOACTIVITY ${}_{}{}^{249}{}_{}{}^{}\mathrm{Es}$ [from ${}_{}{}^{249}{}_{}{}^{}\mathrm{Cf}(d,2n)$ and ${}_{}{}^{249}{}_{}{}^{}\mathrm{Bk}(\alpha ,4n)$]; measured $T_{\frac{1}{2}}$, $E_{\gamma }$, $I_{\gamma }$, $E_{\mathrm{ce}}$, $I_{\mathrm{ce}}$. ${}_{}{}^{249}{}_{}{}^{}\mathrm{Cf}$ deduced levels, $log\mathrm{ft} \left(\mathrm{EC}\right)$, $\gamma$ multipolarity, $I$, $\pi$, 5 single-particle states, one vibrational state, one three-quasiparticle state. Mass-separated ${}_{}{}^{249}{}_{}{}^{}\mathrm{Es}$.

• Received 17 February 1976

DOI:https://doi.org/10.1103/PhysRevC.14.218