One- and three-quasiparticle states inCf249populated by the electron capture decay of 1.70-hEs249

Abstract

The level structure of $^{249}\mathrm{Cf}$ has been investigated by measuring the $\ensuremath{\gamma}$ rays and conversion electrons associated with the electron capture decay of 1.70-h $^{249}\mathrm{Es}$. The $\ensuremath{\gamma}$-ray spectra were measured with a 2-${\mathrm{cm}}^{2}$\ifmmode\times\else\texttimes\fi{}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 $\ensuremath{\gamma}$ 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 \ifmmode\pm\else\textpm\fi{} 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: $\frac{9}{2}\ensuremath{-}[734]$, 0; $\frac{5}{2}+[622]$, 145.0; $\frac{7}{2}+[624]$, 379.5; $\frac{7}{2}+[613]$, 443.0; $K\ensuremath{\pi}=\frac{5}{2}\ensuremath{-}$, 813.2; $\frac{9}{2}+[615]$, 1007.9; and $K\ensuremath{\pi}=\frac{7}{2}\ensuremath{-}$, 1218.5. The $\frac{5}{2}\ensuremath{-}$ band at 813.2 keV has been interpreted as the $\ensuremath{\gamma}$-vibrational band coupled to the $\frac{9}{2}\ensuremath{-}[734]$ ground state. The $\frac{7}{2}\ensuremath{-}$ state at 1218.5 keV has been interpreted as a three-quasiparticle state with configuration ${{p[633]\frac{7}{2} + ; p[521] \frac{3}{2} \ensuremath{-}; n[622] \frac{3}{2} +}}_{\frac{7}{2}\ensuremath{-}}$. This is the first time a three-quasiparticle state in an actinide nucleus has been identified. An enhancement in the $B(E3)$ value of the 145.0\ensuremath{\rightarrow} 0 transition has been observed which suggests collective admixture in the 145.0-keV state. From the experimental $B(E3)$ value the admixture (amplitude squared) of the ${{\frac{9}{2}\ensuremath{-}[734]\ensuremath{\bigotimes}{2}^{\ensuremath{-}}}}_{\frac{5}{2}+}$ configuration in the 145.0-keV state has been derived to be \ensuremath{\sim} 29%, in excellent agreement with the results of ($d,{d}^{\ensuremath{'}}$) reaction studies.RADIOACTIVITY $^{249}\mathrm{Es}$ [from $^{249}\mathrm{Cf}(d,2n)$ and $^{249}\mathrm{Bk}(\ensuremath{\alpha},4n)$]; measured ${T}_{\frac{1}{2}}$, ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$, ${E}_{\mathrm{ce}}$, ${I}_{\mathrm{ce}}$. $^{249}\mathrm{Cf}$ deduced levels, $log\mathrm{ft} (\mathrm{EC})$, $\ensuremath{\gamma}$ multipolarity, $I$, $\ensuremath{\pi}$, 5 single-particle states, one vibrational state, one three-quasiparticle state. Mass-separated $^{249}\mathrm{Es}$.