Level structure of the $Z=99\mathrm{}$ nucleus, ${}_{}{}^{251}{}_{}{}^{}\mathrm{Es}$, has been investigated by measuring the $\gamma$ rays and conversion electrons associated with the electron capture decay of 5.3-h ${}_{}{}^{251}{}_{}{}^{}\mathrm{Fm}$, and from triton spectra produced in the ${}_{}{}^{250}{}_{}{}^{}\mathrm{Cf}(\alpha ,t){}_{}{}^{251}{}_{}{}^{}\mathrm{Es}$ reaction. 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 electron spectra were measured with a cooled Si(Li) detector. Fortyfive $\gamma$ rays were identified in ${}_{}{}^{251}{}_{}{}^{}\mathrm{Es}$ and the multipolarities of most of the transitions were deduced. $\mathrm{Log}\mathrm{ft}$ values for the observed electron capture transitions were derived from the measured electron capture transition intensities. A two-parameter $\gamma -\gamma$ coincidence experiment was performed to determine the relationships between various $\gamma$ transitions. Triton spectra from the ${}_{}{}^{250}{}_{}{}^{}\mathrm{Cf}(\alpha ,t){}_{}{}^{251}{}_{}{}^{}\mathrm{Es}$ reaction were analyzed with an Enge split-pole magnetic spectrograph. On the basis of the present investigation the following intrinsic states were identified in ${}_{}{}^{251}{}_{}{}^{}\mathrm{Es}$: 3/2 - [521], 0; 7/2 + [633], 8.3; 1/2 - [521], 411; 7/2 - [514], 461.4; 9/2 + [624], 777.9; ${\mathrm{}\{7/2+[633]\otimes 2+\}}_{11/2+}$ vibrational state, 889.1; ${\mathrm{}\left\{n\right[734\mathrm{}]\mathrm{}\frac{9}{2}-; n[620]\mathrm{}\frac{1}{2}+; p[521]\mathrm{}\frac{3}{2}-\}}_{\frac{11}{2}+}$ three-quasiparticle state, 1239.0 keV. A $K\pi =\frac{11}{2}+$ state at 1264.9 keV was interpreted as a three-quasiparticle state but could not be given a definite assignment. This is the first time a definite identification of the proton orbitals 1/2 - [521] and 7/2 - [514] has been made in any nucleus.

RADIOACTIVITY ${}_{}{}^{251}{}_{}{}^{}\mathrm{Fm}$ [from ${}_{}{}^{249}{}_{}{}^{}\mathrm{Cf}(\alpha ,2n)$]; measured $E_{\gamma }$, $I_{\gamma }$, $E_{\mathrm{ce}}$, $I_{\mathrm{ce}}$, $\gamma \gamma$ coin, $\alpha$ branching.

NUCLEAR REACTIONS ${}_{}{}^{250}{}_{}{}^{}\mathrm{Cf}(\alpha ,t)$, $E=28.0\mathrm{}$ MeV; measured $E$ (excitation), $\sigma$. ${}_{}{}^{251}{}_{}{}^{}\mathrm{Es}$ deduced levels, $logft\left(\mathrm{EC}\right)$, $\gamma$ multipolarity, $I$, $\pi$, five single-particle states, one vibrational state, and two three-quasiparticle states. Mass-separated ${}_{}{}^{251}{}_{}{}^{}\mathrm{Fm}$ and ${}_{}{}^{250}{}_{}{}^{}\mathrm{Cf}$.

• Received 30 December 1977

DOI:https://doi.org/10.1103/PhysRevC.17.2163