Proton states in theZ=99nucleusEs251excited byFm251electron capture decay andCf250(α,t)reaction

Abstract

Level structure of the $Z=99$ nucleus, $^{251}\mathrm{Es}$, has been investigated by measuring the $\ensuremath{\gamma}$ rays and conversion electrons associated with the electron capture decay of 5.3-h $^{251}\mathrm{Fm}$, and from triton spectra produced in the $^{250}\mathrm{Cf}(\ensuremath{\alpha},t)^{251}\mathrm{Es}$ reaction. 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 electron spectra were measured with a cooled Si(Li) detector. Fortyfive $\ensuremath{\gamma}$ 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 $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence experiment was performed to determine the relationships between various $\ensuremath{\gamma}$ transitions. Triton spectra from the $^{250}\mathrm{Cf}(\ensuremath{\alpha},t)^{251}\mathrm{Es}$ 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; ${{7/2\phantom{\rule{0ex}{0ex}}+\phantom{\rule{0ex}{0ex}}[633]\phantom{\rule{0ex}{0ex}}\ensuremath{\bigotimes}\phantom{\rule{0ex}{0ex}}2\phantom{\rule{0ex}{0ex}}+}}_{11/2+}$ vibrational state, 889.1; ${{n[734]\frac{9}{2}\ensuremath{-}; n[620]\frac{1}{2}+; p[521]\frac{3}{2}\ensuremath{-}}}_{\frac{11}{2}+}$ three-quasiparticle state, 1239.0 keV. A $K\ensuremath{\pi}=\frac{11}{2}+$ 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 $^{249}\mathrm{Cf}(\ensuremath{\alpha},2n)$]; measured ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$, ${E}_{\mathrm{ce}}$, ${I}_{\mathrm{ce}}$, $\ensuremath{\gamma}\ensuremath{\gamma}$ coin, $\ensuremath{\alpha}$ branching.NUCLEAR REACTIONS $^{250}\mathrm{Cf}(\ensuremath{\alpha},t)$, $E=28.0$ MeV; measured $E$ (excitation), $\ensuremath{\sigma}$. $^{251}\mathrm{Es}$ deduced levels, $logft(\mathrm{EC})$, $\ensuremath{\gamma}$ multipolarity, $I$, $\ensuremath{\pi}$, five single-particle states, one vibrational state, and two three-quasiparticle states. Mass-separated $^{251}\mathrm{Fm}$ and $^{250}\mathrm{Cf}$.