Three-Quasiparticle States inLa139

Abstract

Absolute differential cross sections of transitions to levels in the $N=82$ nucleus $^{139}\mathrm{La}$ populated by the reaction $^{138}\mathrm{La}(d,p)^{139}\mathrm{La}$ at 12-MeV bombarding energy have been measured with a magnetic spectrograph. The $Q$ value of the reaction is 6553 \ifmmode\pm\else\textpm\fi{} 3 keV. The reaction strongly populates the one-quasiproton ground state of $^{139}\mathrm{La}$ by $l=2$ neutron transfer, which establishes positive parity for the $^{138}\mathrm{La}$ ground state. Of the many known states in the region from 1200 to 2000 keV in $^{139}\mathrm{La}$, only the 1536.3-keV state is weakly populated. The reaction strongly populates a high density of three-quasiparticle states in $^{139}\mathrm{La}$, beginning at 3355-keV excitation energy, by $l=3$ neutron transfer. The shell-model multiplet structure of $^{139}\mathrm{La}$ has been calculated using a modified surface $\ensuremath{\delta}$ interaction within a configuration space having ($1{g}_{\frac{7}{2}}$) and ($2{d}_{\frac{5}{2}}$) active proton orbitals and ($3{s}_{\frac{1}{2}}$), ($2{d}_{\frac{3}{2}}$), ($2{f}_{\frac{7}{2}}$), and ($3{p}_{\frac{3}{2}}$) active neutron orbitals. Comparisons between experimental and theoretical multiplet structure are presented.