Collective States in theN=82NuclidePr141Excited by the (α,α′) Reaction at 45 MeV

The ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pr}(\alpha ,{\alpha }^{\prime })$ reaction at $E_{\alpha }=45.0\mathrm{}$ MeV and $\Delta E_{{\alpha }^{\prime }}\approx 35$ keV was studied to investigate the particle-vibration interaction in this $N=82\mathrm{}$ nuclide and to elucidate the ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pr}$ level structure in the 0-4-MeV region. The $\alpha$-particle angular distributions were measured in ≤2° steps over ranges of 7-80° for elastic scattering and 12-66° for inelastic scattering. A ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pr}(d,d^{\prime })$ spectrum at $E_d=28.7\mathrm{}$ MeV and with higher resolution (20 keV) was also recorded. These measurements, in combination with other high-resolution studies, establish the existence of approximately 50 levels with excitation energies of 1.1-3.0 MeV. Approximately 20 of these levels can be identified in the ($\alpha ,{\alpha }^{\prime }$) spectra and are interpreted as states with significant collective components. Although most of these levels are in either a positive-parity group between 1.1 and 1.9 MeV or a negative-parity group between 2.0 and 2.5 MeV, it is found that the assumption of a weak particle-vibration interaction based on the $2^{+}$ and $3^{-}$ core states of ${}_{}{}^{140}{}_{}{}^{}\mathrm{Ce}$ and the $d_{\frac{5}{2}}$ ground state of ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pr}$ is inadequate to explain the results. Qualitative agreement is found with the results of recent shell-model studies which assumed nine active protons outside an inert $Z=50\mathrm{}$, $N=82\mathrm{}$ core. In addition, some evidence is found for the approximate validity of the $\Delta B=0\mathrm{}$ selection rule for inelastic scattering transitions which was recently proposed by Hecht and Adler.