Inelastic Alpha-Particle Excitation in the Even Tin Isotopes

Abstract

Angular distributions were measured for elastic and inelastic alpha-particle scattering by isotopically enriched targets of ${}_{}{}^{116}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{118}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{120}{}_{}{}^{}\mathrm{Sn}$, and ${}_{}{}^{122}{}_{}{}^{}\mathrm{Sn}$ using the 40-MeV alpha beam of the NASA 60-inch cyclotron. Two states of energies close to 1.2 and 2.4 MeV were the only strong excitations observed. The inelastically scattered groups of alpha particles were analyzed using a distorted-wave Born approximation (DWBA) and the nuclear vibrational model. The results of these calculations are in good agreement with the known spins and parities: $2^{+}$ for the first excited state and $3^{-}$ for the second. DWBA calculations were also carried out for the $2^{+}$ excitations using eigenfunctions of the pairing plus $Q·Q$ potentials. The calculated cross sections show a tendency to decrease with increasing neutron number. The experimental trend is qualitatively similar but much less regular. The absolute cross sections calculated using only cloud configurations are short of experimental values by about a factor of 4 when the Gammel-Thaler 40-MeV proton-alpha interaction is used. The relation of this discrepancy to the contribution from extracloud configurations is discussed.