Spin-isovector giant resonances induced by(n,p)reactions on heavy nuclei

Abstract

Double differential cross sections from the ${}^{120}\mathrm{Sn}{(n,p)}^{120}\mathrm{In}$ and ${}^{181}\mathrm{Ta}{(n,p)}^{181}\mathrm{Hf}$ reactions at 298 MeV and the ${}^{238}\mathrm{U}{(n,p)}^{238}\mathrm{Pa}$ reaction at 318 MeV have been measured for excitation energies up to 50 MeV in the residual nucleus. These data, together with the previously published data from the ${}^{90}\mathrm{Zr}{(n,p)}^{90}\mathrm{Y}$ and ${}^{208}\mathrm{Pb}{(n,p)}^{208}\mathrm{Tl}$ reactions at 198 MeV, have been analyzed for spin-isovector resonances of multipolarities less than 7, using the multipole decomposition method. The strengths due to spin-isovector excitations of multipolarity less than 4 have been extracted. The anomalous behavior of the extracted spin-isovector quadrupole strength with target mass number is discussed with reference to the calculations of Leonardi et al. The cross section due to quasifree processes was calculated and subtracted from the data. The data after this subtraction were reanalyzed for spin-isovector resonances and the strengths due to multipolarities up to 3 were extracted. The strengths due to spin-isovector dipole and octupole excitations were compared to values calculated, for 1\ensuremath{\Elzxh}\ensuremath{\omega} transitions only, using the sum rules of Macfarlane. The behavior with target mass number is well represented by these sum rules.