SPIN-ISOVECTOR GIANT-RESONANCES INDUCED BY (N,P) REACTIONS ON HEAVY-NUCLEI

Double differential cross sections from the Sn-120(n,p)In-120 and Ta-1 81(n,p)Hf-181 reactions at 298 MeV and the U-238(n,p)Pa-238 reaction a t 318 MeV have been measured for excitation energies up to 50 MeV in t he residual nucleus. These data, together with the previously publishe d data from the Zr-90(n,p)Y-90 and Pb-208(n,p)Tl-208 reactions at 198 MeV, have been analyzed for spin-isovector resonances of multipolariti es less than 7, using the multipole decomposition method. The strength s 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 referen ce to the calculations of Leonardi et al. The cross section due to qua sifree 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 s trengths due to spin-isovector dipole and octupole excitations were co mpared to values calculated, for 1 (h) over bar omega transitions only , using the sum rules of Macfarlane. The behavior with target mass num ber is well represented by these sum rules.