Excitation functions of fission products for 22 fragment masses and neutron-evaporation products, ($p$,$n$), ($p$,$2n$) and ($p$,$3n$) were measured in detail for the system of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$+$p$ in the proton energy range of 8 to 22 MeV. It was found that the excitation functions of symmetrically divided fission fragments were apparently different from those of asymmetrically divided ones. For the interpretation of the experimental results, a statistical evaporation-and-fission calculation was performed by taking into consideration symmetric and asymmetric fission barriers. The experimental data could be reproduced by this statistical calculation, with fission barrier heights that were in accord with the reported experimental ones for the asymmetric fragments and the theoretically predicted one for the symmetric mass division. The level density parameter at the symmetric saddle had to be about 13% larger than that for the asymmetric saddle in order to reproduce the energy dependence of the cross section ratios of asymmetric to symmetric product yields.

NUCLEAR REACTIONS, FISSION ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$($p$,$f$), $E_p=8-22\mathrm{}$ MeV, stacked-foil method, excitation functions of fission products and neutron-evaporation products, statistical calculation, symmetric and asymmetric fission barriers.

• Received 19 May 1981

DOI:https://doi.org/10.1103/PhysRevC.25.3011