Systematic analysis of mass yield curves in low-energy fission of actinides

Mass yield curves in low-energy proton-induced fissions of ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$, ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$, ${}_{}{}^{242}{}_{}{}^{}\mathrm{Pu}$, ${}_{}{}^{244}{}_{}{}^{}\mathrm{Pu}$, ${}_{}{}^{241}{}_{}{}^{}\mathrm{Am}$, and ${}_{}{}^{243}{}_{}{}^{}\mathrm{Am}$ were measured. The full widths at half maximum (FWHM) of the heavier asymmetric peak of the mass yield curves showed a sudden dip in the region of $A_f$=240–245. The correlation between the P/V ratio and the FWHM of the asymmetric mass yield peak was obtained as a function of proton energy. The FWHM values were independent of the incident proton energy as long as the P/V ratio was larger than 10 for all the fissioning systems studied. The excitation energy dependence of the mass yield curves was investigated. It was found that the excitation energy dependence of symmetrically divided fission products was apparently different from that of asymmetrically divided ones. With a simple assumption of the two-mode hypothesis, the observed mass yield curves could be decomposed into two mass yield curves. Asymmetric mass yield peaks could be best represented by the sum of two Gaussians, and their variation could be expressed by the variation of Gaussian parameters. The peak positions of the two Gaussians were always A=133–135 and A=140–142 for all the investigated fissioning nuclides. The preference of those fragment masses is discussed in terms of both the scission-point model and the saddle-point model.