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

Abstract

Mass yield curves of spontaneous fissions, thermal-neutron induced fissions, monochromatic-neutron and proton induced fissions of actinides are reviewed. Mean masses of light and heavy asymmetric mass yield peaks are listed. The subtle change of the shape of the asymmetric heavy peak is expressed in terms of the parameters of two Gaussians that fit the observed data well. One Gaussian is narrow in width and always peaks at A=133–136 while the other is broader and peaks at A=140−144. The FWHM of the heavy asymmetric peak becomes minimum for the fissioning mass A=240−245. An analysis of the projectile energy dependence of mass yield curves indicated, at least, two distinctively different behaviors, one for asymmetric products and the other for symmetric products. From the energy dependence of the peak-to-valley rations, the “extra-energy” required for symmetric mass division, or the difference in the fission barrier heights, in the nomeuclature of the two-mode hypothesis, is deduced for a wide range of fission nuclides.