Mass-Energy Relations in the Fission of Highly Excited Heavy Nuclei

Mass-energy relations in the fission of the compound nucleus Fm²⁵⁴ have been studied at excitation energies in the range from 58 to 116 MeV. This compound nucleus was produced by bombardments of Th²³², U²³⁸, and Pu²⁴² with the heavy ions Ne²², O¹⁶, and C¹², respectively. The kinetic energies of the two fission fragments from each event were measured by using semiconductor detectors. From these energies the masses and total kinetic energies were calculated.The mass distributions were studied as a function of the total kinetic energy. The bell-shaped mass distributions narrowed rapidly as the kinetic energies of the fragments increased. The quantitative behavior of this narrowing suggests that most of the initial excitation energy of the compound nucleus is not available either for conversion into kinetic energy of the fragments or for the production of asymmetric mass divisions that are energetically unfavorable.