Unusually low fragment energies in the symmetric fission of 259Md

The 103-min isotope 259Md has been identified as the daughter of an electron-capture decay branch of 259No produced via the 248Cm(18O,α3n) reaction. Chemical separations were used to confirm the identity of 259Md, which decays by spontaneous fission. The kinetic energies of coincident fission fragments were measured, corresponding to a fragment mass which is highly symmetric, similar to those of 258Fm and 259Fm. However, the total kinetic energy distribution for 259Md is considerably broader (FWHM approx.60 MeV) than those of 258Fm and 259Fm, and peaks at 201 MeV, about 35--40 MeV lower in energy. Furthermore, the maximum total Kinetic energy of 215 MeV for mass-symmetric events is about 30 MeV lower than for similar events from the spontaneous fission of 258Fm and 259Fm. A hypothesis that this energy difference resulted from the emission of light, hydrogen-like particles at scission in a large fraction of 259Md spontaneous fission decays was shown to be unfounded. From experiments to observe such particles with counter telescopes, an upper limit of 5% was determined for the fraction of fission events accompanied by light-particle emission. The total kinetic energy deficit at mass symmetry must, therefore, be distributed between internal excitation energy and fragment deformation energy at scission. Although the presence of a large amount of fragment deformation energy seems incompatible with symmetric fission into spherical Sn-like fragments, we prefer this explanation because the low total kinetic energy suggests a lowered Coulomb energy resulting from greater separation of the charge centers of deformed fragments at scission