Unusually low fragment energies in the symmetric fission ofMd259

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Abstract
The 103-min isotope Md259 has been identified as the daughter of an electron-capture decay branch of No259 produced via the Cm248(O18,α 3n) reaction. Chemical separations were used to confirm the identity of Md259, 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 Fm258 and Fm259. However, the total kinetic energy distribution for Md259 is considerably broader (FWHM ∼60 MeV) than those of Fm258 and Fm259, 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 Fm258 and Fm259. A hypothesis that this energy difference resulted from the emission of light, hydrogen-like particles at scission in a large fraction of Md259 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.