Unusually low fragment energies in the symmetric fission ofMd259

Abstract

The 103-min isotope $^{259}\mathrm{Md}$ has been identified as the daughter of an electron-capture decay branch of $^{259}\mathrm{No}$ produced via the $^{248}\mathrm{Cm}(^{18}\mathrm{O},\ensuremath{\alpha} 3n)$ reaction. Chemical separations were used to confirm the identity of $^{259}\mathrm{Md}$, 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 $^{258}\mathrm{Fm}$ and $^{259}\mathrm{Fm}$. However, the total kinetic energy distribution for $^{259}\mathrm{Md}$ is considerably broader (FWHM \ensuremath{\sim}60 MeV) than those of $^{258}\mathrm{Fm}$ and $^{259}\mathrm{Fm}$, 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 $^{258}\mathrm{Fm}$ and $^{259}\mathrm{Fm}$. A hypothesis that this energy difference resulted from the emission of light, hydrogen-like particles at scission in a large fraction of $^{259}\mathrm{Md}$ 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.RADIOACTIVITY, FISSION $^{259}\mathrm{Md}$ (SF); measured ${T}_{\frac{1}{2}}$, fragment-fragment coin, deduced mass, TKE distributions. $^{259}\mathrm{No}$; measured EC decay to $^{259}\mathrm{Md}$, upper limit to SF decay.