Mass and Energy Distributions in the Spontaneous Fission of Some Heavy Isotopes

A back-to-back semiconductor counter system was used to study the energy and mass distributions of the fission fragments in the spontaneous fission of the isotopes, ${\mathrm{Fm}}^{254}$, ${\mathrm{E}}^{253}$, ${\mathrm{Cf}}^{254}$, and ${\mathrm{Cm}}^{248}$. The results are compared to the fission fragments produced by the spontaneous fission of ${\mathrm{Cf}}^{252}$, whose fission properties are well known. All distributions (including those of the odd-mass isotope ${\mathrm{E}}^{253}$) are rather similar, but not identical with the standard ${\mathrm{Cf}}^{252}$. The mean prompt kinetic energy of the fragments increases with $Z$, and this trend is compared with the results of other related experiments. The asymmetry of the mass distributions shows only small differences between the isotopes studied here. Some theoretical aspects of the asymmetries are discussed. The variance (widths) of nearly all the distributions increases with $Z$ and seems to increase with $A$. A new semiempirical correlation between the variances and $\frac{Z^2}{A}$ is proposed and interpreted on a qualitative basis. The effects of neutron emission on the results are briefly discussed and the values of the mean total kinetic energy release are corrected for the effect of neutron emission.