Origin of the narrow, single peak in the fission-fragment mass distribution for ²⁵⁸Fm

We discuss the origin of the narrowness of the single peak at mass-symmetric division in the fragment mass-yield curve for spontaneous fission of ²⁵⁸Fm. For this purpose, we employ the macroscopic-microscopic model and calculate a potential-energy curve at the mass-symmetric compact scission configuration, as a function of the fragment mass number, which is obtained from the single-particle wave-function densities. In the calculations, we minimize total energies by varying the deformations of the two fragments, with constraints on the mass quadrupole moment, and by keeping the neck radius zero. The energies thus become functions of mass asymmetry. Using the obtained potential, we solve the one-dimensional Schrödinger equation with a microscopic coordinate-dependent inertial mass to calculate the fragment mass-yield curve. The calculated mass yield, expressed in terms of the microscopic mass density, is consistent with the extremely narrow experimental mass distribution.