Combined statistical and dynamical model of ternary nuclear fission

The statistical theory of particle evaporation from hot compound nuclei can be used to calculate the probability that particles are evaporated from the nuclear surface with not enough energy to surmount the Coulomb barrier. These quasievaporated particles exist between the nuclear surface and the Coulomb barrier for a short period of time before returning to the nuclear fluid. Occasionally, a quasievaporated charged particle emitted into the region surrounding the pre-scission neck material, fails to be reabsorbed by either of the main fragments as they accelerate away from each other after scission. This new particle emission mechanism can be used to explain many of the properties of ternary nuclear fission.

Figure 1

Ternary-fission probabilities for ${}^{235}\mathrm{U}\left(n_{\mathrm{th},}f\right)$ [4].

Figure 2

A scission configuration. The thicker curve shows the shape of a nucleus just before the neck snaps. The semicircles show the location of spherical fragments immediately following scission, assuming an instantaneous retraction of the neck stubs. The small solid circles highlight locations on the nuclear surface that do not undergo high acceleration during the collapse of the neck stubs. The dashed parabola represents the surface beyond which particles fail to be reabsorbed by the retracting neck stubs.

Figure 3

The values for ${\Delta }_p$ needed to reproduce ${}^{235}\mathrm{U}\left(n_{\mathrm{th},}f\right)$ ternary-fission probabilities [4]. The line is only to guide the eye.

Figure 4

Measured [26] and calculated (solid curves) energy spectra for ${}^4\mathrm{He}$, ${}^6\mathrm{He}$, and ${}^3\mathrm{H}$ in ${}^{235}\mathrm{U}\left(n_{\mathrm{th},}f\right)$ ternary fission. The disagreement on the low-energy side of the ${}^4\mathrm{He}$ spectrum is due to the breakup of ${}^5\mathrm{He}$ emission into ${}^4\mathrm{He}$ plus a neutron. The data and curves are normalized to have the same maximum height.

Figure 5

Measured [4] and calculated (solid curves) mean energies (a), and the full width at half maximum (FWHM) of the energy spectra (b) of the light particles in ${}^{235}\mathrm{U}\left(n_{\mathrm{th},}f\right)$ ternary fission.