Ternary Fission Process
Normally, fission is a binary process, in which only two partiles (the primary fission fragments) are formed when the fissioning nucleus splits. Much less frequently, more than two particles are formed and if precisely three particles appear, the fission event is classified as a ternary event. A similar effect can be observed when a cylindrical soap-bubble like the one displayed on the main fission-page is chopped up, a small bubble may remain between two big spherical ones.
Ternary fission occurs once every few hundred fission events. Roughly speaking, about 25% more ternary fission is present in spontaneous fission compared to the same fissioning system formed after thermal neutron capture. The ternary fission emission probability seems to depend on Z and A of the fissioning system, as illustrated in fig..
Figure: Total number of ternary particles emitted per fission (T/B) as a function of of the fission system [Wag91].
About 90% of the ternary particles are -particles and about 7% tritons, the remaining fraction being constituted by a large variety of particles. The relative emission probabilities of the various ternary particles are summarized in table for the spontanous fission of Cf. The hydrogen and helium isotopes are always responsible for 99% of the ternary particle yield, while the heavier particles are very rarely emitted. Nevertheless, strong yield differences are observed between neighboring particles.
In table, both the first and second moments of the energy distribution of ternary particles are also given. The main feature of the spectrum is a fairly wide Gaussian shape, with a slight excess of intensity at low energies. It is apparent that mean energies of the measured ternary particles increase with the nuclear charge and decrease within each element with increasing mass. The observed decrease of kinetic energy with increasing mass and constant Z can be understood by assuming very similar initial conditions for the vatious ternary particles. Indeed, for a given initial energy, the heavier particles will move more slowly, allowing the fission fragments to move further away before the particle is fully accelerated.
Table: Relative emission probabilities of the various ternary particles and first and second moments of the energy distribution for the spontanous fission of Cf.
Most of the ternary particles are emitted about perpendicular to the fission axis; hence they are not evaporated from the accelerated fragments. Fig. displays the double differential yield and the corresponding single angular spectrum [Mut93]. Both figures exhibit besides of the dominant equatorial -particles also the small fraction of predominantly higher energetic polar -particles being emitted along the fission axis and under small angles with respect to the directions of the light and heavy group of fission fragments. The mean angle and the width of the angular distribution for the different light charged particles (LCP) emitted in a ternary fission process are summarized in table. The most probable angle of emission is always determined with respect to the direction of the light (L) fragment. All these data were obtained with lower energy cut-offs which result from the use of absorber foils to prevent the registration of the rare ternary LCP's from interference with the several orders of magnitude more frequent fission fragments or -particles from radioactive decay.
Figure: Polar diagram of -particle energy versus the emission angle with respect to the light fission fragment, for Cf.
Figure: Angular distribution of ternary -particles in spontaneous fission of Cf, as a function of the angle relative to the direction of the light fission fragment.
Table: Mean emission angle and angular width of the light charged particles (LCP) emitted in the ternary fission process of Cf. The most probable angle of emission is always determined with respect to the direction of the light (L) fragment. The used energy cut-offs are also given (see text).