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).