Fission Product Yield Data for the Transmutation of Minor Actinide ...

from higher fission chances [3.1.43] (i.e. from very
neutron deficient U nuclei). We have demonstrated
that a more detailed treatment of the observed
fission cross-section as a sum of the symmetric and
asymmetric fission cross-sections needs stronger
contributions from higher chances, or more fissions
from very neutron deficient isotopes. This situation
is described by the energy dependence of the
asymptotic value ã f (U, A) for the a f parameter at
saddle deformations. The description of the
branching ratio of symmetric to all fission events
needs a relative increase of the contribution of
symmetric fission from neutron deficient isotopes,
which is achieved by the damping of triaxial
rotational mode contributions at outer saddle deformations.
Further refinements of the model (such as
inclusion of the temperature and angular
momentum dependence of fission barriers, the
influence of neutron shell N = 126 on the collective
enhancement in the neutron channel [3.1.41], and
calculation of fission barriers for symmetric and
asymmetric fission modes of U nuclei with the shell
correction model method [3.1.40]) would not
change our main conclusions. Lowering of the
asymptotic value ã f of the a f parameter at saddle
deformations can be considered as reproducing the
lumped effect of all these factors. We also expect
that the possible vanishing of the distinction
between symmetric and asymmetric valleys in the
energy surface at high excitation of fissioning nuclei
[3.1.43] would not have much influence on the
calculated values because of the strong emissive
fission nature of the 238 U(n,f) reaction for E n £
200 MeV.
Damping of the contribution of the axial
collective modes to the level density for both inner
and outer saddle and for equilibrium deformations,
as well as triaxial damping at super long mode outer
saddle deformations, were assumed. A consistent
description of the observed fission cross-sections
and the symmetric fission branching ratio for the
238 U(n,f) reaction was obtained on the basis of this
assumption. Symmetric/asymmetric emissive fission
partitioning of the 235 U(n,f), 233 U(n,f), 237 Np(n,f),
232 Th(n,f) and 238 U(p,f) reaction cross-sections have
been predicted. The dependence of the symmetric
fission branching ratio on the target nuclide fissility
is interpreted as being due to a higher contribution
of lower fission chances in the case of higher target
nuclide fissilities.
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