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

fission barrier remains). Only in the fission of
226 Ra and 232 Th does asymmetric fission persist
in predictions up to higher energies than
found in experiments — possibly caused by
the rather slow disappearance of the
asymmetric fission with increasing excitation
energy or from a lack of symmetric fission
contributions from neutron deficient nuclides.
(b) Post-neutron emission mass yields —
generally too narrow, with the heavy wing
underestimated by an order of magnitude that
may be related to an overestimation of the
post-scission neutron multiplicity. The model
neglects the neutron evaporation between
saddle and scission points, which leaves too
much excitation energy within the fission
fragments. Consequently, the heavier
fragment, which receives a larger portion of
the excitation energy of the fissioning system,
evaporates more neutrons than the light
fragment, reducing the width of the final mass
distribution. However, there is evidence from
a comparison with experiments that the prescission
neutron multiplicity is not completely
incorrect and the calculated temperature of
the fissioning system is not excessively high. A
more likely explanation is that the fragments
evaporate too many neutrons because of an
underestimation of the energy required for
the emission of particles.
5.1.3.7. Summary of the essential features
of the new models and systematics
The five systematic approaches are derived
from fitting functions representing the different
fission modes to the experimental data. Parameters
were then fitted by expressions that describe the
dependences of these parameters on the system
target plus projectile and excitation energy. One
model was also explored in which the fission yields
are calculated from nuclear fission theory to derive
fission mechanisms and yield distributions.
Liu developed systematics solely for postneutron
emission yields from neutron induced 238 U
fission by means of a non-linear least squares
analysis and correlation study of the experimental
data. This systematics approach is based on experimental
data adjusted for mass resolution.
Katakura and Wahl have fitted the experimental
data for actinide nuclei with 3 to 7
Gaussians. They used neutron and proton induced
244
fission reactions, assuming reflection symmetry in
the shape of the mass distribution around a point of
symmetry calculated from the compound nucleus
mass minus the average total number of emitted
post (Katakura) or pre + post (Wahl) scission
neutrons. Katakura can only predict post-neutron
emission mass distributions. Wahl has not included
the extensive measurements of Zöller [5.4], and this
omission may be responsible for the discrepancies
of his predictions compared with others at higher
energies (50–160 MeV).
Kibkalo’s phenomenological model was
originally designed to study mass distribution
dependences on transferred angular momentum for
different projectiles, and was later adapted for
predictions of fission yields. The systematics are
similar to Katakura and Wahl, but without the prior
calculation of a point of mass symmetry in the mass
distribution.
The approach of Zhdanov et al. differs from
others in several respects:
(a) Experimental mass and energy distributions
of fragments are analysed;
(b) Expressions for the different fission modes do
not include assumptions concerning the
shapes of the distributions;
(c) Parameter values for the systematics were
derived by minimizing the differences
between experimental and calculated mass
distributions;
(d) Charlier’s peak functions were used in the
systematics to describe the shapes of the
asymmetric components.
Duijvestijn’s predictions are based entirely on
theoretical models for the fission mechanisms and
yield distributions. The fission cross-sections and
emissive fission contributions are calculated by
means of a modelling code to obtain the contributions
from different fissioning systems. A revised
version of the Brosa model [5.2] includes the
temperature dependence in the potential energy
landscape of the nucleus to derive the probabilities
for different fission modes. This approach is coupled
with a model for the neck rupture to obtain the
mass and charge split for calculating the fission
fragment and product mass and charge distributions.
ALICE-91 [5.6], as well as the newly
developed TALYS code [5.7], were used in the
benchmark exercise.