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

TABLE 4.6.5. ACCURACIES OBTAINED FROM COMPARING THE PREDICTIONS WITH EXPERI-
MENTAL DATA FOR INCIDENT ENERGIES BETWEEN 15 AND 200 MeV AND SEVERAL
ISOTOPES.
Uncertainties in the proton and neutron induced fission cross-sections, as well as in the pre-neutron and postneutron
emission mass yields, are given, and are expressed as percentages.
Mass yields can be predicted with a proper
choice of both the ground state to saddle point level
density parameter ratio and the pre-scission shapes.
The pre-scission shapes are fixed by applying the
same recipe to all nuclei. This leaves a f/a n as the only
parameter which is tuned to reproduce at best both
the fission cross-section and the mass yield curve for
a given reaction. The obtainable accuracy depends
in general on the incident energy, as well as on the
isotope investigated. Table 4.6.5 contains a
summary of the uncertainties. They are extracted
from comparing the predictions for the total fission
cross-sections, as well as for the pre-neutron and
post-neutron emission mass yields with experimental
data. The incident nucleon energies range
from 15 to 200 MeV and the isotopes are either
sub-actinides or actinides. In Table 4.6.5 the actinide
region is subdivided into (a) the nuclides on the
edge marking the transition to low energy
symmetric fission, (b) uranium for which a large
amount of data is available, and (c) the heavier
actinides. The predictive power changes drastically
with observable nuclide and excitation energy.
The prediction of the total fission crosssection
is fairly accurate. Deviations of up to 100%
can occur only in the case of the sub-actinides, for
which the probabilities become very small. The
determination of pre-neutron emission mass yields
is satisfactory. The good agreement for uranium at
energies as high as 200 MeV suggests that the
temperature dependent liquid droplet model does
not break down above a temperature of 2.0 MeV
[4.6.40]. This value has been given as the validity
boundary. Only in the fission of 226 Ra and 232 Th can
the model not correctly describe the experimentally
observed transition to symmetric fission.
Asymmetric fission persists up to incident energies
Sub-actinide region Actinide region
Z < 84 Z < 91 Uranium Z > 93
sF (p,f) 10–100% 10–20% 10% —
sF (n,f) 10–15% 10–20% 10–15% 20%
Ypre (A) 10–15% 10–50% 10% —
Ypost (A) 50–1000% — 10–1000% 30%
that are too high. This leads to deviations between
10% at low energies and 50% at high energies in the
fission fragment mass yields. Whether this stems
from asymmetric fission that vanishes too slowly
with increasing excitation energies or from a lack of
symmetric fission contributions from neutron
deficient nuclides could not be determined from the
results of this work. In general, the calculated postneutron
emission mass yields are too narrow: the
light wing and the top are reproduced within 50%
or better, whereas the heavy wing is underestimated
by one order of magnitude. This is probably related
to an overestimation of the post-scission neutron
multiplicity. The model neglects the neutron
evaporation between the saddle point and the
scission point, leaving too much excitation energy in
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 lighter fragment
and reduces the width of the final mass distribution.
However, since the calculated mean mass of the
pre-neutron emission mass yield curves agrees very
well with the experimental values, the pre-scission
neutron multiplicity, which momentarily excludes
neutron evaporation between the saddle point and
the scission point, cannot be completely wrong.
Furthermore, the temperature of the fissioning
system cannot be far too high because of the
correctly reproduced relative contributions of the
different fission modes in the calculated preneutron
emission mass yield distributions.
Therefore, a more likely explanation is provided by
the supposition that the fragments evaporate too
many neutrons because of an underestimation of
the energy required for the emission of particles.
235