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

FIG. 4.6.20. Pre-neutron emission mass yields in proton
induced fission reactions on 226 Ra and 238 U for the incident
energies given in the graphs. Data taken from Refs [4.6.22,
4.6.52].
to the same extent as the sub-actinides in proton
induced reactions at 190 MeV (Fig. 4.6.13).
Unfortunately, the experimental data for
other nuclides are limited to neutron energies
around 14 MeV. Figure 4.6.22 shows data for four
different nuclides: 232 Th, 233 U, 242 Pu and 241 Am
[4.6.53–4.6.56]. At the corresponding excitation
energies the post-scission neutron evaporation does
not yet spoil the outcomes. The predictions in the
wings of the mass distributions deviate from
experiment to a maximum of 30% for 232 Th, 242 Pu
and 241 Am. However, in the case of 233 U the right
tail is underpredicted by a factor of 10. On the top
of the distribution and in the symmetric valley, the
agreement is within 20%, except in some mass
regions of 232 Th where the calculated yields lack the
small symmetric hump.
4.6.4.3.3. Transition from symmetric
to asymmetric fission
From all mass yield curves seen so far in the
previous two sections, somewhere at the edge of the
actinide region a transition takes place between
FIG. 4.6.21. Post-neutron emission mass yields in neutron
induced fission reactions on 238 U. The neutron incident
energy ranges are denoted in the graphs. Data taken from
Ref. [4.6.51].
symmetric and mixed (symmetric plus asymmetric)
fission. Moreover, observations by Schmidt et al.
[4.6.35] suggest a change towards symmetric fission
in the neutron deficient part of the actinide region.
In addition, an increase in symmetric fission is
observed with increasing excitation energy. The
question arises whether this last effect is mainly due
to the vanishing of asymmetric fission modes with
increasing excitation energy or to the contributions
of more and more neutron poor isotopes with an
intrinsic symmetric behaviour.
Figure 4.6.23 shows mass distributions of
various isotopes between actinium and uranium
labelled by the element name and neutron number.
The calculations are carried out for nuclides with an
excitation energy of 10 MeV. The gradual change
from asymmetric and mixed to symmetric fission is
clearly visible. Near the valley of stability, at the
right hand side of the plot, fissioning isotopes tend
to produce mass distributions with a strong
asymmetric signature. At the left hand side the
231