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

FIG. 4.1.9. Total and mode separated fission cross-section
for 238 U(n,f) together with the corresponding calculated
mode separated fission cross-sections.
FIG. 4.1.11. Predicted mass distribution for 237 Np(n,f) at
E n = 0.4 MeV.
However, the observed fluctuations in the fission
fragment branching ratios from Ref. [4.1.17] are
much too strong to be parameterized by the model.
The maximum of the observed fluctuations also
seems to lie around E n = 1.25 MeV, whereas the
structure in the cross-section is located more
towards E n = 1.2 MeV. More experimental investigations
around this vibrational resonance and
another at E n = 0.9 MeV (where no experimental
data exist) are necessary to verify the theoretical
predictions (experiments are planned for the
coming years).
The consequence of the drastic change in the
mass distribution is obvious. Two examples of
predicted mass distributions are given in Figs 4.1.11
and 4.1.12. The mass distribution for 237 Np at E n =
0.4 MeV is plotted in Fig. 4.1.11 and shows a peak
yield towards the position of the S1 mode
FIG. 4.1.10. Calculated and experimental branching ratios
for 238 U(n,f).
FIG. 4.1.12. Predicted mass distribution for 238 U(n,f) at E n
= 0.9 MeV.
(calculations predict that this mode is dominant at
this incident energy (see Fig. 4.1.8)). This
domination should also have a consequence for the
mean total kinetic energy, since the S1 mode has a
more compact shape and hence a higher total
kinetic energy. A similar conclusion can be drawn
for 238 U. The strength of the S1 mode is also
increasing for incident neutron energies smaller
than 1.3 MeV, and this behaviour should be
observable in state of the art experiments.
4.1.4. Concluding remarks
(a) For the first time, fission mode de-convoluted
fission cross-sections based on branching
ratios measured at IRMM were calculated for
incident neutron energies E n = 0.01–5.5 MeV.
Good agreement with experimental fission
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