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

TABLE 4.6.4. PROTON INDUCED SUB-ACTINIDE FISSION CROSS-SECTIONS OBTAINED FROM
EXPERIMENT AND FROM ALICE-91 CALCULATIONS; VALUES FROM THE DATA COMPILATION
BY EISMONT ET AL. [4.6.25] ARE ALSO INCLUDED FOR COMPARISON
ALICE-91 tends to underestimate the fission crosssection
by 10–15%.
4.6.4.3.2. Mass yields in actinide fission
The full competition between the symmetric
and asymmetric fission modes is taken into account
in the case of actinide fission. With these modes a
variety of shapes can be described that are needed
to cover the whole range of intermediate energy
fission between several and 200 MeV. Pre-neutron
emission mass yields in neutron induced fission of
238 U are plotted in Fig. 4.6.15 for various different
projectile energies. The experimental data come
from the work by Zöller et al. [4.6.51]. At an
average incident neutron energy of approximately
13 MeV the mass distribution exhibits pronounced
asymmetric behaviour. This asymmetric character
persists up to an average neutron incident energy of
roughly 100 MeV. Above this energy a broad mass
yield curve remains, which still suggests some
asymmetric components through the appearance of
a broad and flat top. With increasing excitation
energy the symmetric valley from low energy fission
fills up due to a stronger symmetric contribution.
This forms the main contribution. Another much
smaller effect is the widening of the asymmetric
contributions.
The calculations of Fig. 4.6.15 show that the
agreement is within 10% or even better almost
everywhere. Figure 4.6.16 contains the same data
and calculation results but on a logarithmic scale.
This is done to enable a better comparison of the
experimental data and the calculations in the tails of
the distributions. At very high energies (around
200 MeV), the predictions and data start to deviate
at extremely asymmetric yields by one order of
magnitude. This is a similar effect to the subactinide
post-neutron emission yields at 190 MeV of
the previous section, but is less strong. A possible
explanation is that the predicted pre-scission shapes
are too compact. The fact that the mean mass, the
228
nat W
exp
s f [mb] 4.5 ± 0.5 32.8 ± 3.3 94 ± 9 88 ± 8
ALICE-91
s f [mb] 3.7 31.2 83.6 76.4
Eismont
s f [mb] 3.7 – 88 74
197 Au
nat Pb
208 Pb
FIG. 4.6.15. Pre-neutron emission mass yields in neutron
induced fission reactions on 238 U with the neutron
incoming energy range specified in the graphs. The saddle
point to ground state level density ratios are fitted to
reproduce the shape of the mass yield curve. Data taken
from Ref. [4.6.51].
width, and the relative asymmetric and symmetric
contributions come so close to the results found
experimentally indicates that neither the calculated
temperature at scission can be much too high nor
the calculated pre-scission neutron multiplicity can
be much too low. This observation agrees with the
supposition that the calculated post-scission
neutron multiplicity may be too high due to a wrong
estimation of the energy required for the emission