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

present have a reasonable contribution. This
explains the mixed mass yields visible in all graphs.
Apparently, only at quite low energies, symmetric
fission is preferred by the neutron deficient isotopes
as well as by most actinium isotopes at the edge of
the actinide region. Since the melting away of fission
modes happens in beta stable and very neutron
deficient nuclides at a comparable rate, the situation
of Fig. 4.6.24 persists up to a certain excitation
energy, above which an overall and gradual transition
to symmetry occurs simultaneously for all isotopes.
Hence, in the calculations presented here the
increase of symmetric fission at high energies is fed
more by the disappearance of the asymmetric fission
modes due to the vanishing of shell effects and less by
a larger contribution of neutron deficient nuclides
due to the preceding neutron evaporation.
In the previous section a systematic underprediction
of the symmetric component in the
description of intermediate energy fission of
thorium and radium has been observed. Two effects
may be responsible: the increase of the symmetric
component in these elements may be too slow with
excitation energy or the asymmetric fission modes
do not vanish in time. The alternative possibility is a
lack of symmetric fission in neutron poor nuclides
due to wrong barrier parameters resulting in an
overestimation of asymmetric fission at higher
energies, or due to an overprediction of the
excitation energy in these nuclides. Based on the
comparisons between calculations and experimental
results shown here it is not possible to draw
a final conclusion. Perhaps in future, experiments
can be designed to investigate mass distributions in
a systematic way (similar to the calculated results
presented in Figs 4.6.23 and 4.6.24, but also at
higher excitation energies), so that the results will
help to solve the question of how the transition
between symmetric and asymmetric fission takes
place.
4.6.5. Summary and outlook
4.6.5.1. Summary
This work focused on nucleon induced fission
at intermediate energies between roughly 10 and
200 MeV. Some effort has been invested in the
compilation of existing proton induced fission yield
data in the desired energy range. In order to
supplement the limited experimental data available,
especially regarding the fission product isotopes, an
experiment for 190 MeV proton induced fission has
been performed. However, the major contribution
of this work comprises the development of a new
theoretical approach to calculate the fission product
mass yields in competition with all other reaction
channels for light particle induced nuclear reactions
up to about 200 MeV. This new method is based on
the extension of the original Brosa model, with a
temperature dependence and the coupling of the
resulting model with a nuclear reaction code that
provides the connection between fission and other
reaction mechanisms such as pre-equilibrium and
compound emission.
4.6.5.1.1. Experiment
An activation experiment has been carried out
that aims at the measurement of independent and
cumulative fission product yields originating from
190 MeV proton induced reactions on nat W, 197 Au,
nat Pb, 208 Pb and 232 Th. By placing catchers on both
sides of each target during the irradiation, the
fission products have been separated from the
enormous amounts of evaporation products. In this
manner, rather clean spectra with fission product
g rays have been obtained. With separate experimental
set-ups for a short irradiation (catcher
cylinder) and a long irradiation (catcher foil) it is
possible to observe both short lived and long lived
fission products.
The mass yields of nat W, 197 Au, nat Pb and 208 Pb
are well described by a single Gaussian for both the
mass and the charge distribution. Fission crosssections
extracted from the single Gaussian fits are
comparable with the values obtained from a data
compilation made by Eismont et al. [4.6.25]. Subactinide
fission at this energy is purely symmetric.
A careful look at the mass yields for 232 Th
shows that the mass yield curve has a more
complicated form than that of a single Gaussian.
This indicates the contribution of other asymmetric
fission modes besides the symmetric mode already
encountered in sub-actinide fission. The origin of
this particular form of the mass distribution resides
in the process of multi-chance fission. Particle
evaporation prior to fission results in a wide variety
of fissioning nuclides, each contributing to the mass
distribution with its own fission characteristics. The
suggestion made is that the observed strong
symmetric component in the 232 Th mass distribution
is only partly caused by an enhanced
crossing of the symmetric fission barrier in the
fissioning nuclides near the valley of stability, which
possess both symmetric and asymmetric fission
233