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

FIG. 4.6.5. Decomposition of the 232 Th mass yield curve
into one Gaussian (dotted line) coming from purely
symmetric fission of lighter nuclides, and three Gaussians
representing the mixed symmetric (dot-dashed line) and
asymmetric (dashed line) fission modes of heavier
nuclides. The solid line indicates the sum of the symmetric
and the mixed contributions; triangles represent the experimental
mass yields resulting from dividing the measured
yields by the fractional yields obtained with Eq. (4.6.5).
fission. This is in agreement with the idea that the
purely symmetric Gaussian comes from the
fissioning nuclides that have lost more pre-fission
neutrons. The four Gaussians give only a schematic
description of reality, but enable one to estimate the
part of the fission cross-section originating from
purely symmetric and from mixed fission. A disentanglement
of the contributions of the different
fission modes in all the fissioning nuclides is far
more complicated. From this decomposition the
cross-section is estimated to be 680 ± 70 mb for
purely symmetric and 271 ± 27 mb for mixed fission.
The new total fission cross-section thus becomes
950 ± 100 mb, which is in better agreement with the
Eismont et al. [4.6.25] value of 1236 mb than the
value originating from the single Gaussian fit. The
result from the Eismont fit is based on experimental
data points ranging from 1050 mb to 1250 mb.
Therefore the conclusion is that a decomposition of
the 232 Th fission fragment mass distribution into a
mixed fission and a purely symmetric fission
component gives a better description of the data
than a single Gaussian.
The previously mentioned experiment by
Schmidt et al. [4.6.29] supports the assumptions on
the mixed and purely symmetric fission
components. Other experimental evidence for the
particular form of the fission product mass
218
distribution can be found in the work by Pappas and
Hagebø [4.6.16] and by Lee et al. [4.6.37]. Pappas
and Hagebø arrived at similar findings when
inducing fission of 238 U with 170 MeV protons. They
have proposed a decomposition of the mass yield
curve into a symmetric contribution caused by high
deposition energy events, and two asymmetric
contributions connected to low and high deposition
energy events. Lee et al. have studied fission in the
bombardment of 238 U with 240 MeV C ions. They
have interpreted the mass distribution as a superposition
of three components: asymmetric low energy
fission, symmetric high energy fission (combination
of fusion fission and fast fission) and symmetric
sequential fission (i.e. fission preceded by multiple
nucleon emission). What both these approaches
have in common with this work is the symmetric
component linked with the more neutron deficient
fissioning systems. The treatment of the less neutron
poor nuclides differs. Here both symmetric and
asymmetric fission modes are thought to play a role
in this part of the chart of nuclides, while Pappas
and Hagebø omit the symmetric fission contribution
completely and Lee et al. neglect the asymmetric
fission contribution for excitation energies above 35
MeV (although, according to theoretical calculations,
the shell effects responsible for asymmetric fission
have not yet vanished at these energies, see, e.g., Ref.
[4.6.38]).
4.6.3.3.2. Charge distributions
In Fig. 4.6.6 the charge distributions as they
result from the fit of Eq. (4.6.5) are compared to the
measured independent and cumulative yields in
three different mass regions. Due to the various
asymmetric and symmetric contributions, the
charge distribution is expected to be much broader
than for 208 Pb in Fig. 4.6.2. However, in the region
around the maximum of the mass yield curve (A ~
110), the dominant contribution is purely symmetric
(as can be concluded from Fig. 4.6.5). Therefore, the
corresponding charge distribution should be
narrower in this region. The charge distribution fit
with a single Gaussian is obviously not able to take
into account these changes of the width throughout
the different regions. Moreover, the sum of
different contributing fission modes may result in a
shape for the charge distribution which is no longer
a well defined Gaussian. In the upper graph, the
experimental yields around mass 103 are not
properly described by the fit. The fitted width seems
to be overestimated, whereas in the other two plots