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

predicted effects of multi-chance fission have been
described in Section 1.3.4.2, the impact of this
phenomenon on the observed shape of the
composite mass distribution is briefly outlined
below.
(a) Superposition of preferred fission modes
At 100 MeV, the asymmetric and symmetric
fission modes are about equal in the observed mass
distribution. Asymmetric fission is likely to
dominate below 100 MeV, and expected contributions
from multi-chance fission are probably not
sufficient to change the shape of the mass distribution
significantly. Above 100 MeV, symmetric
fission starts to dominate, contributions from
higher-chance fission are more significant, and
therefore the composite mass distribution is no
longer symmetric in shape.
(b) Change in peak positions and mass symmetry
point in asymmetric fission
The expected effect of the contributions from
lower mass fissioning nuclides — a stable position
for heavy mass peak, and a broadened and lower
light mass peak — are only observed in the
predictions of Duijvestijn with ALICE-91. There
are several possible reasons:
(1) This possible effect is not included in most
systematics, and reflection symmetry is
assumed in the modelling of the mass
distributions;
(2) As above, the predicted effect is too small
below 100 MeV to cause a noticeable change
in the mass distribution;
(3) When the chosen peak functions are too
broad, any such visible effect at 100 MeV and
above would be washed out, resulting in a flat
plateau (even if reflection symmetry in the
peak functions is not assumed).
At higher excitation energies, when symmetric
fission becomes significant, the composite
symmetric mass peak is expected to broaden
towards lower masses. Similarly, the point of
symmetry in the mass distribution, defined
originally as the point where both fragments have
equal mass (symmetric fission), changes from the
position of first chance fission towards lower masses
(broader valleys or symmetric peaks) with
decreasing mass of the fissioning nucleus. When the
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composite mass distribution is considered, the
symmetry point becomes the point above and below
which the sums of the mass yields are equal and
total 100%. However, this new point of mass
symmetry is not defined solely by the mass of the
target, plus projectile, and minus the total number
of neutrons emitted, as assumed in some
systematics.
(c) Multiplicity distribution of neutrons emitted
from fission fragments
The observed multiplicity distribution of
neutrons emitted from fission fragments is the sum
of the contributions from different fissioning
nuclides. Systematics and models have to include
this effect when calculating the unmeasured
neutron distributions derived for target nuclei only.
Since there are hardly any measurements at intermediate
energies, the neutron multiplicity distributions
used in these models to calculate fission yields
are based on crude assumptions.
5.2.3.4. Consequences for model predictions
at higher energies
The following considerations have to be taken
into account in systematics when modelling fission
product mass distributions at higher energies:
(a) Mass distributions are not symmetric in shape;
(b) Light and heavy mass peaks cannot be
described by peak functions that are identical
in shape;
(c) Peak heights and widths are not equal — the
shapes, particularly that of the light mass peak,
cannot be described by symmetric functions
like single Gaussians;
(d) When the asymmetric and symmetric fission
contributions are both significant, the distribution
could be broad and flat;
(e) At still higher energies, symmetric fission
dominates, and the mass distribution should
broaden and not have the shape of a single
Gaussian;
(f) Mass distributions cannot be described by
single Gaussians with identical parameters for
each fission mode, but either by a superposition
of Gaussians accounting for multi-chance
fission or by using ‘effective’ peak functions
that describe the actual peak shapes;
(g) If necessary, an effective point of symmetry
has to be calculated from an effective mass of