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

FIG. 4.6.22. Post-neutron emission mass yields in neutron
induced fission reactions as specified in the graphs. Data
taken from Refs [4.6.53–4.6.56]. The triangles in the upper
graph correspond to data taken from a table, whereas the
circles were read from a figure.
resulting mass yields are entirely symmetric or
possess at least a large symmetric share. A solid line
connects the isotopes for which the symmetric
hump exceeds the asymmetric humps for the first
time in going towards neutron poor nuclides
starting from stability. This is taken as a crude
measure for the transition. The dot–dashed line
represents a condition defined by Chung and Hogan
[4.6.27, 4.6.28], which also marks the transition from
symmetric to asymmetric (mixed) fissioning
isotopes. The dashed line corresponds to a
calculation by Möller [4.6.57], who determined the
stability of the saddle point configuration against
asymmetric deformations. Both lines belonging to
Chung–Hogan and Möller run more or less parallel
to the line of stability. However, the line originating
from the ALICE-91 plus temperature dependent
Brosa model calculations is perpendicular to these
lines. This completely different behaviour is also
observed by Schmidt et al. [4.6.35] in the charge
232
FIG. 4.6.23. Mass distributions of various isotopes labelled
by the element name and neutron number — calculations
are performed at a fixed excitation energy of 10 MeV. The
thick solid line connects isotopes for which the symmetric
hump exceeds the asymmetric humps for the first time,
starting from stability and going towards neutron poor
nuclides; the dotted line is obtained in an equivalent
manner from charge distributions measured by Schmidt et
al. [4.6.35]; the dot–dashed line represents a condition
defined by Chung and Hogan [4.6.27, 4.6.28], and the
dashed line corresponds to a calculation by Möller
[4.6.57].
FIG. 4.6.24. Same as Fig. 4.6.23, but for an excitation
energy of 20 MeV.
distributions of the same fissioning isotopes at
excitation energies peaked around 11 MeV. Here
the transition marked by the dotted line also tends
to occur along a line perpendicular to the Möller
line. Therefore, the prediction by the ALICE-91
plus temperature dependent Brosa model exhibits
the same tendency as experimentally observed. In
conclusion, at low energies the transition seems to
take place at less neutron poor nuclides for thorium
and actinium than for uranium.
The preportions of symmetric and asymmetric
fission depend on the excitation energy. Therefore,
similar calculations are carried out for an excitation
energy of 20 MeV (Fig. 4.6.24). Because the excitation
energy lies well above the barriers, all fission modes