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

compound nucleus) was analysed recently within
the Hauser–Feshbach statistical model [3.1.9].
Individual contributions of the super long fission
mode to the observed fission cross-sections were
reproduced up to the emissive fission threshold
[3.1.10, 3.1.11]. A separate outer fission barrier was
assumed for the super long mode, while a common
inner barrier was assumed for the symmetric super
long and asymmetric S1 and S2 modes. This
assumption is supported by fission mode calculations
[3.1.12, 3.1.13] that use the multimodal
random neck rupture model [3.1.2–3.1.5], and
reveal that the final bifurcation point is close to the
second minimum of the actinide double humped
fission barrier. The super long mode 235 U(n,f) and
238 U(n,f) fission cross-sections were shown to be
controlled by a rather high outer fission barrier with
significant transparency. A steeply rising shape of
the super long mode contribution was found to be
compatible with outer saddle point axial asymmetry
that accompanied mass symmetry. Outer saddle
shapes for S1 and S2 modes were assumed to be
axially symmetric and mass asymmetric.
Above the emissive fission threshold, a
number of nuclei contribute to the fission observables:
the 238 U(n,f) reaction at E n ~ 200 MeV may
involve an appreciable contribution from about
20 nuclides. The ratio of symmetric to the sum of
symmetric and asymmetric fission events was
obtained in the reaction 238 U(n,f) by Zöller et al.
[3.1.14] for incident neutron energies of up to E n ~
500 MeV. They deduced the ratio of symmetric to
all fission events from the fission fragment total
kinetic energy distribution as well as from the mass
total kinetic energy distributions. A simultaneous
analysis of this ratio and the observed neutron
induced fission cross-section for a 238 U target
[3.1.15, 3.1.16] up to E n ~ 200 MeV might be
attempted within statistical theory. The observed
238 U neutron induced fission cross-section data
[3.1.15, 3.1.16] were analysed recently in a fission/
neutron evaporation approximation up to E n ~
200 MeV, in which first chance pre-equilibrium
neutron emission was taken into account [3.1.17].
Fission probabilities of U nuclei in the domain of
the emissive fission reaction 238 U(n,xnf) were
estimated without making any distinction regarding
symmetric or asymmetric fission (i.e. super long, S1
or S2 modes). These fission probabilities provide
correct estimates of lumped fission competition
against neutron emission. Damping of the collective
26
modes’ contribution to the level density at
excitation energies higher than ~20 MeV for axial
symmetric saddle and equilibrium deformations was
shown to be essential for the description of the
observed fission cross-section. Effective estimates
of intrinsic level densities were obtained, which
define the competition between fission and neutron
emission at high excitation energies when rotational
collective modes are completely damped.
Competition of the symmetric and asymmetric
fission of U nuclei would depend essentially on the
excitation energies of the U nuclei emerging after
the emission of pre-fission (n,xnf) neutrons. Level
densities can be anticipated to play a key role in the
relevant saddle deformation. We can probe the
sensitivity of the calculated super long mode and
lumped asymmetric (S1 + S2) mode yields to the
damping of triaxial deformations at the outer
saddles of U nuclei. The range of mass numbers of
U nuclei that make an appreciable contribution to
the fission observables depends on fission barriers
and intrinsic level densities at saddle and
equilibrium deformation. We anticipate that the
ratio of symmetric to all fission events will strongly
depend on the relative contributions to the
observed cross-section of fission chances with
different numbers of pre-fission neutrons. The
analysis of the ratio of symmetric to all fission
events might be a sensitive tool for the partitioning
of the fission (n,xnf) chance contribution to the
fission observables.
With an increase of the target nuclide fissility,
the number of nuclei that make an appreciable
contribution to the fission observables will tend to be
lower. We have developed an approach to describe
the 238 U(n,f) cross-section and branching ratio of
symmetric/asymmetric fission up to 200 MeV
incident neutron energy. This model will be applied
to the observed 235 U(n,f), 233 U(n,f), 232 Th(n,f),
237 238 Np(n,f) and U(p,f) fission cross-section data.
235 sym 233 sym
We will predict U(n,f) , U(n,f) ,
232 sym 237 sym 235 asym
Th(n,f) , Np(n,f) and U(n,f) ,
233 asym 232 asym 237 asym U(n,f) , Th(n,f) and Np(n,f) fission
cross-sections, which represent a wide range of
target nuclide fissilities. Analyses of observed
neutron and proton induced fission cross-sections
would provide valuable information about the
relative contributions of emissive fission. The
possibility of using proton induced fission yields can
then be defined to estimate the neutron induced
fission yields at high excitation energies.