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

4.4. PHENOMENOLOGICAL MODEL FOR FRAGMENT MASS
AND CHARGE DISTRIBUTION IN ACTINIDE NUCLEI FISSION
Detailed analyses of experimental data on
nuclear fission show that the process of fragment
formation depends not only on the temperature but
also on the total angular momentum of the
fissioning nucleus. An analysis of the total crosssections
for the fission of 232 Th, 235 U, 236 U and 238 U
by a particles with energies up to 140 MeV has been
carried out to study the dependence of the
fissioning nucleus formation cross-section on
excitation energy and transferred angular
momentum. The energy dependence of the critical
value of the angular momentum for fissioning
nucleus formation has been obtained for the
interaction of a particles with uranium nuclei in this
energy range. A new approach is proposed to
describe fragment mass distributions from actinide
fission that accounts for the influence of the total
nuclear angular momentum. The experimental
fragment mass distributions for the fission of
actinide nuclei formed in spontaneous fission and in
reactions with neutrons, g rays and a particles in the
excitation energy range up to 200 MeV have been
analysed by this approach. The dependence of
fragment mass distribution parameters on the
excitation energy and transferred angular
momentum of fissioning nuclei was derived. A
phenomenological description of fragment mass
and charge distributions has been developed that
accounts for the quantum-mechanical transmission
of real fission barriers.
4.4.1. Introduction
Studies of cross-sections for the interaction of
charged particles and heavy ions with nuclei reveal
that the reaction mechanism for compound system
formation varies with incident particle energy.
Nuclei fission should be studied over a wide energy
range to ensure that the information obtained from
experimental fission data will encompass the full
characteristics of the fissioning system. Some
progress has been made in explaining the energy
dependence of fusion cross-sections for heavy ions
with nuclei on the basis of the dynamic model
[4.4.1].
Yu.V. Kibkalo
SC Institute for Nuclear Research, Ukraine
Detailed analyses of the dependence of experimental
data on actinide nuclei fission at medium
excitation energies show that, at the time of
formation of the angular, mass and energy distributions
of fission fragments, an essential role is played
by the spectrum of transition states above the
fission barrier. The basic characteristics of transition
states are the collective energy and the total angular
momentum of the fissioning nucleus. Thus the
process of scission and formation of fragments is not
only influenced by the temperature, but also by the
total angular momentum of the fissioning nucleus.
The dependence of the integral and differential
fission cross-sections on the total angular
momentum is well acknowledged and investigated,
while only a few publications are devoted to experimental
studies of the influence of the total angular
momentum on fission fragment mass and energy
distributions. The most complete research of that
kind is presented in Ref. [4.4.2]. This work involved
an investigation of the dependence of the fission
fragment mass and energy distributions, and the
formation of 204,206 Po and 260 Ku in reactions of heavy
ions from 12 C up to 48 Ti with targets from 164 Dy up
to 244 Cm. The distinctive peculiarity of the work is
that heavy ion beams at rather high excitation
energies (E* > 40 MeV) and large transferred
angular momentum (·Ò ≥ 30ћ) were used. Analysis
of the experimental data showed a weak
dependence of the fragment mass distributions on
the average angular momentum of fissioning
nucleus, and the total absence of such a dependence
for the average total kinetic energy of fission
fragments in the case of rather strongly heated
fissioning nuclei [4.4.2].
We analysed a large amount of experimental
data on the fission of 240 Pu formed in reactions with
g quanta, neutrons and a particles in order to study
the dependence of the fragment mass distributions
from actinide fission on medium excitation energy
and small transferred angular momentum [4.4.3,
4.4.4]. Figure 4.4.1 shows the fission fragment mass
distributions of 240 Pu formed in reactions with g
quanta [4.4.5], neutrons [4.4.6] and a particles
[4.4.7] at nearly equal excitation energies (E* = 20,
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