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

5.1.2.3. Multi-chance fission contributions
Maslov has performed a theoretical study of
the fission mechanisms and the emissive fission
contributions to the total fission cross-section in
order to obtain the contributions of the fissioning
nuclides to the total fission yield distributions.
Fission cross-sections for neutron and proton
induced reactions have been analysed and
compared with different model calculations to
obtain the best descriptions (Section 3.1). The
statistical model was found to be the most adequate
for this purpose. Also, the emissive fission contribution
to the observed fission cross-section is
dependent on target fissility and fission probability
for high excitation energies. A method of partitioning
the observed neutron induced fission crosssection
into emissive and non-emissive fission has
been validated for all neutron energies up to
200 MeV on Th, U, Np and Pu target nuclides.
Emissive fission contributions (also for symmetric
and asymmetric fission separately) have been
calculated for several target nuclides.
5.1.2.4. Systematics derived from experimental data
The systematic behaviour of the energy
dependence of the experimental fission yields has
been studied for several fissioning nuclides:
(a) Global systematics of the dependences of
fission product mass distributions on fissioning
nuclide and excitation energy have been
derived from experimental data for neutron,
photon and charged particle induced fission;
(b) Experimental data are insufficient to derive
global systematics for fission product charge
distributions, and theoretical models do not
provide adequate descriptions — therefore
the empirical Z p model as developed from low
energy fission data was used, and parameter
values were determined by the least squares
method from fractional independent and
cumulative yield values derived from the few
available experimental data.
5.1.2.5. Phenomenological models
for the fission yield distributions
derived from experimental data
Two new phenomenological models have been
developed for the analysis of experimental yield
distributions from neutron, proton and alpha
240
particle induced fission (see Sections 5.1.3.4 and
5.1.3.5). Detailed studies of the dependences of
model parameters on target nuclide, projectile and
incident particle energy have revealed regularities
in their behaviour that can be used for systematics.
Thus the models and computer programs have been
successfully adapted during the course of the CRP to
allow predictive calculations of mass distributions.
5.1.2.6. Theoretical prediction of fission yields
at intermediate incident particle energies
A new theoretical approach has been developed
to predict fission yields at intermediate energies and
solve the problems of emissive fission, as well as the
changing fission characteristics with excitation energy
(Section 5.1.3.6). This approach is based on a newly
developed version of the Brosa model for the
calculation of fission yield distributions [5.2], coupled
to a nuclear reaction code for the calculation of fission
cross-sections and emissive fission contributions.
5.1.2.7. Progress towards computer programs
for evaluations
Models and systematics have been developed
that can in principle be used for an evaluation of
energy dependent fission yields. However, the
results of the benchmark exercise are not conclusive
enough to recommend one analytical method at the
present time, and the same is true for the computer
programs. CYF (Section 5.1.3.3) and PYF (Section
5.1.3.5; also available on the Internet [5.3]) are
designed to be user friendly.
5.1.3. Models and systematics
5.1.3.1. Fission yield systematics and
covariance study of 238 U (Liu Tingjin)
The work of Liu Tingjin was restricted to 238 U
fission yields, and no attempt was made to derive
parameters for global systematics in order to predict
fission yields for other fissioning nuclides. Mass
distribution data from only one experiment (Zöller
[5.4] over the energy range from 2 to 200 MeV)
were used, adjusted for mass resolution and fitted
with five (or three at higher energies) Gaussian
functions. Gaussian parameters and their energy
dependence were obtained from a non-linear least
squares fit over the whole energy range, and uncertainties
and correlations in the systematics were also
studied. Liu adjusted for mass resolution, and