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