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

experimental method in order to derive an
appropriate and valid adjustment procedure.
6.2.1.2. Presentation of the analyses
Intercomparison plots for part A of the
benchmark exercise were prepared for 238 U + n,
238 U + p, 239 Pu + n and 242 Pu + n, including mass
distributions and various ratios. For reasons given in
Section 5 we present here only the intercomparison
for 238 U + n. Figures comparing measured and
calculated mass distributions provide the best
picture of differences in terms of absolute yields,
and they are presented for energy values at which
the shapes of the mass distributions and/or the
agreement among data change significantly. Some
parameters of the mass distributions are tabulated
and plotted as a function of the incident neutron
energy.
6.2.2. Post-neutron emission
There are more experimental data and more
calculated results for post-neutron emission than for
the pre-neutron emission yields. A survey of
measured and calculated asymmetric mass peak
positions and heights, valley heights at symmetry
and peak to valley ratios as a function of incident
neutron energy is presented in Table 6.2.1 and
Figs 6.2.1–6.2.4. Intercomparisons of actual mass
distributions are plotted in Figs 6.2.5–6.2.11 for
E n = 1.5, 8, 14–15, 27–28, 50, 100 and 160 MeV. A
more detailed discussion of the intercomparison is
restricted to energies up to 28 (sometimes 50) MeV
because above that value the shapes of the mass
distributions are too divergent (e.g. the same plot
shows distributions that contain 1, 2 or 3 peaks or
are broad and flat, so that terms like asymmetric
peak or valley lose their meaning). The energy
range 50–160 MeV is discussed in a separate
analysis.
6.2.2.1. Experimental data
The experimental data can be subdivided into
two groups:
(1) Yields determined either by g ray
spectroscopy of unseparated fission
products, or by b-g spectrometry of
individual fission products after chemical
separation, E n = 1.5, 5.5, 8.1 + 8.3, 11.3, 14.4
+ 14.9 and 22 MeV;
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(2) Directly recorded mass distributions of Zöller
[6.3] at 13, 20, 27–28, 50, 100 and 160 MeV.
Yields determined by method (1) exhibit large
fluctuations and fine structure in the peak regions,
not all of which are necessarily real and reproduced
by model calculations. Furthermore, the experimental
peak maxima are always closer to symmetry
than the calculated profiles, and lead to narrower
valleys. However, low yield products in the valley
and wing regions are generally not included in these
measurements because the intensities of the emitted
radiations are too low.
Mass distributions from Zöller [6.3] are much
smoother because of the poor and unadjusted mass
and energy resolutions. For example, at an energy
quoted to be 27.5 MeV, the actual energy range is
22–33 MeV and the given fragment mass number
covers 3 mass units. Similarly, for ‘100 MeV’, the
energy range is 89–110 MeV and the mass range is
5 mass units. The better agreement with model
calculations can be attributed to most of the
systematics being based on Zöller’s data. At higher
energies (50–160 MeV) these data favour the results
of Kibkalo and Zhdanov, and also Liu’s systematic
calculations without any adjustment for mass
resolution. Finally, a striking observation is that the
agreement between the measurements of Zöller
and the experimental data from method (1) is worse
than between Zöller and the calculated results.
6.2.2.2. Model calculations
(a) Discrepant results
ALICE-91 (8–160 MeV):
The distribution is in most cases narrower, the
valley higher, and the wings and peaks lower than in
other calculations. While the heavy mass peak
becomes lower than the light mass peak with
increasing neutron energy and disappears
completely at 160 MeV, the light mass peak starts to
rise above all others at 50 MeV. The position of the
light mass peak is consistently two mass units lower
than all other calculations, whereas there is
agreement for the position of the heavy mass peak.
TALYS (1.6–28 MeV):
Similar to ALICE-91, but fails above about 30 MeV.