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

FIG. 2.2.1. Independent charge yields per 100 fissions
determined from the X ray yield measurements for
238 U(n,f) involving four incident neutron energy bins
(black dots): threshold-6 MeV, 11–20 MeV, 20–50 MeV,
50 MeV and above. Average energies are labelled: 3 MeV,
14 MeV, 30 MeV, 190 MeV, and the histograms correspond
to calculations obtained at the average energies with Wahl
systematics [2.2.7].
2.2.3. Results
2.2.3.1. Charge yields
Coincidence between photon and fission
fragment detection in the active target experiment
permitted prompt K X ray measurements [2.2.3].
Due to the large number of K X ray lines (five per
element), the X ray energy spectrum is quite
complex. Nevertheless, the energies and relative
intensities of these lines are known to a high
precision [2.2.4]. Within the limits of the recorded
statistics, the contributions of the different elements
produced for different groups of incident neutron
energy can be de-convolted by fitting the data with
a parametric function of one free parameter per
element. In the 1970s, Reisdorf and collaborators
measured the X ray yields per fission fragment
Y(Z) in spontaneous and low energy neutron
induced fission [2.2.5]. Y(Z) would appear to
depend only weakly on the nature of the fissioning
system. By applying Reisdorf’s prescription to our
X ray yields it is possible to extract an elemental
distribution of the primary fission fragments, as
shown in Fig. 2.2.1. The X ray data have been
corrected for self-absorption and detection
efficiency with point source calibrations and a
Monte Carlo N Particle Transport Code (MCNP)
simulation [2.2.6]. The preliminary results exhibit
the expected increase in the probability for
symmetric fission as one goes towards high incident
neutron energies. Moreover, they agree quite well
with the systematics from Wahl for 14 MeV
neutrons [2.2.7], particularly at 14 MeV. At 50 MeV
and above, the symmetric region gap fills up. These
results agree with the mass distribution measurements
of Zöller et al. [2.2.8] for the same system.
2.2.3.2. Fragment excitation functions
From the inclusive gamma ray measurements,
more than one hundred post-neutron fission
fragments were identified from Ge to Nd. Among
them the even–even nuclei are particularly interesting.
Indeed, the 2 + to 0 + (G.S.) transition of the
ground state rotational band of an even–even
fragment is usually fed at 100% in the de-excitation
process [2.2.9]. Measuring the corresponding
gamma ray intensities gives a measurement of the
fragment production for 23 bins of incident neutron
energies. In some cases, because of pollution or
conversion of the 2 + to 0 + (G.S.) transition, higher
transitions in the G.S. band are used. In cases where
no singles data are usable, coincidence data best
describe the production cross-section, but with
much lower statistics (only three bins of incident
neutron energies). In this way the fragment
production cross-section has been extracted for
about thirty even–even post-neutron emission
fragments as a function of incident neutron energy.
The fragment production excitation functions are
extracted from Ref. [2.2.10] and shown in Figs 2.2.2–
2.2.31 for 92 Sr to 150 Ce. Up to three experimental
FIG. 2.2.2. 92 Sr: best experimental data from the cyan
points divided by 1 × 1 (BR from adopted levels of the
evaluated nuclear structure data file (ENSDF)).
17