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

FIG. 3.3.8. Comparison of regular and symmetric spline fit:
heavy mass peak.
observation is unlikely to be attributed to experimental
uncertainties, but can rather be interpreted
as a systematic trend.
A more likely explanation involves the small
additional contribution of ternary fission or other a
or light particle fission processes. Investigations by
Mehta et al. [3.3.15] have shown that a particle
fission is about 1% of binary fission, and there is a
shift to the left in the mass distribution of such
processes compared with ordinary binary fission.
However, the shift is larger for the light mass peak
and smaller for the heavy mass peak. Furthermore,
the light mass peak becomes narrower, with the half
width at the half maximum changing from about 7.5
to 6.8 amu (see Fig. 4 of Ref. [3.3.15]). a or other
light particle fission also increases v – , which is
reflected in the value derived from the fitted yield
data (Table 3.3.2, normal fit), which is larger than
the internationally recommended value based on
neutron emission measurements.
The experimental and fitted data exhibit a
shoulder on the right hand side of the light mass
peak (at mass numbers 109–112), and correspondingly
on the left hand side of heavy mass peak (at
mass numbers 136–139). Discussions in many
papers suggest that there is a fine structure in the
mass distribution of 252 Cf spontaneous fission [3.3.3,
FIG. 3.3.9. Schmitt et al. data: comparison of heavy and
reflected light mass peak.
3.3.6, 3.3.10]. However, the observed fine structures
extend only over a few (2–4) mass numbers, are very
narrow, and differ in their shapes and positions in
the different measurements. Therefore, such fine
structures could not be confirmed — measured data
may not be real but could be the result of statistical
fluctuations. Due to the disagreement among
measurements, the fine structure was washed out in
our evaluation, where seven sets of experimental
data were used; shoulders appear in the mass ranges
A = 109–112 and 136–139, where there should be
structures. These shoulders are systematically
consistent with the mass ranges where the
experiments exhibit fine structure, and hence this
effect may exist in reality. More definite conclusions
could be made about the existence and positions of
possible fine structure and permit their evaluation if
new measurements were made with high accuracy
and/or better mass resolution.
Comparing the uncertainties of spline fitted data
(without and with symmetric reflection) with the
uncertainties of the evaluated experimental data (see
Table 3.3.4), the uncertainties of the fitted data are
shown to be considerably reduced, especially in the
regions of the peaks (from 3–10% to about 1.1–1.5%).
TABLE 3.3.3. INTEGRAL YIELDS UNDER LIGHT AND HEAVY
MASS PEAK BY DIFFERENT AUTHORS
Author
Li Ze et al. Chen et al. Schmitt et al. Fraser et al.
Light 99.76 98.43 99.88 97.34
Heavy 102.08 100.97 99.99 97.98
93