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

FIG. 3.3.4. 252 Cf spontaneous fission: light mass peak.
using the monitor yields obtained in our evaluations.
Therefore, the yields for masses 99 and 140
were evaluated first, using the absolutely measured
yields and uncertainties accepted by us, which are
listed in Table 3.3.1. Weighted averages were
calculated using the AVERAG code [3.3.11]. The
recommended yields are listed as ‘mean’ in Table
3.3.1, together with their external uncertainties. One
point of note is that the reduced c 2 (1.4) is larger
than 1 for A = 99, which is due to the deviation of
Schmitt’s data from the others.
3.3.3.2. Spline fitting for evaluated
experimental data
The evaluated data were fitted by means of
SPF [3.3.12], a spline fit code for multiple data sets
with knot optimization. Primary knots were
carefully selected on the basis of the shape of the
curve and were automatically optimized by the
code. The best result was obtained with 26 knots,
and the reduced c 2 was 1.551.
FIG. 3.3.5. 252 Cf spontaneous fission: heavy mass peak.
3.3.3.3. Symmetric spline fitting
TABLE 3.3.1. FISSION YIELD DATA AT A = 99, 140
Author A = 99 A = 140
All models for Gaussian fission fragment mass
distributions are based on the assumption that all
fissions are binary [3.3.13, 3.3.14]. This means that
the primary fragment mass distribution should be
symmetric around mass number A – = (A f – v – )/2 =
124.1, where A f is the mass of the fissioning nucleus
and v – = 3.7661 is the internationally recommended
value of the average number of neutrons emitted
per fission [3.3.1]. A symmetric fit of all the data is
achieved by fitting the experimental yields at
masses A under the light mass peak around A – to
obtain the reflected yields at masses A¢ = 252 – v – –
A, which were then combined with the data for the
heavy mass peak. The combined data were fitted
with a spline function (see Section 3.3.3.2), with a
careful selection and adjustment of the knots to
obtain the best fit (reduced c 2 = 1.795). Finally, the
fitted data were reflected back to the light mass
peak to give a symmetrically fitted mass
distribution.
Chen et al. 2.52 0.13 5.77 0.20
Li Ze et al. 2.55 0.11 5.76 0.18
Flynn et al. 2.48 0.13 5.50 0.28
Nervik 2.57 0.21 6.32 0.51
Schmitt et al. 3.10 0.22 5.81 0.31
Thierens et al. 2.67 0.12 5.35 0.22
Mean 2.583 0.062 5.674 0.098
91