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

3.3. EVALUATION OF MASS DISTRIBUTION DATA
FROM 252 Cf SPONTANEOUS FISSION
The mass distribution of 252 Cf spontaneous
fission was evaluated on the basis of seven sets of
available experimental data. The measured data
were modified on the basis of the authors’ adopted
standards and g ray intensities by using our newly
evaluated values, and the originally assigned uncertainties
were adjusted. The evaluated experimental
data were fitted with a spline function. In addition, a
symmetric spline function fit of all experimental
data was performed by symmetric reflection of the
light mass peak into the heavy mass peak. These two
sets of fitted data are recommended as reference for
the mass distribution of 252 Cf spontaneous fission.
Uncertainties in the recommended data were
considerably reduced compared to the measured
values. The light and heavy mass peaks are not
completely symmetric. Fine structure has been
observed in experimental mass distributions around
A = 109–112 and A = 136–139, which is probably
real, but could not be reproduced in the evaluation
due to inconsistencies in the measurement results.
These phenomena and their physical background
should be investigated further.
3.3.1. Introduction
The average number and spectrum of
neutrons from the spontaneous fission of 252 Cf are
used as standards in neutron data measurements
and evaluations, and internationally recommended
data have been published [3.3.1]. The same is true
for the mass distribution data of 252 Cf spontaneous
fission that are used as reference in fission yield
data measurements and evaluations. As requested
by the CRP, these yield data were evaluated and
represent a continuation of our reference fission
yield data evaluation following the completion of
235 U and 238 U yield evaluations in 2001 [3.3.2].
All data were evaluated on the basis of the
available experimental data, and the results are
summarized in this section. The collection,
evaluation and processing of the experimental data
are described in Sections 3.3.2 and 3.3.3, the results
are given in Section 3.3.4 together with a discussion
Liu Tingjin
China Institute of Atomic Energy, China
of some problems encountered, and the conclusions
are given in Section 3.3.5.
3.3.2. Collection and evaluation
of experimental data
As part of our evaluation effort to derive 252 Cf
mass distributions, we collected experimental chain
yield, CU yield and fragment mass yield data, which
we retrieved from the EXFOR data library or
collected as publications found in CINDA and
elsewhere. Since our goal is to evaluate the mass
distribution, we primarily collected and used
experiments in which chain yields and fragment
mass yields were measured for a wider range of
product nuclides. However, chain yields for low
yield product nuclides and CU yields were also
collected and reviewed in case they were needed
(due to the lack of primary data), or to see whether
some supplementary information and data could be
obtained for the evaluation. As a result of this
selection, the following eight sets of data were
considered for the evaluation.
3.3.2.1. Schmitt et al. [3.3.3]
The kinetic energy and flight time of fission
fragments were measured by means of surface
barrier semiconductor detectors. The linear signal
of the detector was used to determine the fragment
energy E, and the flight time was determined by
means of a ‘time pickoff’ scheme in which
transformer coupling was employed. The energy
and mass distributions were deduced from these
energy and time of flight spectra, in which the
energy was calibrated with 79,81 Br and 127 I ions
produced from the ORNL tandem van de Graaff
accelerator and the mass resolution was 1.8 amu for
79 Br (2.25%) and 3.2 amu for 127 I (2.5%). The postneutron
emission mass distribution of the fission
fragments was also given. The data were adjusted
for mass resolution with the following formula:
s d NC( A)
NC( A) = NU( A)
- ¥
dA
2
2 2
2
(3.3.1)
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