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

TABLE 4.6.3. FIT PARAMETER VALUES FOR THE 232 Th MASS YIELD CURVES
The first two columns contain the values of the fit parameters for the single Gaussian mass and charge distribution
fit, see Eq. (4.6.1) (referred to as symmetric). In the third and fourth column the parameter values that
describe the decomposition of the mass distribution can be found, see Eq. (4.6.5) (referred to as decomposed).
The meaning of the parameters is explained in the text. The fission cross-sections as obtained from the fit
parameters are given as well as the value of Ref. [4.6.25]. The uncertainties in the fit parameters indicate the
sensitivity of the fit. The uncertainties in the fission cross-sections include a 10% systematic uncertainty that
arises from the uncertainty in the proton flux.
Symmetric fit parameters Value Decomposed fit parameters Value
lA 30.7 ± 0.5 s1 52 ± 4
MA 111.5 ± 0.2 s2 107.3 ± 0.4
GA 23.15 ± 0.14 s3 14.7 ± 0.6
— — m15.3 ± 1.0
— — m2110.0 ± 0.3
— — m330.2 ± 2.2
— — m411.2 ± 0.7
— — m527.7 ± 0.3
— — m66.5 ± 0.4
g1 0.820 ± 0.012 g1 0.345 ± 0.012
g2 0.00786 ± 0.00013 g2 0.0129 ± 0.0002
m1 1.92 ± 0.02 m1 2.07 ± 0.02
m2 0.3910 ± 0.0002 m2 0.38953 ± 0.0004
c 2
12.8 c 2
5.9
exp
s f [mb] 630 ± 60
exp
s f [mb] 950 ± 95
Eismont
s f [mb] 1236
Eismont
s f [mb] 1236
symmetric component increases with excitation
energy, which is coupled to an increase of neutron
evaporation resulting in a larger contribution of the
neutron deficient nuclides with a higher fissility
parameter. The excitation energy for which
symmetric fission starts to contribute gives rise to a
number of evaporated nucleons, corresponding to a
certain neutron deficient nucleus. This nucleus is
held responsible for the symmetric contribution
and, hence, marks the position of the critical fissility
parameter. From the data they have extracted an
expression for this critical value as a function of
nuclide charge:
(Z 2 /A) C = 35.5 + 0.4(Z – 90). (4.6.4)
This criterion is compared to theoretical results in
Section 4.6.4.
When compared to other experimental data,
this criterion turns out to contradict these results.
216
Measurements of proton induced fission of Th
between 8 and 22 MeV have been performed by
Kudo et al. [4.6.12]. According to the assumption
made by Chung and Hogan, the mass distributions
in their experiment should be purely asymmetric,
since all the fissioning systems that contribute
have a Z 2 /A smaller than the critical value.
However, the mass distributions in their experiment
reveal a symmetric component as well. Another
experiment by Schmidt et al. [4.6.29] on fission of
neutron deficient actinides shows a clear transition
of mixed symmetrical and asymmetrical charge
distributions towards purely symmetrical charge
distributions with decreasing neutron number.
Several attempts other than the one by Chung
and Hogan have been made in the past to describe
and understand the varying fission characteristics of
nuclides with excitation energy (e.g. Refs [4.6.30–
4.6.34]). An overview can be found in Ref. [4.6.35].
Moreover, Brosa et al. [4.6.36] have developed a