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

(dot–dashed curve in Fig. 3.1.4) — the calculated
contribution of increases rapidly; and at En ≥
40 MeV, the contribution is higher than that of
. The assumption of rather weak damping of
the triaxial collective contributions at the outer
saddle deformations with Ur = 30 MeV and dr =
10 MeV (Eq. (3.1.13)) is not helpful: calculated r sym
sym
s nF
sym
s nF
asym
s nF
overestimates data in the energy range 30 to
100 MeV, but underestimates severely the data at En ~ 100 to 200 MeV (double dot–dashed curve in Fig.
sym
3.1.4). The contribution of s reaches that of
nF
asym at En ~ 45 MeV, but decreases sharply at En ≥
100 MeV (see Fig. 3.1.4).
The r sym data measured with a tagged photon
beam in the range Eg = 60–240 MeV give a much lower
contribution of symmetric fission to the observed
photo-fission cross-section [3.1.42]. This low level for
the r sym data can only be reproduced if a much
higher contribution of fissions from very neutron
deficient U nuclides can be adopted, and this
requirement can only be achieved by introducing a
much stronger and unrealistic energy dependence
of the asymptotic value of the af parameter.
The observed r sym s nF
ratio corresponds to
different masses of fissioning U nuclei due to prefission
neutron emission. As a consequence of the
higher symmetric fission barrier, the number of
nuclides that make an appreciable contribution to
the ‘observed’ symmetric fission cross-section is
lower than in the case of asymmetric fission
sym sym
events. Partial branching ratios rx = s nxnf , /
sym asym
238 238
(s + s for U(n,nf), U(n,3nf),
238 238
U(n,8nf) and U(n,17nf) are shown in Fig. 3.1.5.
Partial contributions to the 238U(n,xnf) sym nxnf , nxnf , )
and
238 asym
U(n,xnf) reactions correspond to the fission
sym
cross-sections shown in Fig. 3.1.1. rx for lower
fission chances reaches higher values at lower
incident neutron energies; and for the 238 U(n,nf)
reaction, the maximum of r
is reached at En ~ 40 MeV. The ratio
sym
rx for higher chances x = 3 or x = 8 has a similar
sym
shape, while rx at x = 17 never reaches the
maximum value in the excitation energy range
under consideration.
sym sym
1 = s nnf , /
sym asym
(s nnf , + s nnf , )
3.1.3.3. Other actinides
3.1.3.3.1. 235 U(n,f)
Simple systematics of the level density
parameters ((dW fBSL – dW fBS1(S2) ) ~ 3.5 MeV,
dW fS1(S2) ~ 0.6 MeV) and the fission barrier ((E fBSL-
E fBS1(S2) ) ~ 3.5 MeV, ћw BSL ~ 2.25 MeV) for U
34
FIG. 3.1.5. Ratio r sym of symmetric to symmetric plus asymmetric
fission events for 238 U(n,f). The solid line is the same
as in Fig. 3.1.4; dashed lines correspond to the respective
ratios r s x ym for partial 238 U(n,xnf) reactions with x = 1, 3, 8, 17.
nuclides reproduces the observed 235 U(n,f) [3.1.15]
and 233 U(n,f) [3.1.16] fission cross-sections.
Figure 3.1.6 shows calculated symmetric
235 U(n,f) sym , asymmetric 235 U(n,f) asym and symmetric
+ asymmetric 235 U(n,f) fission cross-sections for the
damping of axial and triaxial deformations at outer
super long mode saddles. The energy dependent
asymptotic value ã f (A, U) of the a f parameter (see
Eq. (3.1.16)) is employed. Data by Lisowski et al.
[3.1.15] for the 235 U(n,f) reaction below 20 MeV are
compatible with earlier data by Poenitz [3.1.44],
Czirr and Sidhu [3.1.45] and Leugers et al. [3.1.46].
Solid lines show the symmetric cross-sections
( 235 U(n,f) sym ), and dashed lines show the
asymmetric cross-sections ( 235 U(n,f) asym ) for the
neutron induced fission of 235 U. The sum of
235 U(n,f) sym and 235 U(n,f) asym (dash–dotted line)
agrees reasonably well with the observed
235 U(n,f) sym up to En < 200 MeV.
3.1.3.3.2. 233 U(n,f)
The fissility of 233 U is much higher that that of
235 U. Figure 3.1.7 shows symmetric 233 U(n,f) sym ,
asymmetric 233 U(n,f) asym and symmetric plus
asymmetric 233 U(n,xnf) fission cross-sections. The
lines in Fig. 3.1.7 have the same meanings as in
Fig. 3.1.6. There is a fair number of measured data
for the 233 U(n,f) reaction cross-section at incident