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

FIG. 4.5.8. Average proton ·Z LÒ i and neutron ·N LÒ i
numbers of the light fragment group for modes S1 and S2
for E p = 10.3 MeV. Full squares: S1, open circles: S2.
the modes S1 and S2 are more stable against
variations of A than the average neutron numbers.
As the peak positions ·Z peakÒ for the two main
asymmetric fission modes are conserved within the
isotopes of an element, and the width of the peaks is
practically independent of A, the composite
asymmetric distribution can be expected to be
Y A (Z) = SY i (Z) (i = S1, S2 and S3) for isotopes of
the same element. Since the total contribution of
asymmetric fission at E p = 10.3 MeV exceeds 80%,
the grouping effect for Y(Z) from the fission of
isotopes of the same element can be observed
directly in experimental distributions without any
preliminary decomposition.
Figure 4.5.9 shows the dependence of the
experimental relative fragment yields at E p =
10.3 MeV on M, N F and Z F of the fragments. As
known for the asymmetric fission of actinides, the
positions of the heavy fragment peaks of the mass
yields stay practically constant for all A, making it
difficult to observe the details of the isotopic
dependences in this part of the distributions. On the
other hand, for the light fragments the peak
positions exhibit a clear dependence on the
fissioning actinide, and there is a distinct grouping
of yield distributions for Np and Am isotopes for
Y(M) and Y(Z F) that is not evident for Y(N F).
Visible differences of the Y(Z F ) curves inside the
Np and Am isotopes are observed only in the
limited regions close to the tops of the peaks and in
the valleys of these distributions, which could be
conditional on the competition between symmetric
and asymmetric fission modes.
The observation of the strong grouping in the
Y(Z F ) distributions for isotopes of the same
200
FIG. 4.5.9. Fragment mass (Y(M)), neutron number
(Y(N F)) and charge (Y(Z F)) yields from fission of
compound nuclei 233 Pa to 245 Bk at E p = 10.3 MeV. Solid
lines: 233 Pa, open circles: 234,236,237,239 Np, full squares:
239,240,241,243 Am, dashed lines: 245 Bk.
element is supported by measurements of the
charge distributions of the Pa isotopes by electromagnetic
excitation of relativistic nuclei on a Pb
target [4.5.30, 4.5.31]. The results are shown in
Fig. 4.5.10 for the four heaviest Pa isotopes (which
have the least contribution of symmetric fission).
This grouping of yields is evident in the coincidence
of all distributions over the whole charge range,
with differences only in the heights of the peaks and