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

FIG. 4.5.10. Fission fragment charge distributions Y(Z F )
for nuclei 229, 230, 231, 232 Pa from Refs [4.5.30, 4.5.31].
the valley, as also observed in the Y(Z F) distributions
for Np and Am isotopes (Fig. 4.5.9).
It can be seen that the main features of the
UCD charge distributions in Fig. 4.5.9 and the
measured fragment charge yields presented in Fig.
4.5.10 (such as coincidence of the curves for
different isotopes at the sides of peaks) look similar.
This result supports the validity of the approximations
used (non-emissive fission and UCD). The
decomposition of the experimental fragment charge
yields into symmetric and asymmetric fission
components [4.5.30, 4.5.31] showed that the
positions of the heavy fragment (asymmetric) peaks
stay constant (Z FH ≈ 54) not only for all Pa isotopes,
but also for the isotopes of U and Th, and more or
less for all actinides with predominantly asymmetric
fission.
Features of the shapes of the asymmetric
fission yields are presented in detail in Figs 4.5.11
and 4.5.12, where the symmetric mode yields are
subtracted and the asymmetric fission yields Y a (Z F )
and Y a (N F ) at E p = 10.3 and 22.0 MeV are displayed
in linear and logarithmic scales. Clearly, the Y a(Z F)
curves for all the nuclides studied coincide for both
light fragments (Z F ≈ 30) close to the magic number
N = 28 and for heavy fragments with Z F ≈ 50 (see
arrows). The mass distributions for the actinide
nuclei are concentrated between fragments with Z FL
≈ 28 and Z FH ≈ 50, and therefore the widths of these
distributions are mainly defined by ΔZ ~ Z CN – 50 –
28, and are almost independent of N CN . Recently, a
similar global grouping effect in fission fragment
yields was revealed in Refs [4.5.32, 4.5.33] for Y(M)
in the thermal neutron induced fission of actinides.
This effect was interpreted as evidence that spherical
closed shells in asymmetric fission fragment
formation determine the position and slopes of the
peaks in the fragment mass distributions.
Figures 4.5.11 and 4.5.12 show even more
clearly the coincidence of the Y a (Z F ) curves from
the fission of isotopes of the same element (Np and
Am), whereas the Y a(N F) curves do not exhibit any
visible coincidence, not even in the vicinity of the
magic neutron numbers 50 and 82 that are marked
by arrows.
The different effects of fragment proton and
neutron numbers on the shapes of Y a (Z F ) and
Y a(N F) are shown in more detail in Fig. 4.5.13,
where the distributions are presented for the fission
of 237,239 Np and 239,241 Am at E p = 10.3 MeV. This set
of compound nuclei consists of two isotopic (Z CN =
93, 95) and two isotonic (N CN = 144, 146) pairs.
Whereas the charge yields within both isotopic pairs
coincide practically over the whole range of
fragment proton numbers, the curves Y a (N F ) for
both these isotonic pairs coincide only in a narrow
range in the vicinities of neutron numbers 52 and 82.
For other ranges of fragment neutron numbers, the
difference of the yields is also rather significant in
the vicinity of N = 88 (arrows). As mentioned
above, the deformed neutron shell 88 is normally
used to explain the properties of S2 as the
predominant asymmetric fission mode. Under these
circumstances, some stabilizing influence of this
shell on the positions and/or shapes of the heavy
fragment asymmetric distributions would be
observed, at least for nuclei with equal N CN . Instead,
very strong stabilizing effects can be detected in
Y a(Z F) for nuclei with equal Z CN.
4.5.5. Systematics of fragment mass yields
Studies of the asymmetric fission modes
have shown that the unchanged charge density
distributions for asymmetric fission Y a (Z F ) =
SY i(Z F) (i = S1, S2, S3) practically coincide for all
isotopes of one element. Therefore, the relative
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