Fission Product Yield Data for the Transmutation of Minor Actinide ...
Fission Product Yield Data for the Transmutation of Minor Actinide ...
Fission Product Yield Data for the Transmutation of Minor Actinide ...
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FIG. 4.5.10. <strong>Fission</strong> fragment charge distributions Y(Z F )<br />
<strong>for</strong> nuclei 229, 230, 231, 232 Pa from Refs [4.5.30, 4.5.31].<br />
<strong>the</strong> valley, as also observed in <strong>the</strong> Y(Z F) distributions<br />
<strong>for</strong> Np and Am isotopes (Fig. 4.5.9).<br />
It can be seen that <strong>the</strong> main features <strong>of</strong> <strong>the</strong><br />
UCD charge distributions in Fig. 4.5.9 and <strong>the</strong><br />
measured fragment charge yields presented in Fig.<br />
4.5.10 (such as coincidence <strong>of</strong> <strong>the</strong> curves <strong>for</strong><br />
different isotopes at <strong>the</strong> sides <strong>of</strong> peaks) look similar.<br />
This result supports <strong>the</strong> validity <strong>of</strong> <strong>the</strong> approximations<br />
used (non-emissive fission and UCD). The<br />
decomposition <strong>of</strong> <strong>the</strong> experimental fragment charge<br />
yields into symmetric and asymmetric fission<br />
components [4.5.30, 4.5.31] showed that <strong>the</strong><br />
positions <strong>of</strong> <strong>the</strong> heavy fragment (asymmetric) peaks<br />
stay constant (Z FH ≈ 54) not only <strong>for</strong> all Pa isotopes,<br />
but also <strong>for</strong> <strong>the</strong> isotopes <strong>of</strong> U and Th, and more or<br />
less <strong>for</strong> all actinides with predominantly asymmetric<br />
fission.<br />
Features <strong>of</strong> <strong>the</strong> shapes <strong>of</strong> <strong>the</strong> asymmetric<br />
fission yields are presented in detail in Figs 4.5.11<br />
and 4.5.12, where <strong>the</strong> symmetric mode yields are<br />
subtracted and <strong>the</strong> asymmetric fission yields Y a (Z F )<br />
and Y a (N F ) at E p = 10.3 and 22.0 MeV are displayed<br />
in linear and logarithmic scales. Clearly, <strong>the</strong> Y a(Z F)<br />
curves <strong>for</strong> all <strong>the</strong> nuclides studied coincide <strong>for</strong> both<br />
light fragments (Z F ≈ 30) close to <strong>the</strong> magic number<br />
N = 28 and <strong>for</strong> heavy fragments with Z F ≈ 50 (see<br />
arrows). The mass distributions <strong>for</strong> <strong>the</strong> actinide<br />
nuclei are concentrated between fragments with Z FL<br />
≈ 28 and Z FH ≈ 50, and <strong>the</strong>re<strong>for</strong>e <strong>the</strong> widths <strong>of</strong> <strong>the</strong>se<br />
distributions are mainly defined by ΔZ ~ Z CN – 50 –<br />
28, and are almost independent <strong>of</strong> N CN . Recently, a<br />
similar global grouping effect in fission fragment<br />
yields was revealed in Refs [4.5.32, 4.5.33] <strong>for</strong> Y(M)<br />
in <strong>the</strong> <strong>the</strong>rmal neutron induced fission <strong>of</strong> actinides.<br />
This effect was interpreted as evidence that spherical<br />
closed shells in asymmetric fission fragment<br />
<strong>for</strong>mation determine <strong>the</strong> position and slopes <strong>of</strong> <strong>the</strong><br />
peaks in <strong>the</strong> fragment mass distributions.<br />
Figures 4.5.11 and 4.5.12 show even more<br />
clearly <strong>the</strong> coincidence <strong>of</strong> <strong>the</strong> Y a (Z F ) curves from<br />
<strong>the</strong> fission <strong>of</strong> isotopes <strong>of</strong> <strong>the</strong> same element (Np and<br />
Am), whereas <strong>the</strong> Y a(N F) curves do not exhibit any<br />
visible coincidence, not even in <strong>the</strong> vicinity <strong>of</strong> <strong>the</strong><br />
magic neutron numbers 50 and 82 that are marked<br />
by arrows.<br />
The different effects <strong>of</strong> fragment proton and<br />
neutron numbers on <strong>the</strong> shapes <strong>of</strong> Y a (Z F ) and<br />
Y a(N F) are shown in more detail in Fig. 4.5.13,<br />
where <strong>the</strong> distributions are presented <strong>for</strong> <strong>the</strong> fission<br />
<strong>of</strong> 237,239 Np and 239,241 Am at E p = 10.3 MeV. This set<br />
<strong>of</strong> compound nuclei consists <strong>of</strong> two isotopic (Z CN =<br />
93, 95) and two isotonic (N CN = 144, 146) pairs.<br />
Whereas <strong>the</strong> charge yields within both isotopic pairs<br />
coincide practically over <strong>the</strong> whole range <strong>of</strong><br />
fragment proton numbers, <strong>the</strong> curves Y a (N F ) <strong>for</strong><br />
both <strong>the</strong>se isotonic pairs coincide only in a narrow<br />
range in <strong>the</strong> vicinities <strong>of</strong> neutron numbers 52 and 82.<br />
For o<strong>the</strong>r ranges <strong>of</strong> fragment neutron numbers, <strong>the</strong><br />
difference <strong>of</strong> <strong>the</strong> yields is also ra<strong>the</strong>r significant in<br />
<strong>the</strong> vicinity <strong>of</strong> N = 88 (arrows). As mentioned<br />
above, <strong>the</strong> de<strong>for</strong>med neutron shell 88 is normally<br />
used to explain <strong>the</strong> properties <strong>of</strong> S2 as <strong>the</strong><br />
predominant asymmetric fission mode. Under <strong>the</strong>se<br />
circumstances, some stabilizing influence <strong>of</strong> this<br />
shell on <strong>the</strong> positions and/or shapes <strong>of</strong> <strong>the</strong> heavy<br />
fragment asymmetric distributions would be<br />
observed, at least <strong>for</strong> nuclei with equal N CN . Instead,<br />
very strong stabilizing effects can be detected in<br />
Y a(Z F) <strong>for</strong> nuclei with equal Z CN.<br />
4.5.5. Systematics <strong>of</strong> fragment mass yields<br />
Studies <strong>of</strong> <strong>the</strong> asymmetric fission modes<br />
have shown that <strong>the</strong> unchanged charge density<br />
distributions <strong>for</strong> asymmetric fission Y a (Z F ) =<br />
SY i(Z F) (i = S1, S2, S3) practically coincide <strong>for</strong> all<br />
isotopes <strong>of</strong> one element. There<strong>for</strong>e, <strong>the</strong> relative<br />
201