Fission product yield
Nuclear fissionsplits a heavy nucleus such as uraniumor plutoniuminto two lighter nuclei, which are called fission products. Yield refers to the fraction of a fission product produced per fission.
Yield can be broken down by:
Chemical elementspanning several isotopes of different mass numberbut same atomic number.
#Nuclei of a given
mass numberregardless of atomic number. Known as "chain yield" because it represents a decay chainof beta decay.
Isotope and element yields will change as the fission products undergo beta decay, while chain yields do not change after completion of
neutron emissionby a few neutron-rich initial fission products ( delayed neutrons), with halflifemeasured in seconds.
A few isotopes can be produced directly by fission, but not by beta decay because the would-be precursor with atomic number one greater is stable and does not decay. Chain yields do not account for these "shadowed" isotopes; however, they have very low yields (less than a millionth as much as common fission products) because they are far less neutron-rich than the original heavy nuclei.
Yield is usually stated as
percentageper fission, so that the total yield percentages sum to 200%. Less often, it is stated as percentage of all fission products, so that the percentages sum to 100%.
Mass vs. yield curve
If a graph of the
massor mole yield of fission productsagainst the atomic numberof the fragments is drawn then it has two peaks, one in the area zirconiumthrough to palladiumand one at xenonthrough to neodymium. This is because the fission event causes the nucleus to split in an asymmetric manner. [http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/fissionyield.html]
Yield vs. Z - This is a typical distribution for the fission of
uranium. Note that in the calculations used to make this graph the activation of fission products was ignored and the fission was assumed to occur in a single moment rather than a length of time. In this bar chart results are shown for different cooling times (time after fission).
Because of the stability of nuclei with even numbers of
protons and/or neutrons the curve of yield against element is not a smooth curve. It tends to alternate.
In general, the higher the energy of the state that undergoes nuclear fission, the more likely a symmetric fission is, hence as the
neutron energyincreases and/or the energy of the fissileatom increases, the valley between the two peaks becomes more shallow; for instance, the curve of yield against mass for Pu-239has a more shallow valley than that observed for U-235, when the neutrons are thermal neutrons. The curves for the fission of the later actinidestend to make even more shallow valleys. In extreme cases such as 259Fm, only one peak is seen.
Yield is usually expressed relative to number of fissioning nuclei, not the number of fission product nuclei, that is, yields should sum to 200%.
The table in the next section gives yields for notable
radioactive(with halflifegreater than one year, plus iodine-131) fission products, and (the few most absorptive) neutron poisonfission products, from thermal neutronfission of U-235(typical of nuclear powerreactors), computed from [http://books.elsevier.com/companions/075067136X/pdfs/Yield.bas?mscssid=HAX80JCKT7RB8LS6F675GU2LM83N1CL6] .
The yields in the table sum to only 45.5522%, including 34.8401% which have halflife greater than one year:
thermal neutron neutron absorption cross section
* [http://www-nds.iaea.org/sgnucdat/c1.htm Chain Fission Yields] For 90-Th-232 92-U-233 92-U-235 92-U-238 94-Pu-239 94-Pu-241, and Thermal, Fast, 14MeV.
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