- Fission product yield
Nuclear fission splits a heavy nucleus such asuranium orplutonium into two lighter nuclei, which are calledfission products .**Yield**refers to the fraction of a fission product produced per fission.Yield can be broken down by:

#Individualisotope

#Chemical element spanning several isotopes of differentmass number but sameatomic number .

#Nuclei of a givenmass number regardless ofatomic number . Known as "chain yield" because it represents adecay chain ofbeta decay .Isotope and element yields will change as the fission products undergo beta decay, while chain yields do not change after completion of

neutron emission by a few neutron-rich initial fission products (delayed neutron s), withhalflife measured 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

percentage per 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

mass or mole yield offission products against theatomic number of the fragments is drawn then it has two peaks, one in the areazirconium through topalladium and one atxenon through toneodymium . 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 ofuranium . 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

proton s and/orneutron s 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 energy increases and/or the energy of thefissile atom increases, the valley between the two peaks becomes more shallow; for instance, the curve of yield against mass forPu-239 has a more shallow valley than that observed forU-235 , when the neutrons arethermal neutron s. The curves for the fission of the lateractinides tend to make even more shallow valleys. In extreme cases such as^{259}Fm, 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 (withhalflife greater than one year, plusiodine-131 )fission product s, and (the few most absorptive)neutron poison fission products, fromthermal neutron fission ofU-235 (typical ofnuclear power reactors), 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:

**Ordered by**halflife **Ordered by**thermal neutron neutron absorption cross section **References*** [

*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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