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The isobaric radionuclide chains formed in the explosion are known
to be distributed on a mass scale in a way generally similar to the products of asymmetric fission of y235 by thermal neutrons, but with some
important differences,
The experimental yield curve for slow neutron
fission has a broad minimum for mass numbers approximately one-half
that of the original nucleus and maxima on cither side at mass numbers
in the neighborhood of 95 and 139 (Reference 7).
Comparing the chain
yields for megaton-range detonations with this curve, it is noted that there
is a small drop in the peak yields accompanied by an increase in the symmetric fission probability.
The same nuclide distribution might be ex-
pected in the fallout material and this is found to be roughly true under
certain conditions,
In other cases, the elements formed initially partially separat
with respect to
/one another so that samples of fallout may differ in composition
among themselves and also from the distribution curve characteristic for
the event,
Fractionation is a term which has been applied to this phenomenon
and it is used to signify an alteration in nuclide composition of some portion
of the debris which renders it non-representative of the bomb products as
a whole,
The R-values, which are commonly used for reporting radiochemi-
cal data on cloud and fallout samples, are useful indices of fractionation.
The R-value for any nuclide is defined as the ratio of the number of atoms