products formed per KT of fission yield, plus 1.1 x 10° curies of beta-
active products, for a total of 4.1 x 10° curies of fission product
activity.
The average effective gamma ray energy in a fission product
field is about 0.7 Mev, so that if 1 million curies of mixed fission
products are spread uniformly over a one square mile plane surface, the
gamma radiation intensity measured 3 feet above that surface would be
about 4 r/hr.
The specific yields of radioactive materials have been studied in
%
a variety of ways.
The Hunter-Ballou studies?!
.
on slow neutron fission,
and Coryell-Sugarman compilation2/ ean be used to obtain percentage
values.
Zinc-72 is the atom of lowest mass and gadolynium-160 the
greatest of those found to result from fission.
Figure 1 indicates the
variation of fission yield with mass number, with fission yield exe
pressed as a per cent, for y> uv, and Pu’? ,
The total adds up to
200% since each fission gives two fission products. Fast fission of
uranium-238, plutonium-239, and uranium-235 results in approximately
the same fission yield values with the largest difference in the middle
zone where the value varies from 0.01% for uranium-235 to 0.05% for
uranium-238.
Thus, the various mixtures of fissionable material and
uranium-238 tamper material which may be present in a weapon have relatively little effect percentage-wise on the relative amounts of radioactive isotopes formed.
The first peak in Figure 1 includes strontium-
-89 and strontium-90 and the second peak contains iodine-131, which are
¥
—
Hunter-Ballou studies are an accurate measurement of the percentage
of radioisotopes obtained from a laboratory bombardment of uranium
and plutonium by slow neutrons. The results of the observations in
the laboratory are compared with the percentages of radioisotopes
found in fall-out particles and variations between the labcratory
and test percentages are an indication of fractionation. This
approach is considered the only accurate method possible to calculate percentage yield. ‘The Coryell-Sugarman studies provide similer information about the percentages of isotopes produced in the
laboratory from the bombardment of fissionable materials with fast
neutrons.
a
2/ Hunter, H.F., Ballou, N.E., Simultaneous Slow Neutron Fission of
U-235 Atoms, I. Individual and Total Rates of Decay of the Fission
s, USNRDL ADC-65, 1949.
3/ Coryell, C.D., Sugarman, N., Radiochemical Studies, The Fission Products, ine National Nuclear Energy Series, Book 2;
6
Part V, 1951.