source should be very small because of the inherent low energy of
these secondary particles and the short half-life of most of the
isotopes concerned with these processes.
3.3
EXTRAPOLATION OF BETA ACTIVITIES TO SAMPLING TIME
The activities Ay were extrapolated to the sampling time of each
intermittent fallout collector tray. Ths method of extrapolation was
determined by the amount of decay data obtained from each shot and
varied for each shot.
In gereral, the activity consisted of fission activity decay
and the decay from uranium capture products. The fission products
decayed in a manner which can be represented as:
Ag =
where
Ay,t™
(324)
Ayp= Activity when t =1
Ag
n
Activity at later tim
Time after shot
Decay Exponert
The uranium neutron capture products activity decay can be represented
best as a sum of individual nuclides which can be determined by radio=chemical analysis. The form of the equation would be:
A= bool = Ceti )
where
(325)
A, = Activity due to capture products at
time t
;
= Proportionality constant such that
Keo
where
Age is the zero time activity of the
uranium neutron capture activities
Cy = relative initial activity of nuclide
uy = semi-log decay constant of ith nuclide
The ratio of yield of the various uranium neutron capture nuclides can
be expected not to vary from sample to sample. This is because they
are all uranium isotopes during the time of fallout formation. These
ratios (which determine the Cy s) may be found from capture to fission
ratios determined by radiochemical means.
The relative amount of the uranium neutron capture activity with
respect to the fission activity varies from sample to sample. The
values for Ay¢ and n of Eqe 3.4 were found from the decay curve after
2000 hours when the neutron capture activity no longer contributed
significantly to the sample activity. The difference between this
activity (Eqe 34) and the experimentally determined activity at tims
earlier than 2000 hours was used as a-measure of uranium capture
activity. From this the value of Ag, (in Eq. 305) could be determineds
This difference was measured at the earliest possible time when the
difference was greatest.
37