and *°*T exposure rates were obtained by interpolating from
the data in Table 2 for the source energies in Table l,
multiplying by the number of y-rays emitted per 23°y and
Th disintegration and summing.
232
s
*
.
.
Calculations of the exposure rates
for uniformly
distributed sources in the soil are discussed and evaluated
together with the differential energy spectra,
exposure rate spectra,
A.
integral
and angular exposure rates.
Exposure Rates
The total exposure rates as well as the direct beam or
unscattered component are given in Table 2 as functions of
source energy for various detector heights.
Figure l
illustrates the variation of exposure rates with height for
three different source energies.
Note that the total expoSure rate changes very little in the first 10 meters above
the interface but then begins to drop off fairly rapidly with
height with the lower energy sources falling more rapidly
than the higher energy sources due in large part to the
more rapid decrease of the unscattered component for low
source energies.
The scattered components fall off less
rapidly than the total exposure rate.
Since the calculated
expesure rates for *°K. **°u series, and the 7°*tTh series all
showed
though
single
height
almost exactly the same variation with height even
their source spectra varied considerably (Table 3),
curve is given in Figure 1 for the variation with
of the natural gamma emitters.
The variation with height of
exposure rate, given in Figure 1,
us at a single location by making
measurements at h - 1 meter and h
the natural emitter
was crudely verified by
ionization chamber
= 7 meters above a
predominantly (95%) natural y-radiation field.
The
measured variation in exposure rate with height of 14%
compares reasonably well with the 11% reduction predicted
by Figure 1, and is within the experimental error.
i
te oe
a