Our ionization chamber measurements at several of the
lake sites were quite consistent with simultaneous measurements by the NYU group although they do appear to be slightly
lower at the higher altitudes.”
This may be due to the
effect on our thicker-walled chambers of a slight change in
the cosmic ray secondary energy spectrum with altitude.
Spectrometric Determination of 137¢s Concentrations in Soils
Spectrometric
number of U.
sites.
S.
measurements were made
at
or near
a
Department of Agriculture - HASL soil samplinc
Estimates of the total activity of 137cs
present in
the soil at these locations were made from the 13’cs dose
rates determined by our spectrometric techniques.*
These
estimates will be compared with radiochemical determinations
of 137cs and 90Sr in recent soil samples taken at these sites
in order to determine the validity of applying our field
spectrometric technigues to study fallout deposition.
We assume fallout to be distributed with depth in the
soil according to the relationship S = So e-2/4 where S
(mc/cn
is the activity at depth z (cm) below the surface and So is
the activity at the surface.*
We routinely use a relaxation
length a = 3 cm and an in situ density of 1.6 gm/em3 (average
over top 6" of soil) to represent a typical situation.
The
flux and total dose rate from such an exponential distributior
depend on the quantity ap, where p is the density.® For a
given a and p the dose rate at one meter above the ground
per 100 mc/mi? of activity can be calculated as can the
expected number of 0.66 Mev 137’cs absorption peak counts for
our detector per unit dose rate and the expected number of
counts per 100 mc/mi2 total soil activity.
In order to illustrate the magnitude of the error
involved in determining the 137¢s dose rate and soil activity
from a spectrum for a particular site the values of the above
guantities are given below for 4 different products of a and p