‘
However, two problems arise with the peak rnothod as
applied to fall-out that are not encountered in the case
of the natural emitters. First, the source distribution in
the groundis less well defined than for the natural emitters,
where the assumption of uniform concentration in_ the
ground half-space is a reasonable one on the scale of our
measurements.
Fission products are, of course, not noces-
sarily distributed uniformly in the upper layers of the
ground, nor is the assumption of a uniform plane source
distribution adequate for any but the most recently
deposited fall-out. Measured soil-depth distributions of
the significant y-emitters!*-*? indicate that an exponontial
source distribution as a function of depth with a ��relaxation length’ of 3 cm may be a reasonable model. Of
course, significant deviations from this average situation
would not be unexpected at some locations, particularly
where there has been substantial recent deposition.
Secondly, several radionuclides contribute to the
0-5-MeV peak (Table 1). Since the total y-dose rate per
unit 0-5-MeV flux depends on the decay scheme of the
nuclide or series of nuclides under consideration, the dose-
rate calibration of the 0-5-MeV peak area depends on the
relative population of these nuclides, which in turn is a
function of the mean age of the fall-out. Since rhodium106 dominates the 0-5-MeV activity for fall-out more than
several months old and has an intermediate value for its
peak calibration factor, the use of the rhodium-106
peak calibration and the exponential source distribution
provides a reasonable estimate for the dose contribution
of the 0-5-MeV emitters in most circumstances.
The peak calibration factors for the main fall-out
y-emitters are given in Table 2, along with those for the
natural emitters. Part of the dose rate contribution of
cesium-137 (0-66 MeV) would be included with that of
zirconium-95 {0-75 MeY), since the two peaks overlap in
our spectra, the 0-66-MeV peak generally being completely hidden by the larger 0-75 MeV peak. Otherfall-out
y-emitters generally give only a very small proportion of
the dose rate.
The overall consistency of the two methods for determining the fall-out dose rates, obtained over a wide range
of natural y-fields, and the apparently high degree of
precision of individual total y-dose-rate measurements
suggest that a standard deviation of about + 0-5 pur./h
would be appropriate for individual fall-ont dose-rate
estimates. The individual natural dose-rate components
have standard deviations conservatively estimated at
+ 10 per cent; the total dose-rate values have somewhat
smaller deviations.
Since July 1962, a number of survey trips have been
undertaken to various parts of the United States. These
surveys have generally been motivated by an interest in
natural radiation-levels in certain areas (for example,
6
rC