3 arise with the peak methodas
northern New England**, New York State (to be published), and Denver1*). But spectrometric measurements
are not encountered in the case
First, the source distribution in
ned than for the natural emitters,
£ uniform concentration in the
asonable one on the scale of our
were also carried out en route so that we now have spot
readings of natural and fall-out radiation-levels at more
than 200 different locations, fairly well scattered throughout the United States. Since such readings might be
expected to vary quite considerably with time and quite
roduets are, of course, not noces-
mly in the upper layers of the
iption of a uniform plane source
x any but the most recently
ured soil-depth distributions of
'?-3? indicate that an exponential
‘unction of depth with a ‘relaxaay be a reasonable model. Of
ions from this average situation
1 at some locations, particularly
ibstantial recent deposition.
dionuclides contribute to the
possibly with small shifts in location, little significance
can be attached to these data oxcept in so far as they show
# common pattern. The exceptions to this statement are
those regions where a large number of measurements
have keen carried out at nearly the same time (New
England, New York and Denver) and those locations
where the variations with time have been followed (New
York). In such cases, we have a reasonably adequate
measuro of typical natural and fall-out y-levels at particu-
lar times. Unfortunately, to carry out such measurements
on @ national scale would be a tremendous undertaking.
Since the total y-doso rate per
At present, we must rely on the relatively few data
ds on the decay scheme of the
les under consideration, the dose5-MeV peak area depends on the
se nuclides, which in turn is a
that are available and reasonable inferences therefrom to
construct a coherent picture of the influence of deposited
fission products on environmental +-radiation-levels
in the United States.
In order to make data obtained in different locales com-
Of the fall-out. Since rhodium.
’V activity for fall-out more than
parable, every effort was made to conduct our measurements at similar kinds of locations, namely, over flat,
open ground, generally grass-covered, at a distance of at
Jeast 50 ft. from other surfaces. In every case, a survey
of the immediate area was conducted with portable
1as an intermediate value for its
the use of the rhodium-106
'exponential source distribution
“mate for the dose contribution
n most circumstances.
factors for the main fall-out
‘able 2, along with those for the
of the dose rate contribution of
would be included with that of
, since the two peaks overlap in
eV peak generally being com.
scintillation detectors to ascertain that the measured
total dose rate was representative for the area. The
requirement for flatness is an important one, for ground
depressions have often shown elevated y-ray-levels.
Several spectrometer readings have been taken over
depressions of large area where such elevated readings
were observed. As expected, the resulting spectra exhib-
or 0-75 MeV peak. Otherfall-out
ited large fall-out peaks (0-5 and 0-75 MeV). The observed
only a very small proportion of
dose rate increases above the values for adjacent flat
areas generally were consistent with those predicted from
‘ of tho two methods for determites, obtained over a wide range
the apparently high degree of
total y-dose-rate measurements
deviation of about + 0-5 ur. fh
or individual fall-out dose-rate
ul natural dose-rato components
18 conservatively estimated at
dose-rate values have somewhat
the differences in peak areas.
It should be noted that open-field fall-out determinations
may be representative of areas of bare or grass-covered
ground, but not necessarily of all areas which are of
significance with respect to the radiation exposure of the
human population in their daily rounds. Thus, any
consideration of the influence of fall-out on general
population exposure-levels must take into account a
numberof factors not discussed here, such as the distribu-
mber of survey trips havo beon
rts of the United States. These
tion of fall-out on roadways, sidewalks and buildings,
the degree to which the activity is distributed to other
een motivated by an interest in
locations, and the shielding effect of man-madestructures.
For example. Franke et al.24 have reported finding very
In certain areas (for example,
little artificial radioactivity in homes, and then only
6
7
oe.