reduction in dose rate caused by radon-222 emanation from the
soil.*
The reproducability of individual measurements has also
been investigated at several standard locations in the New York
City area.
In addition, our ionization chamber measurements
have been directly compared with those of other investigators,®
Based on these findings we conservatively estimate stand-
ard deviations of an individual determination of + 10% for th232
and K40, + 20% for u238, and + 0.5 ur/hr for total fallout for
our individual component dose rate values, while + 7% and + 0.5
uxr/hr would be reasonable uncertainties to assign to the total
natural and "best value"
respectively.
terrestrial gamma dose rate values,
These error estimates are of course only educated
guesses and include both systematic uncertainties in the various
calibration factors and experimental and statistical uncertain-
ties involved in individual determinations.
Further comparison
of our data with soil sample measurements, as well as the expected
decrease in fallout contributions as a result of the test ban
treaty, should enable us to evaluate more accurately the validity
of our dose rate estimates, especially the contribution from
the u*38 series.
RESULTS AND DISCUSSION
Table IlIcontains a detailed presentation of the data for
the surveys covered by this report.
Any interpretation of
these data in terms of radiation exposure to the population
should consider carefully the significance of open field dose
rate measurements in terms of their relation to dose rates in
homes, on concrete roadways, in buildings, and at other locations.
We have found that the total dose rate levels inside
typical New England homes are about 70% of the outdoor natural
levels.”
This factor does of course depend strongly on the
type of building material, and in these areas wood frame houses
were quite prevalent.
Furthermore, natural radiation levels
may vary considerably with time due to such factors as snow
cover, soil moisture, and natural fallout.”
Thus, individual
measurements are not necessarily representative of the average
gamma radiation field.
This is particularly true of the isolated spot measurements which constitute a large proportion
of the data reported.
These measurements are of significance
only so far as they fit into a meaningful pattern.