INTRODUCTION
For the past several years, HASL has maintained a continuing interest in the natural radiation environment, with particular
emphasis on: its more penetrating components.’
Sensitive ionization chambers have been developed to measure the total dose
rate from both terrestrial and cosmic radiation sources.*
The
presence of significant quantities of gamma-emitting fission
products in the environment as a result of nuclear weapons test-
ing necessitated the development of techniques for inferring the
contribution of fallout to the total measured dose rate.
Relatively simple methods of analysis have been developed which allow
reasonably precise estimates of the individual components of the
terrestrial gamma radiation field to be made from a pulse height
spectrum obtained in the field with a 5" x 3" NaI(T1l) detector.
This report summarizes these methods® and presents the
total terrestrial and individual component dose rates determined
from spectrometric and ionization chamber measurements taken in
1962 and 1963 at numerous locations in the southeastern, central
and western United States.
Most of these locations were grouped
in a few areas of particular interest (e.g., Denver and Colorado
Springs, Colo.;
the San Francisco Bay area;
the Olympic Peninsula,
Washington; and a region near Aiken, S.C.) for studying the nat-
ural radiation environment and for verifying our methods of
determining dose rates.
A number of spot measurements were also
taken enroute to these areas from our laboratory in New York City.
SURVEY TECHNIQUES
Choice of Site
Survey locations were usually city parks, school or church
lawns, or vacant lots providing fairly large (at least 30 ft.
in diameter), grassy, flat areas, whose soil appeared to be
fairly typical of the locale.
The requirement for flatness is
important for two reasons: first, the interpretation of the
pulse height spectra in terms of dose rate relies on the assumption of relatively uniform distribution of emitters ina flat
"half space"; second, ground depressions often show elevated
fallout gamma dose rate levels.