EXTERNAL ENVIRONMENTAL RADIATION
MEASUREMENTS IN THE UNITED STATES
L. R. Solon*
W. M. Lowder
A. V. Zila
H. D. LeVine
H. Blatz
M. Eisenbud
Recent interest in the dose to man from natural radioactivity has been
stimulated by the assumption by many geneticists of a linear relationship between radiation dose and the incidence of genetic mutations.
Although this has not been demonstrated at the low dose rates prevailing
in nature, the likelihood of such a relationship has led to the suggestion
that geographical variations in the frequency of spontaneous mutations
may be correlated ultimately with differences in the radiation dose to
populations.+ This question has recently been reviewed by Gopal-Ayengar .@
The studies of the dose received by man from naturally occurring ionizing
radiations can be divided into that received from external and internal
sources. The dose to the germplasm is primarily due to the external
radiation, although one internal source, potassium-40, does deliver a
dose $0,the reproductive organs amounting to about fifteen millircentgens/
year.?
Studies of the radiagion dose from external natural sources have been
reviewed by Sievert,’
Libby,’
and Lowder,? and extensive sets of measure-
ments with particular emphasis on dwellings have been reported by Hultqvist
in Sweden.
6
Although measurements have been made in this country by Hess!
and Neher, © no systematic study of the environmental radiation dose rate
over an extensive area of the United States has been reported previously.
During the Summer of 1957 this Laboratory made measurements in the United
States to establish the approximate range of population exposures to
cosmic and terrestrial gamma radiation. An effort was made to obtain
results which would be representative of the unperturbed natural backGround, affected as little as possible by the occasional substantial
variations in the observed natural radiation levels produced by localized
sources (e.g., proximity of granite buildings, brick paving, fallout, etc.).
Measurements were made with a 20-liter, air-filled, polyethylene-walled
jonization chamber at atmospheric pressure. The chamber was kept inside
an automobile under essentially identical field conditions of loading and
ionization chanbder orlentation.
It had been established previously that
the attenuation by the vehicle did not affect the measured values in an
important way (sbout five percent). The ionization current was measured
* United States Atomic Energy Commission, Health and Safety Laboratory,
aa? York.
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