GAMMA RADIATION FROM DEPOSITED FISSION PRODUCTS
243
rates; it is expected that this percentage will remain nearly constant
for some time because of the important contribution from long-lived
137Cs5, Any consideration of the degree of exposure of the general pop-
ulation to external gamma radiation from fallout in the environment
must take into account a number of factors that have not been discussed
here, one of the most important of which is that open-field dose rates
are certainly not typical of highly urbanized areas where structural
shielding and the removal of the fission products from hard surfaces
play important roles in reducing the exposure of large numbers of
people. However, our observations during and shortly after the recent
period of nuclear testing indicate that at such times the exposure of the
general population to fallout-gamma radiation cannot be considered
negligible in comparison with that from natural sources,
CONCLUSIONS
The techniques briefly described in this paper enable total environmental radiation dose rates to be measured to an accuracy of close
to 15% (standard deviation) in a typical situation and the individual
components to an accuracy of approximately + 10%. Itseems quite prob-
able that real changes of 0.5 ur/hr or less in the total radiation field
would be unambiguously detected by both the high-pressure ionization
chambers and the spectrometer and that the appearance of the field
spectrum would be noticeably changed if the contributing gamma emit-
ters had not been previously present in significant quantities. It follows
then that such techniques are particularly applicable to any situation
where the possibility of widespread low-level contamination of the environment by gamma-emitting radioisotopes is of concern and the
rapid accumulation of information is of special value,
ACKNOWLEDGMENTS
We would like to acknowledge the assistance and encouragementof
James E, McLaughlin, Director, Radiation Physics Division, in many
phases of the work described herein, as well as the participation of a
_ number of our colleagues, including Keran O’Brien, Robert Sanna, and
Stephen Samson, in some of the survey work. Data on the radioactivity
of soil samples were kindly furnished by L. T. Alexander, U.S. De-
partment
of Agriculture,
J.
A. S. Adams, Rice University, P. F.
Gustafson, Argonne National Laboratory, and A. R. Smith and H. A.
Wollenberg, Lawrence Radiation Laboratory.