?
a
%Zr—-“Nb contributing 60-80 per cent of the fall-out dose-
rate. This is consistent with Gustafson’s results?? during
a comparable period in 1959, when more than 90 per cent
of the inferred fall-out y-dose rate at a location at Argonne
National Laboratory was found to be derived from these
isotopes. Peirson and Salmon? obtained similar results at
several locations in Great Britain at about the same time.
The Health and Safety Laboratory has therefore developed techniques for the accurate determination of environmental y-dose rates and the partial dose-rate contributions
of the significant natural and fall-out emitters utilizing
high-pressure ionization chamber and y-spectrometric
ao
“a
measurements directly in the field.
These techniques
depend on reasonable assumptions as to the distributions
in the environment of the various radioisotopes and on
relatively unsophisticated procedures for interpreting the
spectral data.
Measurements at more than 200 sites in the United
States indicate that the open-field y-dose rates from
natural sources do not vary over a wide range from place
to place in this country, generally falling between 5 and
9 ur.fh. In those areas where high natural levels (up to
20 ur.fh) have been observed, high thorium and potassium
content of the soil is generally responsible. The relatively
small magnitude and narrow range of the observed doserate contributions from the uranium series are interesting
results deserving of further study, and are probably
related to the leaching of uranium-238 and radium-226
from weathered material and the migration of free
radon-222 from the upper layers of the soil’.
During the second half of 1962 and 1963 it was observed
that y-emitting fall-out in the environment had elevated
the y-background by a measurable amount at every location and that increases of 50 per cent or morein the total
y-level appeared to be quite common over open ground.
The highest fall-out levels were observed during the spring
and summerof 1963; the autumn 1963 readings were lower,
and generally comparable with those a year earlier in the
western States and in northern New England. <A decline
in fall-out levels set in at the New York City area sites in
Septemberof 1963 after at least four months of quite large
elevations above the normal background-levels.
These
results are consistent with those of Burch et al.44 in England between August 1962 and April 1963, who recorded
with an ionization chamber somewhat elevated readings
at one location during August-November 1962 and then
a jump to roughly 50 per cont above normal y-background
in November, which was maintained except for the effect
of snow coveruntil] at least April 1963. There is also rough
agreement with Gustafson’s data®*, which seem to indicate
a rather steady increase in fission product dose rate at a
site at Argonne National Laboratory from 2 ur./h to
7 pr.fh between September 1962 and July 1963 and then a
i2