tions will be published in future reports from the Health
and Safety Laboratory.
Most of the inferred natural y-dose rates are in the
range 5-9 pur./h, and the average forall locations surveyed
is 7-0 ur.jh. The dose-rate contributions from potassium40 and the thorium series are generally comparable.
usually amounting to approximately 80 per cent of the
total natural level. The uranium series contribution
seldom accounts for more than 25 per cent of the total
natural y-dose rate.
High natural levels have been
encountered in northern New Hampshire, Denver and
Colorado Springs, Colorado, and in an srea of North
Carolina north-east of Raleigh. These high levels are
attributable to high thorium and potassium content of the
soils, particularly the former.
At almost every location, the observed fall-out y-dose
rate is a significant fraction of the total. Between July
1962 and September 1963. dose rates of 3-4 ur./h seemed
to be typical of the eastern United States. Somewhat
lower levels were encountered in some areas in the western
States, which may be related to relatively less rainfall.
For example, the measurements in the San Francisco Bay
area were carried out after periods of very little rainfall.
The highest fall-out readings were observed during the
spring and summerof 1963, when levels of 4-6 ur./h were
found at a numberoflocations scattered throughout the
eastern states.
Beginning in May 1963. an effort was made to monitor
tho changes in fall-out y-levels in the New York City area
by repeated measurements at several locations. Three
flat grassy areas in Westchester County, New York, were
chosen. separated by about 5 miles from one another.
The spectrometer readings indicated that the natural
y-dose rates at the three locations were all approximately
7 ur./h. The measured total y-dose rates were generally
slightly more than 12 ur./h until September. Our best
estimates for the fall-out y-levels at these sites are shown
in Fig. 2. These data provide strong evidence that open
field fall-out y-levels in the New York area remained
roughly constant at a high level (~ 5 pr./h) from at least
May until September 1963 and then decreased, roughly
exponentially with an apparent ‘half-life’ slightly longer
than that of ™Zr-**Nb.
This is consistent with the
evidence provided by the field spectra that ™Zr—*Nb
has been contributing 60-80 per cent of the fall-out dose
rates during the periods of measurement.
During the 1962 survey trip to the Pacific coast, a
specific effort was undertaken to obtain measurements on
the northern part of the Olympic peninsula (Clallam Co.),
Washington. Over a distance of approximately 50 milos,
the mean annual rainfall varies by a factor of nearly ten
from east (Sequim) to west (Forks); under such conditions
one might expect a substantial variation in the fall-out
10
eo