correlation study between radon daughter levels and the 238uU
series dose rate (specifically the counts in the 1.76 Mev
214Ri peak) could not be completed.
The total gamma dose rates were relatively constant
during this four day period with a variation of only a few
tenths of a ur/hr except for the period immediately followinc
a brief heavy shower on the third day.
Measurements
immediately after the shower with our high pressure ion-
ization chambers were significantly higher indicating the
presence of "natural fallout".
{A similar effect was
obtained at the Elko, Nev.
site
(see Table IV) on our recent
field trip.)
The NYU dose rate measurements fell, however,
and remained depressed for several hours.
This was probably
due to the greatly enhanced attenuating effect a small incre:
in soil moisture had on the @-ray component which accounted
for a large proportion of the total ionization in the NYU
chamber.
The dependence of free air B-ray ionization from
emitters in the soil on isotope distribution and soil
moisture suggest that spot measurements of environmental
radiation with thin walled chambers would be very hard to
interpret.
Fluctuations in natural dose rate with time necessitate
that care be taken in interpreting the data obtained from a
Single set of measurements at a site.
The greatly reduced
natural levels encountered at many of our midwestern and
central U.S. locations in 1965 compared to our 1962 and 1963
measurements indicate the danger of misinterpreting spot
measurements.
Such spot measurements are useful when a
large number of them can be used to deduce a pattern over a
large area.
We intend to continue to study the effect of the
migration of radon on natural dose rates, and a more careful,
controlled monitoring experiment may be undertaken.
If
small differences in exposure to penetrating radiation prove
to be biologically significant, then the fluctuations in the
natural gamma dose rate level have to be considered in
studies of population exposure.