The dose rate from the 238U series may also be increased
.,
due to “natural fallout".
and
the free radon in the top few inches of soil escapes into
The resulting dose
the atmosphere and is widely dispersea.**
rate from gamma emitting daughters in the atmosphere usually
cate that
nificantly
so been
iuses
amounts to only a few tenths of aur/hr.*
for
which result |
‘ough the
emitters)
mitters)
ite
from
le ratio of
air concentrations would be expected to
However,
fairly similar to those over nearby land areas.
the radon daughter
these particulate gamma emitting daughters are effectively
fF washed out of the atmosphere during rainfall and thus the
This washout results in a fairly
term “natural fallout".
30 min.) gamma
ft high concentration of short-lived (T 1/2
emitting radon daughters close to the soil surface and a
corresponding sizeable temporary increase in the 238uU series
‘or determining
dose rate.
jes by less
1 zero to
> decrease
jht is not
96 MeV y-rays
reduction
:al dose
ur
1 cases
-e is
‘ters than
scted by
of the
1 the soil
sts for a
up towards
dose rate
-er,
2% unusual
mpanied
1 the
To study the variation of natural gamma radiation with
time experimentally, in May, 1965, HASL and NYU attempted to
monitor the radiation background continuously for several
days over a soil location at Greenwood Lake, N.Y.
Measurements were made hourly of the total gamma dose rate with both
our lonization chambers and an NYU plastic-walled chamber.
+ The temperature, barometric pressure, relative humidity, and
f wind speed were also monitored.
Pulse height spectra were
accumulated every two hours and air samples of a activity at
' ONe meter above ground were taken every two hours.
During a four day period the soil moisture content did
not change appreciably, and no significant changes were
- Observed in the 49K and 232Th dose rates.
238y
The radon daughter
concentrations at one meter above the ground inferred from
the air samples increased markedly during the calm evening
and early morning hours.
rate,
The corresponding 238uU series dose
inferred from the spectra,
' changes,
did not show any significant
indicating the radon daughter distribution in the
SOil air was not building up significantly.
-n the
This was substan-
tiated by our spectral measurements over large lakes where
f be
um series.
1 dose rates
Under normal conditions most of
| COrrelation in the variation of the
The slight
?38u series dose rate
and radon daughter concentration in the air which was detected
was probably due to a slight increase in the dose rate from
the airborne daughters.
Large variations in relative humidity
# Often reaching into the high 90's resulted in a multi-channel
analyzer failure early in the third day and thus the
21 -