Variation of Natural Dose Rates with Time
Repeated measurements at sites at Pelham, N.Y., and
Mamaroneck, N.Y., over the past several years indicate that
the dose rate from the natural emitters varies significantl
with time
(see
Table VI).
Such variations have
also been
The major causes for
reported by other investigators.?%’
such variations are changes in soil water content which res
in increased attenuation of gamma rays passing through the
ground and changes in the dose rate from the uranium series
The effect of increased soil water content on dose rat
from both a uniformly distributed source (natural emitters)
and an exponentially distributed source
(fallout emitters)
was calculated using our eguations for the dose rate from
emitters in the soil.*
For 1.46 Mev 40K y-rays the ratio o
primary flux to total dose rate, which is the factor determ
our field spectrometric dose rate estimates, changes by les
than 5% when the soil water content increases from zero to
The corresponding actual dose rate decrease
30% by weight.
A soil water content of 30% by weight is not
is about 30%.
at all uncommon during wet periods.
For 137Cs 0.66 MeV y-r
the same change in water content results in a 5% reduction
in the flux to total dose rate ratio while the total dose
Thus the effect on our
rate decreases by about 17%.
spectrometric dose rate estimates is small in both cases
while the significant effect on the total dose rate is
greater
for the uniformly distributed natural
emitters
than
for the closer to the surface fallout emitters.
The dose rate from the uranium series is affected by
soil water in two conflicting ways.
The clogging of the
pores in the soil impedes the escape of radon from the soil
air into the atmosphere.
If this condition persists for a
few days, radon levels in the soil air will build up toward:
226Ra-222Rn equilibrium resulting in an increased dose rate
from the radon daughters.
The additional soil water,
however,
also increases the y-ray attenuation as in the
case of the other natural emitters.
It is thus not unusual
to find decreases in 49K and 232th dose rates accompanied
by either a slight increase or no change at all in the 238y
series dose rate.