8
LOCKHART AND PATTERSON
19.000
0.735 are artifacts due to the low accuracy in de-
terming p (primarily a function of the total count
during the 61-71 minute decay period) and that
secular equilibrium did exist, the mitial count
during the 1-11 minute decay period can be emRADON CONCENTRATION (pCi/m3)}
ployed to calculate the radon concentration. Such
calculations have been made for the extended
series of collections made on January 5 and
1900
January 7, 1965; the results of both types of
100
measurement are compared in Figs. 9 and 10.
The horizontal axis of the diamond-shaped pattern surrounding the points mdicates the collection time, while the vertical axis indicates the
standard error (a7) in the determination of the
radon concentration based on the uncertainty in
he
the measured activity ratio. The statistical errors
associated with each of the measurements in the
series which assumes equilibrium conditions to
exist are not shown but are considerably smaller
since the determination is based on a single
measurement made during the period of highest
10
a
|
5
|
_
{
10
i)
20
STANDARD ERROR (PERCENT)
activity.
From Figs. 9 and 10 it may be concluded that no
advantage ensues from use of the more compli-
4
25
Fig. 8 ~— Estrmations of the statistical error inherent in
cated
the determination of radon concentrations using filter
samples counted through 10-mil aluminum absorbers
assuming a 20-min sampling period at the rate of
determinations of radon activity made by numer-
0.5 m/min, a background of 30 counts/min, and a 5%
ous investigators who have mvariably assumed
secular equilibrium to exist in the free atmosphere.
contribution of ThB+C and fission products.
having
the
rather
large,
of the
measurements,
random
errors
as-
sociated with low counting rates.
For values of p tn excess of 0.735 to occur in
the free atmosphere, as is suggested by a few
some
mechanism
procedure, at least under the sampling
conditions existing here. Indeed, it permits one
to place more reliance on the results of the many
must
exist there for removing the gaseous radon from
its solid daughters or for fractionating the daughter products relative to one another. Wilkening (3)
has demonstrated one such fractionation process
in which RaA is preferentially removed by the
strong electrostatic charge developed in the
atmosphere dur.ng thunderstorms. This loss of
RaA would interrupt the radon decay chain with
the result that the RaC/RaB atom ratio (p) would
Increase above the equilibrium value. There was
no indication that such a mechanism was in
operation during any of our sample collections;
however, no electric field measurements were
made.
If the assumption ts made that the observed
departures of p from the equilibrium value of
It would appear that, in general, this approach ts
valid. On the other hand, during the developing
stages of a strong temperature inversion in the
atmosphere, or immediately following ventilation
of a mine, tunnel, or similar closed space, de-
partures from equilibrium would be expected
and the simple approach could sertously under-
estimate the radon concentration.
The validity of this approach for determining
radon concentrations from measurements of Its
filterable decay products was tusted by the chamber studies in which a measured quantity of radon
was released into closed spaces of known volume as
described earlier. The data accumulated in two
experiments are summarized in Tables 5 and 6.
In both instances the expected progressive in-
creases in p with time were found. The experi-
mental values of p obtained through use of the
described procedure are compared in Figs. 1!
and 12 with calculated results which duplicate as nearly as possible the actual conditions