ts
RsitAR“RONNIEmere «=
13
NRL REPORT 6374
of aerosol, little of the radon daughter activity
remains airborne. The progressive decrease in
observed radon with time is in accord with the
loss of aerosol through coagulation and deposition,
during which umethe relative surface area of the
wall increases. The wall effect is greatest in the
large chamber, which contains so much perma-
nently mounted equipment that it has a large
surface-to-volume ratio. The apparentincrease in
radon concentration with introduction of addi-
tional smoke, as noted in Table 5, is to be expected
if the relative surface areas of aerosol and wall
are of significance.
The application of correction factors based on
the p values does give a rather high degree of
internal consistency to the measured radon con-
centrations, except for the first few measurements
involving extreme degrees of nonequilibrium.
Even these fall reasonably well into agreement
when corrections are applied for the rapid changes
ter as
in p occurring during the early collections (see
Appendix). In any case the corrected values are
morerealistic than those derived from the assump-
tion of secular equilibrium between radon and
a ee aes erp
its daughters. The discrepancy between the ex-
perimental and the true radon concentration
should not be encountered in measurements made
of the radon concentration in the free atmosphere,
since in that environment interaction with the
ground surface would be much morerestricted.
On the other hand, use of this technique to
measure radon concentrations in mines, tunnels,
or other closed spaces might be expected to show a
products may be conveniently determined by
10-min counts starting 1.0 min and 61.0 min
after a collection of 20.0: min duration; counting
on calibrated equipment with a 10-mil aluminum
absorber inserted between the sample and count-
ing tube gives a good compromise between sensitivity and the uncertainues introduced by the
complex group of conversion electrons emitted
and by counting statistics.
This method will detect any gross departures
of radon in the atmosphere from secular equilibrium with its daughter products and permits a determination of the apparent age of the mixture.
In the free atmosphere radonis apparently near
secular equilibrium with its daughters at most
times; however, during initial stages of inversions
and in recendy ventilated spaces equilibrium
departures can be expected. Chamber studies
using artificial aerosols indicate a loss of radon
daughter products to the walls or other surfaces
and suggest that underestimation of radon by
this methoct may result when the surface-tovolume ratio is high or when mixing promotes
intimate contact between the air and a surface.
ACKNOWLEDGMENT
The authors wish to express their apprectation
for help given them during the chamberstudies
by Dr. James A. Young and Mr. Wendeli L.
Anderson of the Protective Chemistry Branch,
NRL.
wall effect whose magnitude would depend on the
configuration of the restraining walls and the rate
of mixing within the volume.
CONCLUSIONS
Reasonably accurate estimations can be made of
the radon concentrations in the free atmosphere
by collection on efficient filters of the shortlived descendants of radon and measurement of
their $8 activity during two fixed periods fol-
lowing collection. The measurement of the extent
REFERENCES
1, Lockhart, L.B., Jr, Patterson, R.L., Jr., and Hosler, C.R.,
“Determination of Radon Concentrations in the Air
Through Measurement of Its Solid Decay Products,” NRL
Report 6229, March 1965
2. Lockhart, L.B., Jr., Patterson, R.L., Jr., and Anderson, W.L.,
“Characteristics of Air Filter Media Used for Monituring
Aurborne Radioactivity,” NRL Report 6054, March 1964
. Wilkening, M.H.. “Radon Daughter [ons in the Atmosphere,” Chapter 21 in “The Natural Radiation Environ-
we
*n Was
of secular equilibrium of radon andits daughter
nent,” J.A.S. Adams and W.M. Lowder, editors, Chicago:
The University of Chicago Press (1964)