John Mallinckrodt said, "My guess is that the meter is recording the
rate of beta decay from an initially stable population of Po-218 created
by the alpha decay of Rn-222 in the air. When the replenishing Radon
source is removed, the Po-218 quickly (3 minute half life) alpha decays
to Pb-214."
This is close, but not exactly correct.
If the 222-Rn level would stay constant in a room, then an equilibrium
would establish and eventually all the daughters would be present at the
same level. It is not that one decays building up the next, and then
that decays and builds up the next, etc.
For example, suppose the 222-Rn level in the room would be 20 picocuries
per liter of air (20 pCi/L), and this level is maintained, despite the
3.8-day half life, by a slow influx of radon from natural uranium in the
soil. After only a couple hours, there would be 20 pCi/L of 218-Po, 20
pCi/L of 214-Pb, 20 pCi/L of 214-Bi, and 20 pCi/L of 214-Po because all
these have short half-lifes.
Note... The time for a short-lived daughter to reach equilibrium with a
long-lived parent is determined by the half-life of the daughter. In
this case the 222-Rn essentially has infinite half-life because it is
being replenished. We usually figure an equilibrium is pretty much
established after about five half-lifes of the daughter. Thus, the
218-Po (3.05 min) is essentially at the same level in the room as 222-Rn
after roughly 15 minutes. We will have 214-Pb (26.8 min) in equilibrium
at the 222-Rn level after roughly two hours. Thus, after four or five
hours, all daughters from 218-Po through 214-Po will be at the same
level of activity as the 222-Rn.
The next daughter, 210-Pb (22.26 years) will not reach equilibrium
unless the 222-Rn level is maintained and the room is closed for roughly
100-years.
Thus, if we are monitoring the radon level in a room by looking at the
214-Pb and 214-Bi gammas, we only need to assume that the room air has
been undisturbed for four or five hours so that the level of 214-Pb
measured, or the level of 214-Bi measured, is the same as the 222-Rn
level.
Actually, this is not a very good way to do it because the lead and
bismuth have a stronger tendency to bind to surfaces than the 222-Rn.
Thus, the actual Pb and Bi in the air is generally less than the level
of 222-Rn in the air. Therefore direct measurement of 214-Pb and 214-Bi
in the air is not a very accurate way to measure radon levels.
It would be nice if we could take a swipe of a person's TV or computer
monitor, measure the 214-Pb and 214-Bi on the swipe, and get an accurate
number for the radon level in the room. That would be quick and simple.
But the collection efficiency of the screen is a big, big variable.
We have tried to use electrostatic air filters that operate for a fixed
time, and then we count the collector plates, and try to correlate that
with radon levels measured in a more accurate manner. We have not been
able to get the kind of correlation we would want in order to report a
good radon number to the homeowner. Nonetheless, taking a swipe of the
TV or computer monitor and seeing radon daughters is interesting, and it
is an indication that further radon testing is warranted.
My method for accurately determining radon is a modification of the
charcoal canister and is more accurate than the typical canister method.
I have a calibrated air pump that pulls room air through two canisters
filled with activated charcoal. The pump and canisters are placed in
the room overnight and a known quantity if air passes through the
canisters... Usually about 50 liters. The charcoal traps the radon
itself, as well as the daughters. This can be confirmed by noting the
gammas from the can (after it is sealed and equilibrium has been
established) decays with a 3.8-day half-life.
Unless the room is very humid, the first can grabs all the radon and
daughters and there is no activity in the second can. If the second can
shows activity we might do a retest with three cans, or we can also do a
calculation in which we add both cans together, and also figure a
collection efficiency in the first can based upon how much bled through.
If the various calculation procedures agree, then we don't do a retest.
Most of the time there is no activity in the second can unless the room
was humid. The water apparently fills up a sufficient percentage of the
charcoal sites in the first can at some point allowing radon to get into
the second can.
Anyway, after the 50 L of air have passed through the cans, the inlet
and outlet are capped and the cans are brought back to our lab and
allowed to sit for a couple hours so the 214-Pb and 214-Bi will reach
equilibrium with the 222-Rn. The equilibrium is already mostly
established before the cans come back because the pumping takes place
over about 12 hours. We have a computer program set up in which we
enter the flow rate, the start collection time, the stop collection
time, and the counting time. The program calculates the build-up of Pb
and Bi as well as the decay of radon during the collection, sitting, and
counting times.
Counting is done with a 2" NaI detector connected to a pulse-height
analysis system.
My students and I have been measuring radon in homes for about 10 years.
We do 20-30 homes per year, so we have done over 200 homes. Early on we
got in trouble with the State of Ohio because radon testers in the state
have to be licensed. Licensing mostly consists of taking a test that
almost anyone could pass, plus paying an annual licensing fee of roughly
$1000. I am still not licensed because I refuse to pay what amounts to
an expensive tax. We are not actually in trouble because we do not
charge a fee for our service, and because we inform the homeowner that
we are not licensed and they are not permitted to use our results in any
legal way. We are performing a public service, plus we are doing
research. This is permitted under Ohio law.
However, we believe our measurements are probably the most accurate way
of getting a quick measurement of radon in a home (accurate results in
less than 24 hours). We have eight test kits (pumps and canisters) that
we built into plastic tool boxes. We could test eight homes in one
night, or several rooms in one home. If we find a problem in a home, we
make suggestions about fixes, but we have to be sure to inform the
homeowner that they should not engage in any extensive or costly
remedies based upon our results. There isn't enough space here to
describe the whole process, but we have methods homeowner can use to fix
typical radon problems at a cost to the homeowner (for fans and plumbing
parts) of less than $25, and then we have been able to prove that the
remedy worked. About one-fourth of the homes we have tested have done
some remediation and it has worked in every case except one. One
basement was leaking water through esentially all surfaces of a
flagstone foundation. The radon comes in with the water, and there is
no simple way to fix a basement that is leaking all over.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu