4 RESULTS
4.1 How Calculation Is Made
Th
e RAD7 calculates the sample water concentration
by multiplying the air loop concentration by a
fi
xed
conversion coe
ffi
cient that depends on the sample
size.
Th
is conversion coe
ffi
cient has been derived
from the volume of the air loop, the volume of the
sample, and the equilibrium radon distribution
coe
ffi
cient at room temperature. For the 40mL
sample volume the conversion coe
ffi
cient is around
25. For the 250mL sample volume the conversion
coe
ffi
cient is around 4.
Th
e RAD7 does not presently make any correction
for the temperature of the water sample. In theory,
such correction would slightly improve the analytical
accuracy for the larger (250 mL) sample volume, but
would make little or no di
ff
erence for the smaller
sample volume.
4.2 Decay Correction
If you collect a sample and analyze it at a later time
(rather than immediately), the sample's radon
concentration will decline due to the radioactive
decay. You must correct the result for the sample's
decay from the time the sample was drawn to the
time the sample was counted. If the sample is
properly sealed and stored, and counted within 24
hours, then the decay corrected result should be
almost as accurate as that of a sample counted
immediately. Decay correction can be used for
samples counted up to 10 days a
ft
er sampling, though
analytical precision will decline as the sample gets
weaker and weaker.
Th
e decay correction is a simple exponential function
with a time constant of 132.4 hours. (
Th
e mean life of
a radon-222 atom is 132.4 hours, which is the half-
life of 3.825 days multiplied by 24 hours per day
divided by the natural logarithm of 2.)
Th
e decay
correction factor (DCF) is given by the formula DCF
= exp(T/132.4), where T is the decay time in hours.
You will notice that decay times of under 3 hours
require very small corrections, so you can ordinarily
neglect the decay correction for samples counted
quickly.
To correct your result back to the sampling time,
multiply it by the decay correction factor (DCF) from
the chart, Figure 6 opposite.
4.3 Dilution Correction
If you intend to count samples that have very high
radon concentrations, you may wish to dilute the
sample by a
fi
xed ratio, then correct the result back to
its undiluted concentration.
Example: You take a 4mL sample and dilute it with
36mL of distilled water in a 40mL sample vial.
Overall, this would be a 10:1 ratio of
fi
nal volume to
initial volume, so you must multiply the result by 10
to correct for the dilution. If the RAD H
2
O reports a
result of 9,500 pCi/L for the 10:1 diluted sample, then
the original concentration must have been 10 X
9,500, or 95,000 pCi/L. Great care must be taken in
this process to avoid loss of radon from the sample.
Th
e syringe should be
fi
lled and re
fi
lled several times
from under water that is a true sample, see method 2
in section 1.
Th
e 40mL vial should contain 36 mL of
radon-free water. 4mL of the undiluted sample
should be injected slowly at the bottom of the vial,
and the vial quickly capped. Any air bubble should be
very small.
Section 4
Results
19
