when measuring samples below 1,000 pCi/L, but if
you are measuring only water samples above 1,000
pCi/L, you may consider this amount of error to be
acceptable. To reduce the error due to purge air radon
you may either subtract o
ff
the background from
every measurement, or adopt strategies to reduce the
background to acceptable levels. In any case, for
levels below 1,000 pCi/L you should preferably use
250mL vials when ambient air of 4 pCi/L will give
only 20 pCi/L additional radon concentration to the
next water result.
Th
e best way to determine the background is to
measure a "blank", a water sample containing no
radon. To get radon free water, purchase distilled
water from your local pharmacy, or
fi
ll a container
with tap water, and allow the container to stand
closed and undisturbed for 4 weeks or more.
Th
e 4
week period allows any radon present in the water to
decay away. Store your radon free water in a closed
air-tight container. Remember that the background
due to purge air radon will change when the air
radon concentration changes, so if you intend to
subtract background you should measure a blank
sample at every measurement session.
An alternative method to determine background is to
make a measurement of the air in sni
ff
mode and
note the count rate in window A, a
ft
er 15 minutes.
From a previous printout of a water measurement,
with the format set to medium or long, you can see
the count rate in window A corresponding to the
water radon concentration measured. Typically, for a
250mL vial, 1,000 pCi/L in the water will generate
about 50 cpm in window A. A background count rate
of 0.5 cpm in window A (equivalent to about 2 pCi/L
in air) will then produce an error of 1% in the
fi
nal
reading.
Th
e obvious way to reduce background is to purge
with very low radon air. Outdoor air rarely exceeds
0.5 pCi/L at several feet above the ground, so you can
probably get the water background to below 13 pCi/L
by simply using outdoor air to purge. To get even
lower radon air,
fi
ll a tank or balloon with outdoor air
and let it age for several weeks. If you are using
compressed air or inert gas, be very careful not to
allow the RAD7 to pressurize, as this may cause
internal damage to the pump or seals.
Another method to reduce background is to use
charcoal adsorption to clean the remaining radon
from the system following the purge. A small column
containing 15 grams of activated carbon can remove
up to 98% of the remaining radon from the RAD
H
2
O system when connected in a closed loop.
Th
is
will reduce the system's radon to truly negligible
levels for the most accurate low level radon in water
measurement.
Th
e charcoal
fi
lter works best if you
use it only a
ft
er a complete purge with low radon air,
which avoids overloading the
fi
lter with radon. If the
charcoal
fi
lter becomes badly contaminated with
radon it can give o
ff
some of the radon and actually
increase the background a
ft
er a purge. Store the
charcoal
fi
lter with the end caps installed to allow the
fi
lter to "self-clean" by waiting for adsorbed radon to
decay over several weeks time. Always keep the
charcoal dry by using it in conjunction with a drying
tube, since water vapor can adversely a
ff
ect charcoal's
capacity to adsorb radon.
Even if you choose not to use fancy methods to
reduce the background, you should always purge the
system between samples. It is much better to purge
with ordinary room air than not to purge at all. In
any case, it is also necessary to purge to remove any
accumulated water vapor from the system, and bring
the relative humidity back down to close to 5%.
3.4 Background and Residuals
Purge air is one among several causes for background
counts in the RAD H
2
O.
Th
e most signi
fi
cant other
causes are radon daughters and traces of radon le
ft
from previous measurements.
Th
e RAD7 has the
unusual ability to tell the di
ff
erence between the
"new" radon daughters and the "old" radon daughters
le
ft
from previous tests. Even so, a very high radon
sample can cause daughter activity that can a
ff
ect the
next measurement.
A
ft
er the high radon sample has been purged from
the system, its decay products stay behind until they
decay away.
Th
e polonium-218 isotope decays with a
3 minute half-life. In the 30 minutes following the
purge, the polonium-218 decays to about a
thousandth of its original activity.
Th
at still leaves a
background of 100 pCi/L a
ft
er a 100,000 pCi/L
sample.
In addition to polonium-218, the RAD7 is sensitive
to polonium-214, which can give counts for several
hours a
ft
er the radon has been removed.
Th
e RAD7
uses alpha energy discrimination to reject
polonium-214 counts from a measurement, but a
very small percentage of the polonium-214 decays
slip past the discriminator.
Th
is can add background
to a measurement that follows a high radon sample.
Section 3
RAD H
2
O Technique
17
