Th
e solution to the problem of daughter activity is
time. Simply wait for the activity to decay away.
Check the background with a blank sample. If it is
still too high, keep waiting, and keep checking.
Th
e
length of time you will wait depends on just how
much radon your high radon sample had, and just
how much background you are willing to tolerate
before the next measurement. If you expect the next
sample to be high also, you may want to go ahead
with the next measurement right away, considering a
small amount of background acceptable.
In the case of extremely high radon samples, you may
develop a background that is more persistent than
daughter activity.
Th
at is possibly due to o
ff
-gassing
of residual radon that has absorbed into internal
surfaces. In particular, rubber and plastic parts can
absorb a small fraction of the radon that passes
through the system. A small fraction of a very large
amount can still be a signi
fi
cant amount.
Th
e radon
may desorb from these materials over many hours. In
the worst case you may have to allow the system to sit
idle for a day or more for the absorbed radon to
fi
nish leaking out of these materials, then purge the
system again to remove the radon. A radon
concentration high enough to cause a concern of this
kind is very rare in natural ground water, but is
possible in arti
fi
cial radon sources such as radium
crocks or "Revigators".
Sustained counting of very high radon concentrations
can lead to the buildup of long lived lead-210
contamination of the RAD7's alpha detector.
Th
is
possibility is described in the RAD7 Operator's
Manual. It su
ffi
ces to say that the RAD7's ability to
distinguish alpha particles by energy makes it far less
susceptible to the build up of lead-210 related
background than other radon monitors.
Section 3
RAD H
2
O Technique
18
