thoron concentration. Nevertheless, the presence of a large spill from window C would increase the
uncertainty of the thoron count in window B. It is therefore important that the C window count be low
when performing a thoron measurement or calibration.
If a radon calibration check of a RAD7 has been performed with a Rock Sample, it is good practice to
wait an hour or two for the count rate in window C to drop below 10 cpm, before proceeding with the
thoron calibration check. During this time, the Natural Rock Sample can be le
ft
purging with a RAD7
(set Test > Purge) and a Laboratory Drying Unit, which will ensure that the Natural Rock Sample is
thoroughly dry before the measurement begins.
3.2 Rough Thoron Check
Before closing the ball valves and disassembling the setup, change the protocol to
Th
oron by selecting
Setup,
Protocol,
Thoron,
[ENTER]
.
Th
en turn the RAD7 o
ff
and back on, and let the printer
print out the new header for thoron.
Start a measurement. Let it run for at least 15 minutes (3
fi
ve-minute cycles), or more if you would like
a higher precision average. Average all the thoron readings. Store the thoron average reading with the
radon average.
Th
is thoron reading cannot be used to check the thoron sensitivity of the RAD7, as the reading is
heavily dependent on the pump
fl
ow rate and even the position of items in the setup. For example, if
the desiccant is placed upstream instead of downstream of the Natural Rock Sample it will make no
di
ff
erence to the average radon reading, but will make a big di
ff
erence to the thoron readings. Even just
switching the connections to the Natural Rock Sample can a
ff
ect the thoron readings. However, if the
setup remains identical in every respect from one monthly check to the next, if the RAD7 pump is still
working at close to the same
fl
ow rate, and if the inlet
fi
lter is not blocked, then thoron readings made
with this setup will be similar from one month to the next. A
ft
er several months of experience the user
will know the typical variation and will recognize an anomalous shi
ft
, should it occur.
3.3 Better Thoron Check
A more accurate thoron sensitivity check involves the
Th
oron Calibration Check Kit, which is sold
separately. To check the thoron sensitivity of a RAD7 using this kit, the measurement must be made in
precisely the same conditions and using the same con
fi
guration as thoron measurements in regular use.
In standard thoron protocol, the RAD7 has 1m of 3/16” ID tubing between the inlet
fi
lter and a Small
Drying Tube, which may be used as a wand for sni
ffi
ng purposes.
Th
e thoron check process therefore requires that the sampling point at the end of the Small Drying
Tube must be open to the air. It is the thoron concentration at that sampling point that we measure.
Th
us we put a T-connector at that location, inject thoron at a known rate, using a carrier with less than
the RAD7
fl
ow rate, and with fresh air from the other side of the T-connector supplying the balance of
air to make up the RAD7
fl
ow rate.
Th
is con
fi
guration is illustrated in Figure 7.
Sec$on 3
RAD7 Thoron Sensitivity Check
11
