1 INTRODUCTION
Th
e Natural Rock Sample is a 21oz (600g) sample of granite gravel, enclosed in a container that may be
sealed using a pair of ball valves and end caps.
Granite generally contains trace amounts of both uranium and thorium that has been in the rock since
it was formed, millions if not billions of years ago.
Th
erefore all the progeny of both the uranium and
thorium decay chains are fully supported and in full equilibrium down to 226-Ra and 224-Ra.
When the Natural Rock Sample is kept dry, it constitutes a rock-steady emitter of both radon and
thoron.
Th
is manual shows how to take advantage of that property to check the performance of a
RAD7. Please note that to keep the emission constant it is essential that the Natural Rock Sample be
kept fully dry.
Th
is is because the radon and thoron emanation e
ffi
ciency depends upon the humidity at
the rock-air interface on the surface of the rocks. Always use a Small Drying Tube, with blue, active
desiccant in it, in the closed-loop radon calibration circuit, or upstream of the desiccant in the open-
loop thoron calibration con
fi
guration. If the desiccant in the Small Drying Tube gets used up it should
be replaced or regenerated.
Th
e Natural Rock Sample should be thoroughly dried by purging with dry
air every 30 - 50 days.
Th
is can be done as part of the standard radon calibration check protocol (see
Section 2.3.1) or, if no calibration check is required, as a standalone procedure.
Th
e Natural Rock
Sample has a desiccant pack and humidity indicator built into it for maintaining and verifying dryness.
For thoron sensitivity measurement, the carrier air, pushing the thoron into the sample path, should be
dried before it enters the rock sample.
Th
ese con
fi
gurations are illustrated on the following pages.
A
ft
er calibration at DURRIDGE Company, and with the optional
Th
oron Calibration Check Kit, the
Natural Rock Sample can be used for low-precision checking of both the radon and thoron sensitivities
of any RAD7.
Th
e Natural Rock Sample can provide a means for monitoring long-term changes in both
radon and thoron sensitivity.
Figure 1 Natural Rock Sample Components
Sec$on 1
Introduction
3
