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Chapter 4
for a few hours. It is necessary to know the total
volume of the closed-loop system. For this
purpose, the volume inside the RAD7 may be
taken as 1L. The initial rate of increase in radon
concentration (neglecting the first 15-min cycle),
multiplied by the volume, then gives the rate of
emission of radon. A reduction in the slope, as
the radon level builds up, may be due to leaks in
the system, or to a reduction in the net emission.
the thoron that gets fed back to the enclosure.
Note that, for thoron, both the closed loop and
open loop modes are steady-state measurements.
With the open loop configuration, a steady, and
known, flow rate must be established. If a
cylinder of ‘zero’ air, or nitrogen, is used, then the
RAD7 internal pump may be set to Off, and the
pressure reduction valve used to control the flow
rate. With ambient air, the internal pump may be
set to On, for a continuos flow. In both cases, a
flow meter is required.
The alternative procedures are similar to those for
bulk emission measurement, as described above.
Once the total emission rate, within the enclosure,
has been calculated, it may be divided by the area
of the surface within the sealed boundary, to
determine the emission per unit area.
4.7.4
For these measurements, an enclosure, with two
hose connectors, has one side open, which is
sealed against the surface under investigation.
A steady state should be achieved and a long-term
measurement may be made. The rate of emission
will equal the radon concentration times the flow
rate. The precision will depend on the
concentration and the duration of the
measurement.
4.7.3
Surface Emission
4.8
Thoron Measurement
4.8.1
Thoron and Radon
Thoron is an isotope of the element radon having
an atomic mass of 220, so it is also known as
radon-220. The word "radon" without a mass
number almost always refers to radon-222.
Thoron and radon have very similar properties.
They are both chemically inert radioactive gases
that occur naturally from the decay of radioactive
elements in soils and minerals. Both thoron and
radon are members of decay chains, or long
sequences of radioactive decay.
Bulk Emissions
An airtight box with two hose connectors, well
separated, will usually work fine. The material is
placed in the box. The box is connected, by
tubing from one connector, to the laboratory
drying unit, and thence to the inlet filter on the
RAD7. The other box connector has tubing
attached, which is either connected to the RAD7
outlet, for closed-loop operation, or, for open-
While radon results from the decay of natural
uranium, thoron results from the decay of natural
thorium. Both uranium and thorium are
commonly found in soils and minerals, sometimes
separately, sometimes together. The radioactive
gases radon and thoron that are produced in these
soils and minerals can diffuse out of the material
and travel long distances before they themselves
decay. Both radon and thoron decay into
radioactive decay products, or progeny, of
polonium, lead, and bismuth before finally
reaching stable forms as lead.
loop operation, connected to a cylinder of zero
gas, or left open to ambient air.
Note that bulk emissions are affected by pressure
fluctuations and by temperature and humidity. All
these parameters can, and should, be controlled in
both the closed-loop and open-loop
configurations. Radon emission is also dependent
on the grain size of loose materials, and porosity
of any bulk material.
Thoron and radon and their respective progeny
differ very significantly in their half-lives and in
the energies of their radiations. While radon has a
half-life of nearly 4 days, thoron has a half-life of
only 55 seconds. Since thoron is so short lived, it
cannot travel as far from its source as radon can
before it decays. It is commonly observed that
Thoron emission can also be measured in same
apparatus. In the open loop mode, a correction
will be required for the decay of the thoron during
the time between emission and measurement in
the RAD7. In the closed loop mode, another
correction should be made for the proportion of
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