Wipe test (using glycerin or toluene-saturated filter papers,
then measurement with the probe LB 1239 or in a liquid
scintillation counter)
Washing down or rinsing clothing, followed by measuring
liquid samples in the liquid scintillation counter.
The direct measurement is the first choice for flat, smooth sur-
faces which do not allow
3
H contamination to penetrate. Such
surfaces are specified for the working surfaces in the radionu-
clide laboratory. This probe is ideally suited for this because of
its large window opening.
Rough surfaces such as absorbent paper or wood make the
measurement increasingly inaccurate since self-absorption in
the contaminated medium can be considerable due to the low
energy of the Beta particles. In this case, other methods of de-
tection may have to be used (wipe test, washing down or rins-
ing followed by measurement in the liquid scintillation counter).
To ensure an accurate measurement, the large window opening
of the counter tube LB 1239 requires that the surface to be
monitored should totally cover the opening. For this reason, the
counter tube needs to be placed onto the surface, for example,
the table top, from above. At the same time, this ensures that
the minimum distance is kept.
When surfaces to be monitored are not horizontal or flat, or if
they do not fully cover the window opening, then the aperture
plate supplied has to be used.
In conjunction with the aperture plate the Tritium probe can also
be used for detecting contamination on finger tips and, to a lim-
ited extent, also on specific areas on clothing, for instance after
it has been splashed. However, in this case it is only possible to
give a qualitative indication since the extent of the self-
absorption remains unknown.
Let us use an example to illustrate this: The maximum range of
a
3
H Beta particle of maximum energy (18.6 keV) emitted at
right angles to the surface is only 8 µm in material having a
density of 1; the average range of medium-energy particles, on
the other hand, is 0.2 μm. When taking a direct measurement
one can therefore at best detect the activity in a layer of approx-
imately 0.5 µm corresponding to a surface density of 0.05 mg
per cm².
The methods used in the wipe test or when washing items are
more suitable from this point of view for detecting Tritium activi-
ty on rough surfaces or fabrics.
However, their efficiency is completely indeterminate. What
fraction of the activity is wiped off or removed by washing de-
pends very much on individual conditions.
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