The measurement of tritium in the air presents a problem because the average energy of the beta particles emitted by tritium is so low (5.7 keV) that it is not possible to make a detector with walls thin enough to be penetrated by the tritium betas. Instead the air to be sampled is drawn through an ionisation chamber by a pump so that all of the beta particle energy is used to create ion pairs in the chamber.
The instrument is compensated for use in gamma fields by having a second chamber in the monitor which is sealed and measures only the gamma field. The difference between the readings of the two chambers is a measure of the tritium concentration.
Fig. 6.2
shows the gamma compensated tritium monitor .
Figure 6.2: Tritium in air monitor
In practice the instrument has a number of limitations. The gamma
compensation is adequate for only
relatively low fields (less than
).
Also any radioactive gas present in the air will produce a signal which can be
interpreted as the presence of tritium and because the energy of almost any
betas or positrons emitted by any other radioactive gas is higher than that of
the tritium betas, the monitor usually is more sensitive to other
radioactivities.