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6.0. CALIBRATION
6.1. METHOD
Tritium monitors employing ionization chambers, such as the 400 series portable instruments may
be calibrated with either of two methods.
The first method consists of injecting a known activity of tritium gas; the second method uses
external gamma radiation of known field strength.
To ensure traceability to National Standards, the first method must be employed. This method is
time consuming, and is quite difficult to perform with precision. This first method is, however,
useful as a “type” test, and can serve as a basic accurate calibration from which the gamma
response (the second method) can be cross-correlated.
The second method uses an external gamma field. In this instance, the polarization of the
compensation ionization chambers is reversed to coincide with that of the measurement ionization
chambers.
In this condition, the effect of external gamma radiation now adds rather then cancels, and a known
gamma field should produce a predetermined measurement indication.
6.2. GAS
CALIBRATION
Since the instrument is essentially linear, a relatively high concentration can be used for most
accurate results. Values between 100 – 1,000
μ
Sv/hr are convenient, but any other values from
20 – 5,000
μ
Sv/hr can be used.
The manufacturer of the gas calibrator generally provides instructions for the use of gas calibrators,
and these should be followed.
Some general hints can be given.
It is important that the calibration sample be well circulated through the entire calibration system
loop.
Adequate time should be allowed for the system pressure and temperature to come to
equilibrium, and that no excess pressure is built up.
The inclusion of a previously calibrated "master" or "reference" tritium monitor in the sampling loop
is highly recommended.
The calibration can actually be repeated for several levels of tritium activity. This is not done to
verify the linearity of the tritium monitor (which is highly linear) but to ensure that the calibration
process itself is free from subtle errors.