Fig. 4-8 Changes in the radioactivity distribution due to the diffusion of radioactive gas injected into a long proportional counter This shows that the total radioactivity can accurately be determined in a time of 400 seconds before the injected gas reaches the end of the counter tube.

Fig. 4-9 The system for direct measurement of gaseous radioactivity using the principle shown in Fig.4-8, proving highly reliable calibration of radioactive gas monitors

Accurate calibration of radiation detection instruments is indispensable for ensuring safety in and around nuclear facilities. Instruments measuring airborne radioactive gas concentrations must be calibrated using actual radioactive gases to ensure reliability. It is, however, not easy to obtain the necessary gas standards which have had the level of radioactivity measured with sufficient accurately to permit the desired calibration. A new technique has been developed in which the radioactivity of a standard gas is measured by means of counting the total beta-ray within the diffusion time of the gas injected from a central port into a long proportional counter, as shown in Fig. 4-8. The technique has the merit of simplicity and high accuracy, compared with conventional methods which use multiple counters. Since the air containing a trace amount of radioactive gas has to be flow into the gas monitor to be calibrated, a calibration gas loop is built separately from a standard gas loop having a long proportional counter. The two loops are connected at the sampling chamber as shown in Fig. 4-9. In this system, a known quantity of gas in the calibration loop is sampled and introduced into the standard gas loop, where its activity is determined. This new method has been proven and is in practical use employing various kinds of gas standards and highly-reliable calibration of gas monitors are currently being made.

Reference M. Yoshida et al., A Calibration Technique for Gas-flow Ionization Chamber with Short Half-lived Rare Gases, Nucl. Instrum. Methods, A383, 441 (1996).

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