Radon-induced Background in X and Gamma Radiation Measurements Using Ionization Chambers

Ionization chambers with non-sealed volumes find widespread use in monitoring x and gamma radiation. However, the accuracy of results can be compromised by the presence of 222 Rn in the air. To investigate this influence, two ionization chambers, each with a 600 cm 3 volume and walls constructed of air-equivalent plastic, were exposed to controlled 222 Rn concentrations. Following exposure, chamber readings (expressed in units of exposure rate and air-kerma rate) were monitored for 11 d. Two components of the signal attributed to 222 Rn and its progeny were identified. The first component, which dominates within the first day after exposure, arises from radon diffusion within the volume through gaps in wall joints. The second component results from radon absorption in the plastic construction materials of the chamber, persisting detectably for over 10 d post-exposure. The background induced by airborne 222 Rn can be significant. Concentrations near the reference level in many European countries (300 Bq m -3 ) can generate signal equivalent to air kerma rate of about 1.34 μGy h -1 . This could potentially lead to erroneous radiation protection decisions in radiological departments. Addressing this background can be challenging, as contributions from 222 Rn concentrations in previous hours and days impact correction. It is advisable to store such chambers in locations with low 222 Rn concentrations and to use construction materials for ionization chambers with low radon absorption ability.