Evaluation of electret ion chamber for tritium measurement

Commercial E-PERM radon-in-air monitors modified to detect tritium in air have been evaluated. Each monitor consists of a small ion chamber with access holes around the sides to allow the air to diffuse into the chamber, and an electret at the bottom to establish an electrostatic potential. Radioactive gas inside the volume generates ions, which are collected by the electret. The reduction of charge (and subsequent reduction in surface potential) on the electret is a measure of the integrated exposure. Two types of detectors, a 50 cm3 model and a 200 cm3 model, were tested for tritium-in-air concentrations from 1 to 60 MBq m-3, and gamma-absorbed dose rates from background levels to 50 microGy h-1. The HTO-in-air response of the modified E-PERM monitors was 3.3 +/- 0.3 V MBq-1 m3 h-1 for the 200 cm3 and 0.8 +/- 0.2 V MBq-1 m3 h-1 for the 50 cm3 monitor. For external gamma radiation (226Ra), the 200 cm3 chamber gave a response of 7.6 +/- 1.8 V microGy-1 and the 20 cm3 chamber 1.8 +/- 0.5 V microGy-1. The detection limit was about 3.6 and 15.2 MBq m-3 h for the 200 and 50 cm3 chambers, respectively. The accuracy and limit of detection of the electret detectors are limited by the accuracy in measuring the surface potential on the electret.