Calibration of cellulose nitrate film for measurement of time-integrated concentration of radon-222 in air

Measurement of time-integrated concentration of 222Rn in air by using solid-state nuclear track detectors (SSNTD) is finding increasing application in such diverse fields as radiation protection, exploration of radioactive minerals, prediction of earthquakes etc. While there are several types of SSNTDs sensitive to specific types of particulate radiation, films made from cellulose nitrate (CN) are found to be the best suited for quick and quantitative measurement of alpha radiation. This is because CN films are available in small thicknesses, of the order of 10-12 μm, which can be suitably evaluated by spark counting technique. This report describes the use of a sensitive thin film of CN (Kodak LR 115, Type II) for quantitative estimation of 222Rn. The film (along with the base is exposed in a cylindrical plastic cup closed at one end with a special rubber membrane which permits discrimination of 222Rn against 220Rn, which is also present in air to varying extents. The calibration procedure, including etching and evaluation of track registration efficiency, are described in detail. The mean track registration efficiency works out to 41.9 per cent and the sensitivity of the system is found to be 58.92 tracks cm-2 per (Bq.litre-1) day. The report lists the advantages and limitations of the system for measurement of time-integrated concentration of 222Rn in air. (author)