The Determination of Thorium Lung Content by Measurement of Thoron in Exhaled Air

A method is being developed for assessing intakes by inhalation of insoluble thorium by measurement of thoron (220Rn) in exhaled air. It has been shown that it is possible to collect the short-lived decay products of thoron, 216Po and 212Pb, electrostatically. The subsequent counting is done using a silicon surface barrier detector, which gives a high detection efficiency and good energy resolution. Experiments have been carried out using a calibrated thoron source to determine the effects of parameters such as flow rate, applied collection voltage and humidity on the collection efficiency of thoron daughters. The estimated minimum detectable activity of the prototype system for lung thorium activity is between 1 and 10 Bq for a 45 min. exhaled air collection period. With this method it will be possible to measure thorium levels which correspond to intakes of a fraction of an ALI (Annual Limit on Intake).