Various factors affecting the calibration of alpha track detectors: a Monte Carlo study

Abstract

Low levels of indoor radon are frequently monitored using alpha track detectors (ATD). A common detector configuration consists of cellulose nitrate film (Kodak LR-115 II(TM)) enclosed in a cylindrical metallic canister. The ATD system must be properly calibrated to provide accurate estimations of indoor radon levels. Calibration of the ATD system requires relating the average observed track density on the film (tracks cm') to the integrated radon exposure (kBq hr m-'). This relationship is termed the ATD calibration constant (tracks CM-2 per kBq hr m-'). The ATD calibration constant for LR 115 II type detectors was calculated using a Monte Carlo computer code. The alpha particle propagation was simulated using random trajectories and the range-energy curves for alpha particles in air (ICRU 1993). Film response calculations were based upon an analytical expression for the track forming probability of LR 115 II (Damkjaer 1986). The fractions of radon progeny plating out on the inner surfaces of the container were estimated using diffusion theory. The alpha particle energy/angular spectrum found at the detector film surface was characterized. Investigations into the effects of changes in ambient atmospheric pressure (simulating changes in altitude) and canister geometry on the calibration coefficient were also performed. The calibration coefficient was determined to be 1.65 (tracks cm' per kBq hr m-'). The detector response varied inversely with the ambient pressure. The detector response was non-linear producing a 20% increase in the response for a 17 kPa (5 in. Hg) drop in ambient pressure, and a 14% decrease in the response for a 17 kPa (5 in. Hg) increase in ambient pressure. Effects from differences in altitude or ambient pressure can be compensated through the use of these correction factors. The optimum canister geometry was determined to be 3.75 cm (1.5 inches) in height with a radius of at least 5 cm (2 inches). This ATD configuration results in an increase of the calibration coefficient to 1.81 tracks CM-2 per kBq hr m-').