In this paper an analysis of sensitivity of LR 115 II track etch detector for radon measurements in cylindrical diffusion chambers is performed using a Monte Carlo method. The response (sensitivity) of LR 115 II track detectors, placed at the bottom of the tube, depends on the tube height and radius. These dependences are studied for radon and its alpha emitting progeny. The total sensitivity decreases as the dimensions of the tube decrease. It is shown that in narrow chambers, with a radius less than 1.5 cm, sensitivity is not influenced by the fraction of deposited radon progeny on the walls. In chambers with a height smaller than 0.65 cm, only alpha particles emitted by 222Rn can be detected, so that radon determination is realizable without any influence of the radon progeny. © 1995.