CR-39 detectors are widely used to measure environmental levels of Rn-222, Rn-220 and their progeny. Prior research reported the CR-39 detection efficiency for alpha particles from Rn-222, Rn-220 and their progeny under a variety of etching conditions. This paper provides an explanation for interesting observations included in that work, namely that the critical incidence angle decreases with the increasing particle energy and the detection efficiency for 8.78 MeV alpha particles is zero. This paper explains these phenomena from a consideration of the interaction of alpha particles with the CR...
