Long-term measurements of radon progeny concentrations with solid-state nuclear track detectors

Abstract

In this paper, we review existing methods for long-term measurements of radon decay products with solid-state nuclear track detectors. We then propose a method to determine the equilibrium factor using the bare LR 115 detector. The partial sensitivities p i of the LR 115 detector to Rn 222 and its α -emitting short-lived progeny, Po 218 and Po 214 , were investigated. We determined the distributions of lengths of major and minor axes of the perforated α -tracks in the LR 115 detector produced by Rn 222 , Po 218 and Po 214 through Monte Carlo simulations. The track parameters were first calculated using a track development model with a published V function, and by assuming a removed active layer of 6.54 μ m . The distributions determined for different α -emitters were found to be completely overlapping with one another. This implied equality of partial sensitivities for radon and its progeny. Equality of partial sensitivities makes convenient measurements of a proxy equilibrium factor F p possible which is defined in the present work as ( f 1 + f 3 ) and is equal to the ratio between the sum of concentrations of the two α -emitting radon progeny ( Po 218 + Po 214 ) to the concentration of radon gas ( Rn 222 ) . In particular, we have found F p = ( ρ / ρ i tC 0 ) - 1 , where ρ (track/m 2) is the total track density on the detector, ρ i = 0.288 × 10 - 2 m (for the V function mentioned above and for a removed active layer of 6.54 μ m ), t is the exposure time and C 0 (Bq/m 3) is the concentration of Rn 222 . If C 0 is known (e.g., from a separate measurement), we can obtain F p . The proxy equilibrium factor F p is also found to be well correlated with the equilibrium factor between radon gas and its progeny through the Jacobi room model. This leads to a novel method for long-term determination of the equilibrium factor. Experimental irradiation of LR 115 detectors to known Rn 222 concentrations as well as known equilibrium factors were carried out to verify the present method. The relationship between ρ i and the removed layer was then derived for the V function specifically determined for the LR 115 detectors we were using for the experiments. The actual removed layers for individual detectors after etching were measured accurately using surface profilometry. A curve showing the relationship between the removed layer and the track diameter of normally incident 3 MeV α -particles is also provided for other researchers, who do not have access to surface profilometry, to use the present technique conveniently.