226Ra measurement via gamma-ray spectrometry of 222Rn progeny - quantification of radon losses from sample capsules.

Abstract

Radium-226 detection in sediment samples is generally executed by means of gamma-ray spectrometry. Data evaluation relies (besides the 186.2 keV ²²⁶Ra gamma peak) on the combined analysis of major gamma peaks that are produced by the short-lived radon (²²²Rn) daughters ²¹⁴Pb and ²¹⁴Bi. Precondition for this detection approach is equilibrium decay of all members of the decay chain between ²²⁶Ra and ²¹⁴Bi. In closed systems, this equilibrium is reached after about five half-lives of ²²²Rn (19 days). However, a closed system can only be guaranteed if the capsule which contains the sample prevents diffusive escape of radon. Such radon-tightness cannot be guaranteed for a wide range of plastic materials. Due to its polymer structure, plastic material generally tends to allow radon diffusion and hence radon loss from the sample resulting in a disturbance of the required decay equilibrium. The paper introduces an approach that allows quantifying radon loss from sample capsules by direct radon measurements using mobile radon detection equipment. The experimental findings are supported by theoretical considerations. An examined alternative approach based on the offset of the 186.2 keV data point from an efficiency function that is calculated exclusively from short-lived radon progeny peaks in the gamma-ray spectrum did not prove to be applicable due to a lack of supporting peaks in the low-energy section of the spectrum.