Measuring radon progeny and thoron progeny in air by absolute beta counting subsequent to grab sampling

摘要

A new grab sampling method for the simultaneous determination of radon progeny and thoron progeny in air has been developed and demonstrated by the first author of this paper a few years ago. The characteristics and capabilities of this method are discussed in the present paper in detail. The method is based on gross beta counting of filtered aerosol samples counted over successive time intervals using an end-window Geiger-Muller counter. The measured counting rate versus time curve is analyzed by fitting to it some linear combination of the theoretical decay curves of the decay products with the knowledge of the counting efficiencies and some other quantities. Defined solid angle absolute beta counting was used to evaluate the counting efficiencies for the decay products one by one. So, the method is an absolute one that does not demand further calibration. The decay products of radon and thoron can be determined separately and simultaneously. The systematic error of the results, deriving from the inaccurate evaluation of the counting efficiencies, is probably lower and can be estimated more surely than that of the methods based on alpha counting. The method is highly sensitive: accurate results can be obtained for Pb-214, Bi-214, Pb-212, EEDC222 and EEDC220 even outdoors. However, the results for Po-218 and Bi-212 are much more inaccurate and Tl-208 cannot be determined at all. The reliability of the determinations can be raised significantly using conditional computations based on the information obtainable from some theoretical considerations and models. The method can be used in a broad activity concentration range, equally in caves, mines, houses or outdoors. The above characteristics and capabilities of the method are demonstrated in this paper through the detailed presentation and analysis of the results obtained for three samplings performed indoors in a house, in a cave, and outdoors, respectively. The known advantages and drawbacks of the method are also surveyed in detail. (c) 2005 Elsevier Ltd. All rights reserved.