Measurement of 210Pb airborne activity concentration by gamma-ray spectrometry of air particulate samplings

Lead-210 (half-life 22.3 y), is a natural radionuclide, being produced in the atmosphere through radon decay. Due to its affinity for particles, it is rapidly fixed by aerosols and, with its long half-life is the most important tracer of long range atmospheric aerosol transport and removal processes. Main measurement techniques adopted to evaluate 210Pb air activity concentration differ in analysis and sample preparation methods. Some of them involved complex chemical processes or decay product detection (for example 210Po), with a long decay time period between sampling and experimental measurement. The availability of an high-volume particulate sampler (about 900 m3 h-1) combined with a low background gamma-ray spectrometric system based on an Ortec HPGe, type GLP Series Planar Low-Energy Photon Spectrometer (LEPS) allows the evaluation of 210Pb airborne activity concentration by gamma-ray spectrometry of particulate samplings after a short time period from the end of the sampling. In this work, a measurement procedure able to distinguish the airborne 210Pb from the one product on filter by decay of 222Rn products is proposed. Indeed, during the sampling, the number of 210Pb nuclei on filter increases for both filtration of those present in air and for decay of radon products fixed on the filter. The method consists on the contemporary evaluation of an equivalent radon concentration by a series of gamma ray spectrometric measurements after a minimum decay time period from the end of sampling. A comparison with a model allows the evaluation of 210Pb activity percentage produced by decay of short-lived radon daughters on the filter matrix during and after the sampling. A gamma-ray spectrometric measurement, with reference to 46.5 keV gamma emission of 210Pb, after a few days from the sampling allows the determination of total 210Pb . The airborne 210Pb activity and derived air concentration was at last computed by a simple differentiation of the two contributions. Experimental tests have allowed obtaining a range of 2÷10 per cent for 210Pb activity percentage on a filter due to decay of short-lived radon daughters, with higher values when 210Pb air activity concentration is very low. All the measurements resulted above detection limit, even in the case of a short sampling time period.