Continuous measurement of radon progeny in atmospheric aerosol using a NaI(Tl) detector and a High Volume Air Sampler

Radon short-lived progeny may be used as tracers of environmental processes. Monitoring their activity in ambient air is a challenging process, and full interpretation of their fluctuations remains unclear. In this work, an enhanced technique for monitoring radon progeny in ambient air is presented. The technique consists of continuous gamma measurements with a 3″x3″ NaI(Tl) detector placed over an air filter during aerosol sampling outdoors with a High Volume Air Sampler. Measured signals are magnified, thus allowing for relatively small collection times and essentially continuous measurements. The technique was applied for a period of eight months and information about radon progeny activity in outdoor air was obtained, focusing on ²¹⁴Bi. Recorded signal timeseries confirmed elevated ²¹⁴Bi activity measured during occurrence of precipitation, although a positive relationship between rain rate and elevated count rate was indicated but not confirmed. Results pointed towards a positive correlation between recorded ²¹⁴Bi count rate and particulate matter levels, as well as relative humidity in outdoor air. Further investigation demonstrated the effect of wind conditions (speed, direction) on recorded signals: continental Northern and, to some extent, Eastern winds were found to correlate radon progeny activity with relative humidity, in contrast to marine Southern winds. High wind speed was found to affect recorded signals as well. The technique presented in this work may be used to investigate the relation between environmental parameters and radon progeny activity levels in atmospheric air, proving of great value for on-line environmental monitoring.