An investigation of the decay curves for radon daughters with active collection of radon on activated charcoal.

Abstract

The focus of this investigation was to develop a rapid "on site active" collection technique for the concentration of radon on activated charcoal. The radon was adsorbed on the charcoal and its analysis was based on gamma spectroscopy, detecting primarily the decay daughters 21tpb and 21tBi. The results obtained were then used to relate the activity of radon (in secular equilibrium with its decay daughters) on the charcoal at a given time (t) to the initial activity of radon at the time of collection. The decay curves for active and passive collection samples were analyzed over various time intervals for any deviations from the theoretically expected logarithmic decay curve using the computer spreadsheet QUATTRO. Results obtained from the passive collection method indicated the expected linear decay curve for samples with moderately high activities. The active collection method gave numerically increased initial slopes during the interval of 250 -5,000 minutes apparently due to radon gas transport. The portion of the decay curve past 20,000 minutes resulted in a slope slightly less than the theoretical slope probably due to the increasing effects of background variations and statistical variations of random decay. Active collection was revealed to be a rapid method for determining radon concentrations with reasonable accuracy, provided the optimum portion of the decay curve (from 5,000 -10,000 minutes) was used to determine original radon concentrations.