Minimum detectable activity concentration in direct alpha spectrometry from outdoor air samples: continuous monitoring versus separate sampling and counting

Rapid method for identifying the presence of alpha particle emitting radionuclides in outdoor air is of paramount importance should a nuclear or radiological incident occur. Minimum detectable activity concentrations of 2 3 8 U, 2 3 4 U, 2 3 9 Pu, and 2 3 8 Pu in outdoor air are calculated for two direct alpha spectrometry methods: continuous air monitoring is compared with separate sampling and subsequent alpha particle counting in a vacuum chamber. The radon progeny activity concentration typical for outdoor air and the effects for the alpha particle spectra caused by the properties of the filter and the aerosol particles are taken into account using measurements and Monte Carlo simulations. Continuous air monitoring is a faster method for identifying the presence of (trans)uranium elements when their activity concentration is considerably higher than the typical detection limit. Separate sampling and counting in a vacuum chamber is a more sensitive method when concentrations are close to the detection limit and when the duration of the sampling-counting cycle is greater than approximately 2 h. The method may serve as a tool for rapid field measurements.