Design and evaluation of a low-level (0.24μCi) radioactive aerosol charger based on 241Am

A low-level radioactive charger based on 241Am was developed and tested. Its activity (0.24μCi) is below the exemption limit of IAEA. The design leads to values of the NitNit product (∼1013m-3s) comparable to current radioactive ionizers with activities up to three orders of magnitude higher. The performance of the weak radioactive 241Am ionizer was assessed experimentally and by means of a numerical model based on Fuchs theory. The results show that the 241Am ionizer charges to steady state particles less than 250 nm at number concentrations up to 106cm-3 at a maximum flow rate of 2.5lmin-1. These conditions correspond to values of the particle to ion concentration ratio Np/NoNp/No less than unity and a residence time of 5 s. For particle to ion concentrations above unity, the model predicts that the time to steady state increases strongly with both particle size and aerosol particle concentration. For Np/No≅10Np/No≅10, the minimum time to achieve stationary charging conditions in the 241Am ionizer is 10 s, for particles of 50 nm. In accordance with the numerical results, a simple analytical criterion is derived, which predicts whether an aerosol of given concentration and mean particle size is charged to steady state in an arbitrary radioactive bipolar ionizer.