A low-level radioactive charger based on Am-241 was developed and tested. Its activity (0.24 mu Ci) is below the exemption limit of IAEA. The design leads to values of the Nit product (similar to 10(13) M-3 S) comparable to current radioactive ionizers with activities up to three orders of magnitude higher. The performance of the weak radioactive Am-241 ionizer was assessed experimentally and by means of a numerical model based on Fuchs theory. The results show that the Am-241 ionizer charges to steady state particles less than 250 nm at number concentrations up to 10(6) cm(-3) at a maximum flow rate of 2.51 min(-1). These conditions correspond to values of the particle to ion concentration ratio N-p/N-o 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 congruent to 10, the minimum time to achieve stationary charging conditions in the Am-241 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. (c) 2007 Elsevier Ltd. All rights reserved.