A theoretical and experimental study of an aerosol particle charging apparatus that utilizes a 407-MBq (11-mCi) 90Sr-90Y beta source and electric and magnetic fields has been performed. Fluid models of electron trajectories in the presence of the magnetic field, ion generation due to electron energy deposition, and particle charge acquisition due to ion transport are developed and applied to the experimental apparatus. Calculated average axial ion generation rates on the order of 1014/m3· s are confirmed by experimental measurements, and calculated radial profiles are in good agreement with experiments. Calculated and experimental charging rates agree within 30% for 50- to 100-μm-diam glass spheres in an electric field of 100 kV/m and a magnetic field of 0.141 T. It is found that both the magnitude and spatial distribution of the ion generation rate play important roles in determining the rate of charge acquisition by an aerosol particle in a partially ionized gas subjected to an external electric field.