Ion Beam Charging of Aerosol Nanoparticles

In our ongoing efforts to achieve the high-efficiency charging of aerosol nanoparticles under low-pressure conditions, our group has recently developed an ion beam aerosol charger (IBAC) that ionizes aerosol nanoparticles using an He⁺ ion beam (Seto et al. 2003 Seto, T., Orii, T., Hirasawa, M., Aya, N. and Shimura, H. 2003. Ion Beam Charging of Silicon Nanoparticles in Helium Background Gas—Design of the Ion Beam Aerosol Charger (IBAC). Rev. Sci. Inst., 74: 3027–3030. CROSSREF]Crossref] , Google Scholar]). In earlier studies we have observed both increases and decreases in the currents from the charged particles, depending on the pressure when the polydisperse particles were irradiated by the ion beam. None of our previous studies elucidated the mechanisms of the charging itself, however. In the present paper we evaluate the charging probability of monodisperse aerosol nanoparticles using a low-pressure differential mobility analyzer (LP-DMA) and aerosol electrical condenser (AEC). The particles were negatively charged by the attachment of free electrons generated via the ionization of carrier gas by ion beam irradiation under a pressure of more than 350 Pa. A charging probability of more than 60% was obtained experimentally for the particles of 10–40 nm in mobility diameter under the pressure of 350–650 Pa. The mobility of the particles was almost the same before and after charging in a tandem LP-DMA analysis, with no multiply charged particles observed. The charging probability of nanoparticles was estimated based on the diffusion charging theory. Lastly, experiments were performed to demonstrate the performance of the IBAC in the charging of neutral particles in comparison with that of an α-ray source.