Enhanced localised charging of radioactive aerosols

The positive self-charging of radioactive aerosols by the emission of elect rons in the decay process is partly neutralised by the attachment of negati ve ions from the surrounding gas. The net charging is enhanced if the neutr alisation is reduced by lowering the concentration of the ions. This occurs when the ion production rate is reduced by diluting the aerosol or when th e aerosol enters a confined geometry, and also when the ions are removed by electric fields. These cases are discussed using an equation derived for t he mean charge on the aerosol. An expression for the charging of a particle in a dilute radioactive aeroso l in the environment is obtained allowing for background ionisation and the ion cloud produced by the particle, and conditions For enhanced lung depos ition are derived. A comprehensive treatment is given of the ion reduction which rapidly occurs when aerosol enters a confined space, and a simple exp ression is shown to give adequately the resulting increase in aerosol charg e. Ion motion and aerosol charging are briefly discussed for large imposed electric fields and for weak fields produced by movement of decay electrons . Near a negative electrode, enhanced charging of the aerosol arises from r eduction of negative ions in the electrode effect, and could lead to greatl y enhanced deposition of the aerosol. (C) 2000 Elsevier Science Ltd. All ri ghts reserved.