Electrical behaviour of radioactive aerosol in the environment

Radioactive aerosols can charge themselves by the emission of charges in the decay process. The negative small ions present, concentration n, will tend to neutralise the charge so that a good estimate for the mean charge, j, on a 0-decaying particle, in units of e, is: j = 442 η(Bq) / n-(109 m-3) where η is the decay rate for the particle. We calculate mean charges for aerosols of low concentration in the environment, and for isolated particles where n-acquires a minimum value which depends upon η. For spherical particles we write η = ηO R3(μm) and deduce that the mean value for the Chernobyl core was initially ηO = 44 Bq(μm)-3, falling to 13 Bq(μm)-3 after 10 days. open-quotes Hotclose quotes particles from Chernobyl would have been highly charged with j ranging up to 105. A open-quotes hotclose quotes particle of 15 μm size found following the Windscale accident had ηO = 106 Bq(μm)-3 from 140Ba and 89,90Sr. Condensation aerosols from volatile isotopes could have much larger values, for example Cs131I with ηO = 3045 Bq(μm)-3. The high charge levels reached when such aerosols leave highly ionised environments implies that much of their subsequent behaviour would be strongly affected by electric fields, both natural and induced. Deposition on conducting surfaces would be enhanced by the electric image force, and we have obtained quantitative enhancements in the alveolar lung deposition of small particles, which for a radius of 0.2 μm would occur once j exceeds 20