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Radiation Protection Dosimetry 22:97-105 (1988)
© 1988 Oxford University Press
The Electrical and Diffusive Properties of Unattached 218Po in Air Systems
The electrical and diffusive properties of unattached 218Po were determined in air environments containing traces of other gases. Of particular interst was the neutralisation of charged, unattached 218Po. An electrostatic collection apparatus and a pulse width modulated ion mobility analyser were used to determine the fraction of the unattached 218Po having a positive charge at the end of the recoil path (f); the diffusion coefficient of the neutral, unattached 218Po (DA); the mobility of the charged, unattached 218Po (B); and the neutralisation rate constant of charged, unattached 218Po (K). Average values found for f, DA, B and K were similar to those determined earlier for the argon system. The average f was found to be about 0.62. For most unattached 218Po, DA was found to be in the range 0.002 to 0.121 cm2.s-1. Values of B were found to be in the range 0.67 to 1.40 cm2. V-1.s-1. These results are in agreement with other published data. The neutralisation rate constant of unattached 218Po ions was found to be of the order of 10 s-1. Two mechanisms may be responsible for neutralisation, namely, scavenging of electrons from trace gases (charge transfer), and recombination with negative small ions. Which neutralisation mechanism is dominant depends on the amount and type of trace gas or organic vapour present and the degree of gas ionisation. Electron-ion recombination is dominant in inert atmospheres at high radon concentrations (>105 atoms.cm-3). The neutralisation rate constant for the recombination process was found to increase with radon concentration. The presence of water vapour significantly increased the rate constant. The electron scavenging process, on the other hand, is dominant at high trace gas concentrations (NO2( about 4.5 ppm). In an air environment, neutralisation occurs predominantly by the scavenging mechanism and the neutralisation rate constant is relatively independent of the radon concentration. Humidity has a less important effect on the neutralisation rate than the presence of trace gases.