ABSTRACT
The electrostatic and diffusion properties of unattached218Po were investigated in an inert gas system containing trace amounts of hydrocarbons. Hexane (ionization potential, I.P., = 10.18 eV), ethylene (I.P. = 10.5 eV), and methane (I.P. = 12.6 eV) were used as the trace gases. The diffusion coefficient of neutral unattached 218Po was measured at various relative humidities. The low diffusion coefficient found at high humidity was attributed to the formation of clusters. The fraction of 218Po having a positive charge at a gas age of about 0.1 sec was determined. Based on the time-of-flight method, a pulse-width modulated ion mobility analyzer was used to determine the mobility of the 218Po. For a singly charged 218Po ion, the diffusion coefficient and mobility data obtained were consistent with the Einstein equation. The neutralization rate constant of 218Po ions was investigated at various relative humidities and radon concentrations. Three neutralization mechanisms were inferred. The neutralization rate constant was found to be proportional to a fractional power exponent of the radon concentration, consistent with the small ion recombination mechanism. The charge transfer mechanism involving hydrated polonium species and hexane (or ethylene) was considered responsible for lowering the power dependence of the neutralization rate constant on radon concentration from 0.5 to 0.4 (or 0.45). Charge transfer does not occur in the methane-nitrogen gas system because of the high ionization potential of methane. The power dependence was found to increase with increasing ionization potential of trace gas and to level off at 0.5 as the ionization potential becomes too high to allow charge transfer. The ionization potential of hydrated polonium species was estimated to be about 11 eV. The neutralization rate constant was found to increase with relative humidity, suggesting involvement of the hydrated polonium species in charge neutralization.
