A theoretical approach to the study of saturation phenomena of electrostatic collection efficiency of 218Po

• Expression for collection probability of positively charged 218Po has been obtained.
• Expression for total collection efficiency of 218Po has been obtained.
• The mechanism of electrostatic collection efficiency of 218Po has been explained.

Radon monitors based on electrostatic collection have been widely used in different radon studies. Collection efficiency of this kind of instrument first depends on the cell geometry and electrostatic field intensity. In the process of new instrument development, some researchers found that electrostatic collection efficiency of 218Po was almost saturated when the voltage between the cylindrical vessel and the detector was higher than 1500 V. In this paper, we analyze the physical processes of the freshly positively charged 218Po formed from the α decay of 222Rn and the neutralization kinetics for positively charged 218Po cluster. The analytical expressions of the collection probability of any freshly positively charged 218Po cluster and the total collection efficiency in electrostatic collection cell are obtained. The ratio of the relative growth rate of the collection efficiency to the relative growth rate of the voltage is inversely proportional to the voltage. Such an approach is useful to understand the mechanism of the electrostatic collection of 218Po.