Mean Thickness Measurement of Distributed Fine Particles by Using α-ray Absorption Curves

Abstract

The mean superficial density of fine particles, such as polystyrene latex particles or ion exchange resin particles, dispersed on a thin supporting film was measured by means of the thickness gauge which used the absorption curves of α-rays. The measured absorption curves coincided with the ones calculated from the particle diameter, density, number of particles per unit area and equivalent film thickness to an absorption gas of 1 mmHg. On the other hand, when the particle diameter exceeded a certain limit, the measured mean superficial density which was derived from the pressure displacement of absorption curve considerably differed from the true mean superficial density. This limit can be calculated from the shape of absorption curve, which depends on the α-ray spectrum emitted from the source. In the case of the ²⁴¹Am source used in this study, the limit was about 450 μg/cm² for polystyrene latex particles.
The mean superficial density of a sample can easily be obtained by measuring the pressure displacement of absorption curve when the maximum thickness difference is below the limit. However, when the maximum thickness difference exceeds the limit, it is necessary to estimate the whole shape of absorption curves.