[en] Equipment described in the claim is designed for selective detection of alpha particles emitted by the mixture of radium A and radium C'. It is primarily intended for mine contamination control. Air is pumped through a filter collecting radioactive aerosols. Two collimators are placed opposite the filter. Cellulose nitrate is used as the detector. Polycarbonate foils serving the filtration of alpha radiation 6 and 25 microns in thickness, respectively, are placed forward of the detectors. The content of radium A and radium C' is calculated from the readings of the detectors. (Ha)