Abstract: A method and apparatus for detecting .alpha.-rays present in a specified energy range includes a chamber in which is located a filter, an .alpha.detector and a measuring space which is kept at less than 40% of normal atmospheric pressure. The reduced measuring space pressure in turn reduces energy loss of the .alpha.-particles as they travel from the filter to the detector, thereby permitting a high degree of differentiability of the .alpha.-particles according to their energy levels while at the same time providing a large enough sample of .alpha.-particles for .alpha.-activity measurement. Sufficient collimation of the .alpha.-rays is effectuated by limiting the linear distance across the detector receiving surface as compared with the distance between the detector and filter receiving surfaces. The collimation is further improved by tapering conically the inside surface of the chamber toward the detector, and by disposing the average aperture angle of the surface elements of the filter between 90.degree.

Abstract: A measuring head 1 which contains in a shielding housing 3 a measuring chamber 5 with a surface barrier-layer detector 9 is provided for measuring the noble gas activity concentration and the aerosol activity concentration. The gas whose activity concentration is to be measured is compressed and introduced into the measuring chamber 5. Due to the pressure increase of the gas, the volume of the measuring chamber 5 and, therefore, the weight of the shielding housing 3 can be reduced.