PURPOSE:To flatten a frequency distribution curve and to enhance measuring accuracy and sensitivity, by using a beta-ray source and a detector both in specified structures. CONSTITUTION:A beta-ray source 3 is furnished with a ray source collimator 4 having a window 5 and a prescribed thickness, and a ray source filter 6 made of a thin metallic plate. A beta-ray detector 9 arranged with a filtering material 7 between is provided with a detection collimator 10 having a window 11 and a prescribed thickness like the collimator 4. As a result, a flat part is formed on the spectral curve of the beta-rays emitted from the source 3 toward the detector 9. A setting level of a wave height discriminator 15 is fixed in the region of this flat part, and the intensity of the rays transmitted through the material 7 before and after collection of dust powder. Prescribed calculation is made in a signal processor 16 to obtain the thickness of the dust powder layer deposited on the material 7, thus permitting enhancement of measurement accuracy and sensitivity.
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