Abstract: A beta radiation sensor and an X-ray radiation sensor are used to monitor the specific weights of both components of a two-component web, in particular, a web of magnetic recording material, by using the beta radiation sensor to measure the combined specific weight of the material. The measurement by the X-ray sensor permits such combined specific weight to be apportioned between the two components of the web. Cross talk between the two sensors arising from bremstrahlung produced by the beta radiation sensor is suppressed by softening the beta radiation beam with films of polyester, by shielding the housing in which the detectors of the two sensors are disposed with a sheet of phenolic, and by stopping down the aperture through the sheet of phenolic through which X-ray radiation enters the detector housing.