RADIATION GAUGE FOR MONITORING SHEET PROPERTIES EMPLOYING A SCANNED SOURCE

A radiation gauge for monitoring a property of a sheet material includes, in combination with an array of stationary detectors positioned across the sheet width, a source of penetrating radiation scanned across the width of the sheet in response to fluid pressure. The source position is determined in response to signals read out from the several detectors comprising the array. The source pattern is effectively maintained isotropic by a slot and key arrangement or by rotating the source at a relatively high spin velocity. The individual detectors are calibrated for different responses across the sheet width. Source detector standardization is performed by translating the source to an off-sheet position so that the source field impinges on an off-sheet detector. In response to standardizing with the off-sheet detector, each of the other detectors is standardized in a like manner.