Abstract: An apparatus for the nondestructive determination of the thickness of an article, e.g. a rolled steel product such as a beam, comprises a plurality of radiation sources mounted in a sectoral body rotatable about an axis such that the center of gravity of the body is offset from this axis. The body is received within a massive shielding structure so that it can be swung into a trough-like recess formed in the latter when not in use and can be swung out of this recess to bring the respective radiation sources into registry with a window formed in a cover overlying the trough-shaped body and defining a space which is filled with a cooling and absorbing fluid. The device is provided between conveyor rollers of a rolling mill line.