[en] In a filling level measuring installation with a radioactive radiator, which can be rotated within a lead shield from a peripheral operating position opposite the measuring receiver into a central safety position, a second radiaiton detector is arranged on a rearward extension of the measuring path in such a manner that in the safety position, its distance from the radiator is smaller than in its operating position, so that the output signal of the second detector can serve for monitoring the radiator position