a plurality of radiation receiving elements disposed outside a pipe at equal distances from the axis thereof so that radiation from a radiation source is received through the pipe wall . as the thickness of the pipe wall varies , the received radiation varies accordingly . if the received radiation fluctuates in excess of the allowable range , the thickness of the pipe wall is judged to be outside the allowable range . when the fluctuation of the received radiation is within the allowable range , the thickness of the pipe wall is within the preset tolerances .