Using a carefully controlled and precisely measured source, a gamma ray radiation beam is projected into the pipe wall where it is reflected and scattered back by the various surfaces it encounters. A detector unit detects and measures the intensity of reflected gamma rays from the pipe wall. The intensity of back scattered gamma rays varies with the thickness of the pipe wall.

Gamma ray is also used by the “through transmission” technique in which a beam is projected directly through the pipe and out the opposite side where it is measured by a detector unit. By measuring the intensity of the beam after passage through the pipe, the operator can determine the thickness of the steel through which it has passed. TUBOSCOPE uses this technique to measure wall thickness in the inspection of used tubular goods.