The use of Compton backscattering for the contactless measurement of the thickness of steel sections and similar dense materials up to 25 mm thick, when access is available only from one side of the sample, has been investigated. The expected intensity of back-scattered photons in a Compton thickness gauge has been estimated using a simple model, whose predictions have been verified and extended by the use of the GEANT3 Monte Carlo package which allowed the scattering geometry to be modelled accurately and the effects of multiple scattering to be estimated. The simulations have been confirmed by experimental measurements on steel samples 1-45 mm in thickness. With a 5 Ci 60 Co radioactive source, a rapid, reliable measurement of wall thickness can be obtained with steel samples up to 20 mm thick, but for thicker samples the absorption of backscattered photons limits the accuracy that can be achieved.

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