An application of the relative attenuation of X rays and γ rays for the measurement of depth of radioactive contamination in concrete and is described, based upon the photon energy spectrum of caesium-137. This has been tested on two bespoke phantoms with pressed/cast concrete and silica sand highlighting the potential for sub-millimetre depth resolution. This research highlights the importance of the ease of calibration, prior to application in the field, and improved knowledge of materials composition in legacy facilities.