A method for monitoring tubing wall thickness includes conveying a tool through a tubular string in a borehole, the tool including a photon source that directs a photon beam along a radial path toward a wall of the borehole, the tool further including an array of collimated detectors that measure Compton backscattering rates at respective distance bins along the radial path. The method further includes calculating a sequence of ratios between measurements from neighboring detectors. The method further includes identifying one or more local extrema in the sequence, each extremum representing a boundary between different materials. The method further includes determining a wall thickness of the tubular string from absolute or relative positions of the one or more extrema. The method further includes displaying a representation of the wall thickness.