Abstract: A cable insulation eccentricity and diameter monitor comprises a revolving structure mounted coaxially with the cable, a finely collimated source of gamma particles mounted on such revolving structure for generating a beam of gamma particles, a radiation detector fixed to the revolving structure for detecting the gamma particles scattered from such beam by the cable and for generating pulses the amplitude of which is dependent on the energy of the scattered gamma particles reaching the detector, first drive means for moving the source of gamma particles so as to direct the beam at predetermined target positions corresponding to probable conductor and cable/atmosphere boundaries of the cable along a first axis perpendicular to the axis of the cable, second drive means for repositioning the revolving structure to move the source of gamma particles and the radiation detector to a position around the cable so as to direct the beam at target positions corresponding to probable conductor and cable/atmosphere bounda