Abstract: A probe for determining the density of the material forming the wall of a wellhole provided with a selectively actuable source of gamma radiation pulses, including a pulse electron accelerator having an anode and cathode in axial alignment with a control electrode therebetween, the anode being connected to the output of the secondary coil of a Tesla transformer the primary coil of which is connected to the output of a capacitive high voltage charge unit. Emitted and scattered radiation are measured by first and second detectors the outputs of which are fed to a divider to derive a quotient of the outputs as a value representative of the density.