An obsolete, nucleonic coal-sensing instrument that can measure the thickness of coal left on the roof or floor of a seam after the passage of a mining machine. The principle used is the measurement of the density of the strata underlying the machine by a gammaray backscattering unit. Gamma rays from a radioactive source are scattered in all directions by the atomic particles in the coal and rock. The amount of scattered radiation eventually reaching the Geiger counter is, approx., inversely proportional to the density of the scattering medium; i.e., more radiation will come back from coal than from rock. Thus, as the amount of coal between the source and the underlying rock changes, so the amount reaching the Geiger counter and the counting unit (the ratemeter) will change, and consequently the output of the meter can be calibrated in terms of the thickness of the floor coal. This instrument has been replaced by a natural-gamma coal thickness sensor.