This paper describes an X-ray fluorescence technique used for measuring thickness of ultra-thin coated layers absolutely and nondestructively with the non-monoenergetic primary radiation from a radioisotope source. It has been found that the precision for the layer-thickness measurements is better than 1.1%

and the relative deviation is less than 3.8% when the measured results are compared with those obtained from an α-ray thickness gauge based on the air equivalent method.