Measurements of the average thickness of material using scattered radiation

A method for determination of the average thickness of material placed between a gamma-ray source and a detector is presented. The method requires the measurement of the ratio between the number of gamma-rays scattered at small angles in the material and registered in the spectrum and the number of gamma-rays penetrating the material without interacting with it and registering in the full-energy peak. The relation describing that ratio in terms of the average gamma-ray path length in the sample, the energy interval where the scattered radiation is measured and the detector response to unscattered gamma-rays was verified by measurements of gamma-ray path lengths in material with known thicknesses at energies of 662 and 1115 keV. Differences between the measured and known thicknesses were less than 15% for thicknesses of copper between 2.5 and 17 g/cm2.