Influence of humidity on the scattering of γ-rays in soil

Transmission and backscattering of gamma-rays have been used to study soil properties such as its porosity, hydraulic conductivity and also for the detection of organic material buried in soil. These applications require a deeper understanding of the interaction of gamma-rays with soil. In the present work a study of the interaction of 511 keV gamma-rays with layers of sand varying both its humidity and thickness is presented. The experimental set-up uses the positron decay of a Na-22 source, and two gamma detectors connected in fast time-coincidence. Both transmitted and backscattered spectra are obtained. For transmission, the photopeak and the low angle scattered photons are studied, and for backscattering we analyze the number of single and multiple backscattered photons as a function of thickness. Finally, a theoretical model for the derivative of the number of backscattered photons as a function of depth is presented.