A detailed Monte Carlo simulation of the gamma Compton scattering process was performed in order to better understand the global phenomena and to improve the interpretation of a γ-Compton densitometer's response. The experimental set up consists of a mono-energetic gamma ray source ( 137Cs), a BGO detector (sensitive volume of 3″ × 3″), a set of pieces of lead (shielding), a standard electronic signal chain for the energy spectrum acquisition and a material under study (Portland Concrete). The Monte Carlo code, PENELOPE 2008, was modified to obtain additional information, about the "detected" photons, that can be useful to understand the relevance of the number of Compton interactions for each primary photon, and so better interpret the count rate and the shape of the acquired spectrum. It is shown that all this new information added to the total energy spectrum can help not only to determine the bulk density of concrete but also it can help to identify non-homogeneous zones, with very low density in comparison to the concrete density, like cavities, pipes, etc. © 2012 American Institute of Physics.