Interpretation of the gamma-Compton densimeter response by analyzing multi-scattered photons
F. Pino 1, H. Barros 1, L. Sajo-Bohus 1*, D. Palacios 1
1 Simón Bolívar University, Nuclear Physics Laboratory, Caracas, Venezuela
Abstract. A detailed Monte Carlo simulation of the gamma Compton scattering process was performed in order to understand better the global phenomena and to improve the interpretation of a γ-Compton densimeter’s response. The experimental set up consists in a mono-energetic gamma ray source (137Cs) of 0.5 mCi (18.5 MBq), a gamma ray scintillator detector BGO (with a cylindrical sensitive volume of 3” x 3”), a set of pieces of lead (for shielding), an usual nuclear electronic chain for the energy spectrum acquisition (pre-amplifier, amplifier, multi-channel analyzer, PC) 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 interpret better the intensity and the shape of the acquired spectrum. These additional information consists in the distribution of the number of detected photons as a function of the number of Compton scatters occurred in the concrete, the average number of Compton scatters in the concrete as a function of detected photons energy and the individual energy spectra of the photons that suffer “n” Compton scatters (n=1, 2, 3,..., 10 or more) in the concrete before they are detected. It is shown that all these new information added to the total energy spectrum can help not only to determine the bulk density of a concrete (by making a calibration curve of the intensity as a function of the density), but also can help to identify non-homogeneous zones, with very low density in comparison to the concrete density (2,3 g/cm3), like cavities, pipes, etc. This identification is possible by means of a detailed examination of the acquired spectrum shape, which is featured by the presence of a slight distribution in energies corresponding to photons that suffer only one (n=1) Compton scatter. The area of n=1 peak (in comparison with the rest of the spectrum) is a direct consequence of the probability of the photons to trespassing the material after just one collision, i.e. is related with its mean free path and so with the density of the surrounding material.
Preferred presentation: oral
Presented by: Laszlo Sajo-Bohus