Investigation of defects in Fresco substrates by means of the ECoSp imaging system and the principal component image analysis

A new imaging technique is presented, based on the detection of Compton scattering photons and a further statistical analysis of the collected data. The photon detection has been performed by the Enhanced Compton Spectrometer (ECoSp) a recently devised instrument that allows one to collect the backscattered photons by investigating the tested sample from one side only. Photons collected during the planar scanning of a given area are used to describe the electronic density of the sample as a density image; afterwards, the Principal Component Analysis (PCA) was applied to the image. As a case study, a non-destructive testing of plaster substrates supporting mural paintings has been performed using this technique, searching for flaws, defects, fractures and so on. The rationale behind this procedure is that the presence of such gaps, into the otherwise uniform material, can be revealed because the existing correlation between the backscattering radiation detected by ECoSp from each adjacent volume of interest (VOI) during the scanning. The principal component correlation analysis, performed over the resulting data from all VOIs, then reveals each gap in his size, shape and position.