Concentration profiles in paint layers studied by differential PIXE

Differential PIXE measurements varying the proton energy were used to probe the concentration profiles of metal-based pigments in paint layers. The algorithms developed earlier for metal targets were improved and enhanced to include light elements; the necessary information on chemical compounds has to be provided by complimentary methods. The de-convolution method employs slicing the target into layers characterized by mean production depths; the matrix inversion is replaced by a minχ2 problem. Two different methods of normalization are used: setting the sum of weight fractions of particular compounds to unity, and direct measurements of the projectile number, in our case through the argon line excited in the air. The efficiency of the two methods was compared for paint layers in frescoes, showing that smother concentration profiles are obtained using the measured proton numbers. Conversion of the layer areal densities into geometrical thicknesses is discussed.