A Proton Miniprobe For Mineralogy

A mini-PIXE beam line has been installed on our 3.5 MV Van de Graaff accelerator. It was built using components available in the laboratory, in particular an old HVEC quadrupole doublet. The field map of this doublet was measured and first order beam transport calculations were performed to determine the geometric parameters of the beam line, as well as the corresponding magnetic parameters for each quadrupole lens. Using a fixed size object source (0.5 mm horizontal, 3 mm vertical), we obtained beam spots with diameters of 40 μm (FWHM) at incident energy between 2 and 3 MeV and with current intensity larger than 1 nA. Such a small beam spot (40 μm) is appropriate to localize and analyze the presence of trace elements in mineralogical samples. In a preliminary test we measure in a thick ZnS matrix the presence of Fe, Cu, Ag and Cd, as well as profiles of concentration. This test provides information about the sensitivity of our probe.