Experimental characterization by means of a scanning proton micro-beam of the spectral response of a polycapillary optics to be used in PIXE

We have developed a PIXE (Particle Induced X-ray Emission) spectrometer for light element analysis based on a Silicon Drift Detector and on a polycapillary lens. The polycapillary lens is needed to guarantee a good solid angle for X-ray detection while minimizing the rate of back-scattered protons impinging on the detector. To this aim we are using a polycapillary lens with the nominal focal spot on the sample slightly larger than the sample area hit by protons. We verified the effectiveness of the polycapillary lens in stopping back-scattered protons (when a 3 MeV proton beam is used as primary source). However, we have seen a marked dependence of the lens focal spot (i.e. the area of the sample seen by the lens) on the X-ray energy. In order to better characterize the lens performance we have measured the spatial distribution of the collection profile of the lens at different energies with the scanning micro-beam facility of the LABEC accelerator in Florence. In this paper we will report on the detailed characterization of the lens with a two-fold aim: i) asses the performance of the lens for PIXE setups, ii) highlight the potential of the micro-beam facility for precise lens characterization.