Thickness uniformity of beryllium foils derived from energy loss broadening of transmitted MeV protons

The thickness uniformity of beryllium foils commonly in use as entrance win dows of Si(Li) detectors has been determined by measuring the broadening in energy loss of 1.5-2.5 MeV protons transmitted through web foils. The ener gy loss spectra were measured after backscattering of the transmitted proto ns from a thin layer of gold on a polypropylene film. The contribution due to energy loss straggling was assessed in transmission studies on uniform f ilms of polypropylene and polyester (Mylar), The full width at half maximum (FWHM) of the loss peaks for the polymer films increased with the square r oot of the energy loss Delta E, in accordance with theory. By contrast, the FWHM of the energy loss in Be increased linearly with increasing Delta E, with a maximum FWHM = 0.21 Delta E at 2.5 MeV. After correcting for the str aggling contribution in quadrature, the net excess broadening was found to range from 18% to 20%, for Be foils with a nominal thickness between 12.5 a nd 37.5 mu m. The excess broadening is attributed to a corresponding variat ion in thickness across the foils, probably due to a significant porosity g enerated during the fabrication process (sintering). This result supports p revious indirect evidence derived from studies on background generation in proton-induced X-ray emission spectrometry (PIXE). (C) 2000 Elsevier Scienc e B.V. All rights reserved.