Improved fitting of PIXE spectra: the Voigt profile and Si(Li) detector modeling

The true emitted X-ray lineshape as measured by a Si(Li) detector is the convolution of the intrinsic Lorentzian X-ray lineshape and the detector response function. We demonstrate the necessity of using the Voigt profile - the convolution of a Lorentizian and a Gaussian - to fit the full-energy peak portion of directly measured X-ray lines. By incorporating the Voigtian in our PIXE spectrum fitting code, PIXEF, we have found consistent improvement in the quality of fit and calculated elemental yields. We have also found that the Voigtian fit is required to give an accurate ratio of tail to peak intensity. We attribute the tail to a surface layer of incomplete charge collection (ICC) at the front of the detector. Although this model is improved by appropriately accounting for the loss of photoelectrons that travel back to the ICC layer after being emitted from the intrinsic region, it appears to fail when the full-energy peak is fit to a Gaussian. On the other hand, excellent agreement between the improved model and experiment is found when the full-energy peak is fit to a Voigtian.