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 t he convolution of the intrinsic Lorentzian X-ray lineshape and the det ector response function. We demonstrate the necessity of using the Voi gt profile - the convolution of a Lorentzian and a Gaussian- to fit th e full-energy peak portion of directly measured X-ray lines. By incorp orating the Voigtian in our PIXE spectrum fitting code, PIXEF, we have found consistent improvement in the quality of fit and calculated ele mental yields. We have also found that the Voigtian fit is required to give an accurate ratio of tail to peak intensity. We attribute the ta il to a surface layer of incomplete charge collection (ICC) at the fro nt of the detector. Although this model is improved by appropriately a ccounting for the loss of photoelectrons that travel back to the ICC l ayer after bring emitted from the intrinsic region, it appears to fail when the full-energy peak is fit to a Gaussian. On the other hand, ex cellent agreement between the improved model and experiment is found w hen the full-energy peak is fit to a Voigtian.