Use of the gamma function for straggling in simulation of RBS spectra

In standard computer codes for the analysis of RBS data, the energy straggling is taken to be a Gaussian function. This leads to unphysical simulations whenever the energy spread is comparable to the average energy loss and to the system resolution, which may happen in the near surface region. We propose to use the gamma distribution for the energy straggling, since it asymptotically approaches the Gaussian distribution for small values of the energy spread relative to average energy loss, while it has no high energy tail above the initial beam energy. This was implemented in the code NDF. We compare the results with calculations made with the stochastic model for 100 keV protons in Hf delta layers in Si and show that a major improvement is obtained with regard to the Gaussian distribution. We also show calculations for 1.5 MeV 4He+ experiments