Non-Gaussian straggling in SIMNRA 8

An improved description of non-Gaussian energy straggling was introduced in SIMNRA 8. The various energy straggling components (electronic energy-loss straggling, energy spread by multiple scattering, geometrical energy spread) are approximated by asymmetric two-piece super/sub Gaussian functions. A fast algorithm for approximate convolution of these functions has been developed, resulting in convoluted functions with correct mean value, variance and third moment but approximate shape. This allows a more accurate simulation of geometrical energy spread and/or of effects where the Vavilov-shape of electronic energy-loss straggling plays a role, for example in simulating the Lewis effect in PIGE. A new Lewis-like effect in particle scattering is predicted.