In thin layers, the Landau model of energy loss fluctuations is not valid, because the number of collisions is too small. In this case, the atomic structure of the atom has to be taken into account. The photoabsorption ionization (PAI) model uses the photoelectric cross-sections to describe the energy loss distribution. The results (the width and the most probable value of the energy loss distribution function) given by this model are equal to those given by standard GEANT procedure described in [PHYS332]. In addition, however, it gives an estimate of the number of collision in each step ( NICOLL in the common block GCSTRA). PAI model is slower than the standard GEANT method.
PAI model is activated, if the user set the flag STRA = 1. The default value is 0.