MCSHAPE: A MONTE CARLO CODE FOR SIMULATION OF POLARIZED PHOTON TRANSPORT J.E. Fernández, V. G. Molinari, M. Bastiano, and V. Scot National Institute of Physics of Matter (INFM) and Laboratory of Montecuccolino, Department of Energetic, Nuclear and Environmental Control Engineering (DIENCA), University of Bologna, via dei Colli,16, I-40136, Bologna, Italy. ABSTRACT MCSHAPE, a new Monte Carlo code based on the recent analytical solution of the vector transport equations in plane geometry including polarization effects, was developed to provide a proper description of the polarization state evolution of photons through multiple scattering collisions. The code considers a detailed description of the prevailing interactions in the X-ray regime (Rayleigh and Compton scattering, and photoelectric effect) and infinite/finite thickness multi- component homogeneous target. As a result, MCSHAPE can give the final state of polarization for each collision number. Good agreement with both deterministic calculation and experimental data was obtained for excitation with unpolarized and linearly polarized sources. INTRODUCTION It is possible to trace back the atomic properties of a material by studying the secondary radiation generated by the interactions between a source of X-rays and its atoms. One of these properties is the polarization state of the interacting photons which changes as a consequence of the number and type of the undergone interactions. The effect of the polarization state on the intensity spectrum is mainly important when the incident beam is partially or totally polarized. MCSHAPE is a Monte Carlo code developed to describe the diffusion of X-rays into a sample taking into account their full state of polarization. The mathematical model is deduced in the frame of transport theory, using the Boltzmann equation for photons. There are different degrees of approximation to describe the diffusion of photons in the matter. A first approximation consists in the use of the so called ‘scalar’ model in which we assume that the photons interacting with a sample cannot modify an average polarization state. Under this condition, the transport of photons is analogous to that of neutrons, i.e. they are considered as neutral ‘particles’. In this case, photon interactions are described using average polarized differential cross-sections. This is the model adopted in the widely used standard versions of the Monte Carlo codes MCNP and EGS4. A higher level of detail can be reached by introducing the so called ‘vector’ transport equation (Chandrasekhar 1950; Fernandez and Molinari 1993). Such a model describes properly the transport of photons with an arbitrary state of polarization. The code MCSHAPE has been written following this second Copyright©JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46. 363