Viviana Scot

Diffusion of photons with arbitrary state of polarization: the Monte Carlo code MCSHAPE

Abstract When X-rays penetrate in the matter, they interact with the atoms, producing secondary radiation that carries important information about the composition of the target. The polarization state is one of the properties of the incoming photons which changes as a consequence of the number and the type of the undergone interaction. Therefore, to study properly the atomic properties of a material, it is necessary to consider the evolution of the polarization state of radiation. It is presented MCSHAPE, a Monte Carlo code developed to describe the evolution of the polarization state of X-ray photons as a consequence of the multiple scattering collisions undergone during the diffusion into the sample. In order to study properly the transport of photons with an arbitrary state of polarization, the model adopted in this code is derived from the so called ‘vector’ transport equation [Radiative Transfer, Chapter 1, Section 15, Clarendon, Oxford, 1950; Nucl. Instr. and Meth. B 73 (1993) 341]. Using the Stokes parameters I, Q, U and V, having the dimension of an intensity and containing all the physical information about the polarization state, MCSHAPE simulates the full state of polarization of the photons at any given position, wavelength and solid angle.

Deterministic and Monte Carlo codes for multiple scattering photon transport.

Deterministic and Monte Carlo techniques compete to provide the best description of transport problems. This article presents three examples from our experience in photon transport, which illustrate the close complementarity of these two approaches.

Reconstruction of the X-ray tube spectrum from a scattering measurement

An inverse technique has been designed to unfold the x-ray tube spectrum from the measurement of the photons scattered by a target interposed in the path of the beam. A special strategy is necessary to circumvent the ill-conditioning of the forward transport algebraic problem. The proposed method is based on the calculation of both, the forward and adjoint analytical solutions of the Boltzmann transport equation. After testing the method with numerical simulations, a simple prototype built at the Operational Unit of Health Physics of the University of Bologna was used to test the method experimentally. The reconstructed spectrum was validated by comparison with a straightforward measurement of the X-ray beam. The influence of the detector was corrected in both cases using standard unfolding techniques. The method is capable to accurately characterize the intensity distribution of an X-ray tube spectrum, even at low energies where other methods fail.