In this page, some results of the code MCSHAPE are presented. First, it is shown the comparison between codes which use the ‘scalar’ approach to polarization and the code MCSHAPE; then, the results of the code in a sample of water are analyzed in order to underline the influence of the state of polarization of the X- ray source for light elements.

Comparison with ‘scalar’ codes

Fig. 1 shows the comparison between the Monte Carlo models:

• the black line represents the spectrum obtained with the scalar code (which considers a medium state of polarization)
• the blue line represents the spectrum of the scalar model, modified to take into account the polarization ([1])
• the red line is the spectrum obtained with the code MCSHAPE ([2])

Fig. 1 - Comparison between ‘scalar’ models and MCSHAPE. The source is unpolarized and monochromatic. The target is carbonium and the scattering angle is 90°.

Analyzing the highlighted area, it is evident that exist a difference between the results of the code MCSHAPE and those of the ‘scalar’ codes. In fact, for the intensity, the spectrum obtained by MCSHAPE is higher than the one detected by the scalar codes, because of the contribution of the second order of collision. This result is not influenced by statistical phenomena, but, probably, it depends by the correlation between the components of the Stokes vector. This behaviour is more evident for the lightest atoms.

Linear polarized sources

One of the most important characteristics of the code MCSHAPE is the possibility of analyse spectrum of targets radiated by polarized sources. In what follows, it is presented a simple (but very significant) example which shows how the spectrum of water depends on the orientation of the scattering vector related to the plane of scattering, in the case of linear polarized sources.

When light has parallel polarization in the plane of scattering, Compton and Rayleigh scattering decrees. On the other hand, when the polarization of light is perpendicular, the scattering phenomena are emphasized; for unpolarized light we have an intermediate spectrum (fig. 2). Photoelectric effect, instead, is assumed not influenced by the state of polarization of the incident radiation. 

Because the influence of the state of polarization is more evident for light elements, the use of a linear parallel polarized sources can sensibly reduce the noise generated by the scattering in the water when a heavy element is present in solution.

Fig. 2 – Spectrum of a target of water (thickness 10 cm). The energy is 59,54 keV, scattering angle is 90°.

REFERENCES:

1. Namito, Y., Ban, S. and Hirayama, H. “Implementation of linearly-polarized photon into the EGS4 code” Nuclear Instruments and Methods in Physics Research A332, 277-283 (1993).

2. J.E. Fernandez, V.G. Molinari, M. Bastiano and V. Scot “Diffusion of photons with arbitrary state of polarization: the Monte Carlo code MCSHAPE” Nuclear Instruments and Methods in Physics Research B213, 105-107 (2004).

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