Solomon Musibau

Numerical study of the B3G dipole fringe field impact on the spot size used in the Proteus One Gantry

Abstract

In the framework of Master’s project (MA1) in partnership with Ion Beam Applications, a numerical study of proton bunch tracking in the Proteus One compact gantry beamline (CGTR) was conducted. The Zgoubi ray-tracing code and its Python interface, Zgoubidoo, developed at the Service de Métrologie Nucléaire at ULB, has been used. Previous studies show that large fringe fields of the B3G magnet, the last bending dipole of the magnetic line, have an impact on the scanning spotsize at the isocenter. This phenomenon is important in practice because it can modify the size of the beam at the isocenter during clinical treatment of tumors. The effect of high order magnetic field components in the fringe field region of B3G is suspected. This study attempts to emphasize this effect. To investigate further, this work studies numerically the impact of the scanning amplitude on the beam size at the isocenter using the analytical model of B3G dipole implemented in Zgoubi by the means of the Enge fit distribution. We first track individual proton particles through the entire beamline, using optimal fringe field extension parameter for B3G, and we reproduce the experimental scanning pattern of previous studies. This allows us to visualize the deformation of the scanning grid compared to the experimental patch. Then, the tracking of proton bunch between B2G dipole and isocenter has been performed. We present the influence of scanning magnet settings on the beam size with and without the aperture model of the beamline. Statistical errors concerning the sample sizes are discussed. Finally, the coupling between x and y coordinates during scanning procedure is analyzed. The results suggests that the fringe field extension of B3G contains a strong sextupolar component that affects the scanning pattern in x direction.