Geant4 Systematic Study of the FRACAS Apparatus for Hadrontherapy Cross Section Measurements.

In this work, we report on systematic Monte Carlo (MC) studies for the FRACAS apparatus, a large acceptance mass spectrometer that will be used to measure the fragmentation cross sections of $^{12}$C ions for hadrontherapy. The apparatus will be made of a beam monitor, trackers surrounding a magnet and a Time-Of-Flight (TOF) wall. Using Geant4 simulations of the whole system and in-house developed reconstruction algorithms we studied the influence of the tracker positions and spatial resolutions on the charge and mass reconstruction efficiencies. An optimal configuration was found where the upstream tracker positions should be located 6 cm away from the target and spaced by 4 cm and their spatial resolution should be 100 $\mu$m. The downstream trackers positions will have to be changed according to the beam energy mostly to preserve the geometrical efficiency of the system. Their spatial resolutions, even though of a lesser importance compared to the upstream trackers, should be around 1 mm or better. In this optimal configuration, we were able to obtain an overall fragment identification efficiency above 90% for beam energies ranging from 100 to 400 MeV/nucleon.