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

Abstract

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 placed, in a 100 mbar reaction chamber, will be made of a beam monitor, trackers surrounding a magnet and a Time-Of-Flight (TOF) wall. In order to determine the required performances of the trackers, Geant4 simulations of the whole system and in-house developed algorithms were used. While keeping the beam monitor and TOF-wall positions fixed, the effects of the tracker positions and spatial resolutions on the trajectory reconstruction and mass identification efficiencies have been extracted. An optimal configuration was found where the upstream trackers should be located 6 cm away from the target and spaced 4 cm apart whereas their spatial resolutions should be close to 100 $\mu$m. The positions of the downstream trackers will have to be changed according to the beam energy to preserve high identification efficiencies. 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, an overall fragment identification efficiency above 90% has been obtained for beam energies ranging from 100 to 400 MeV/nucleon.