New reconstruction algorithm for the fast neutron MONDO tracker

Summary. — Particle Therapy employs charged light ions to treat cancer with localized energy deposition, so that damages to healthy tissues are significantly reduced with respect to conventional radiotherapy. When using carbon ion beams fragmentation has to be taken into account: the ultra-fast secondary neutrons can be responsible for additional dose also far from the treated volume, and are likely candidates for the occurrence of secondary malignant neoplasms. MONDO is a neutron tracker dedicated to the characterization of the secondary neutrons produced in the interaction between human tissues and the therapeutic beam. It is made of a compact matrix of plastic scintillating fibers coupled with a CMOS-based SPAD sensor (SBAM) developed specifically for MONDO. Its technique of measuring the neutron four-momentum is based on the detection of recoiled protons scattered during two consecutive elastic collision events. A Monte Carlo simulation based on the FLUKA code has been developed to optimize the MONDO detector geometry and study its characteristics. The sensor has been fully implemented in the simulation, and an algorithm for event reconstruction, operating with a cluster building technique, has been written to evaluate detection efficiency, energy resolution and backpointing rejection capability from the chip readout. The algorithm description and first results are presented in this manuscript.