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Description
The measurement of proton and light nuclei fragmentation cross sections with energies up to $400 \ MeV/A$, which is relevant for improving treatment-planning systems in hadron therapy, is the primary goal of the FOOT (FragmentatiOn Of Target) experiment. FOOT will use an inverse-kinematic approach profiting from the Lorentz boost to detect nuclear fragments that would otherwise stop in the target. The momentum, time of flight and energy will be measured by a spectrometer, a thin scintillator and a BGO calorimeter respectively, and will make mass identification possible. In this work we present the design of the BGO calorimeter and performance of its prototype.
The use of SiPM arrays coupled to the crystals allows a compact design for the calorimeter. We tested $2$ different FBK SiPM arrays with $15$ and $20 \ \mu m$ microcell size, respectively, coupled to $24 \ cm$ long BGO crystals in two different configurations (reflective for $15$ and absorbent for $20$). At the CNAO facility in Pavia, Italy, we have measured the detector response to low intensity ($\sim 10 \ KHz$) proton and carbon beams with energies up to $220 \ MeV$ and $400 \ MeV/A$, respectively. The reflective configuration on the $15 \ \mu m$ provides the best results: energy resolution is below $1 \%$ for carbon ions and smaller than $2\%$ for protons down to $120 \ MeV$; it increases to $3\%$ for $70 \ MeV$ protons. Moreover, no saturation effect up to $4.8 \ GeV$ deposited energy is found. Non-linear response to ionizing particles is observed, which can be caused by scintillation quenching effects. The SiPM signals are sampled at $1 \ GHz$ rate to extract temporal properties of the light pulses. Observed differences in shapes of the pulses provide an additional possibility for particle identification and improved energy correction. Therefore, the $15 \ \mu m$ SiPMs are a good candidate for a photosensor in the BGO calorimeter of the FOOT experiment.
