Development and performance assessment of the BGO calorimeter module for the FOOT experiment

FOOT (FragmentatiOn Of Target) is a nuclear physics experiment currently under construction that will measure diff erential cross sections for the production of secondary fragments induced by the interactions of proton and ion beams, up to 400 MeV/u, with human tissues. By extending the energy range to about 800 MeV/u, FOOT will also provide data useful to radio-protection in space, as understanding fragmentation processes that take place in spacecraft shieldings is crucial to their optimisation. The FOOT collaboration is building a detector designed for the identification of heavy fragments in an inverse-kinematics configuration, through the measurement of their momentum, energy and time-of-flight with very high resolution. The kinetic energy of the fragments will be measured with a BGO calorimeter that must cover a dynamic range from tens of MeVs to about 10 GeV and achieve an energy resolution smaller than 2%. In this work we report about the R&D steps that led to the design choices and we assess the performance of the calorimeter prototype. Several beam tests have been performed at CNAO (Pavia, Italy) to choose the best photodetector, crystal wrapping, front-end electronics and readout, in order to achieve the required performance in terms of linearity and energy resolution. Measurements on the first assembled module, made of 3 × 3 BGO crystals with truncated pyramid shape, coupled to SiPM photodetectors, show that, up to at least 5 GeV of deposited energy, there is no saturation effect related to optical photon pileup in the SiPM microcells and the energy resolution ranges from about 2% standard deviation for 70 MeV protons to less than 0.5% for 400 MeV/u carbon ions. This level of performance has been achieved on data collected within a temperature range of about 10◦ C. Deviations from linearity were studied by calibrating the crystals with monochromatic beams impinging both on the front face and at different positions along its side. Correction methods to compensate for the signal loss as a function of the range (i.e., the energy) and for temperature fluctuations, were developed and validated on experimental data. Presently, the full calorimeter construction (320 BGO crystals) is complete.