  Fast procedures for the beam quality assessment and for the monitoring of
beam energy modulations during the irradiation are among the most urgent
improvements in particle therapy. Indeed, the online measurement of the
particle beam energy could allow assessing the range of penetration during
treatments, encouraging the development of new dose delivery techniques for
moving targets. Towards this end, the proof of concept of a new device, able to
measure in a few seconds the energy of clinical proton beams (from 60 to 230
MeV) from the Time of Flight (ToF) of protons, is presented. The prototype
consists of two Ultra Fast Silicon Detector (UFSD) pads, featuring an active
thickness of 80 um and a sensitive area of 3 x 3 mm2, aligned along the beam
direction in a telescope configuration, connected to a broadband amplifier and
readout by a digitizer. Measurements were performed at the Centro Nazionale di
Adroterapia Oncologica (CNAO, Pavia, Italy), at five different clinical beam
energies and four distances between the sensors (from 7 to 97 cm) for each
energy. In order to derive the beam energy from the measured average ToF,
several systematic effects were considered, Monte Carlo simulations were
developed to validate the method and a global fit approach was adopted to
calibrate the system. The results were benchmarked against the energy values
obtained from the water equivalent depths provided by CNAO. Deviations of few
hundreds of keV have been achieved for all considered proton beam energies for
both 67 and 97 cm distances between the sensors and few seconds of irradiation
were necessary to collect the required statistics. These preliminary results
indicate that a telescope of UFSDs could achieve in a few seconds the accuracy
required for the clinical application and therefore encourage further
investigations towards the improvement and the optimization of the present
prototype.
