Fluence Beam Monitor for High-Intensity Particle Beams Based on a Multi-Gap Ionization Chamber and a Method for Ion Recombination Correction

This work presents the tests of a multi-gap detector (MGD), composed of three parallel-plate ionization chambers (ICs) with different gap widths, assembled to prove the capability of correcting for charge volume recombination which is expected to occur when high fluence rates are delivered. Such beam conditions occur with a compact accelerator for charged particle therapy developed to reduce the costs, to accomplish faster treatments and to exploit different beam delivery techniques and dose rates as needed, for example, for range modulation and FLASH irradiations, respectively. The MGD was tested with carbon ions at the Centro Nazionale di Adroterapia Oncologica (CNAO Pavia, Italy), and with protons in two different beam lines: at Bern University Hospital with continuous beams and at the Laboratori Nazionale del Sud (Catania, Italy) of the Italian National Center of Nuclear Physics (INFN) with pulsed beams. For each accelerator, we took measurements with different beam intensities (up to the maximum rate of ionization achievable) and changed the detector bias voltage (V) in order to study the charge collection efficiency. Charge recombination models were used to evaluate the expected collected charge and to measure the linearity of the rate of ionization with the beam fluence rate. A phenomenological approach was used to determine the collection efficiency (f1) of the chamber with thinnest gap from the relative efficiencies, f1/f2 and f1/f3, exploiting the condition that, for each measurement, the three chambers were exposed to the same rate of ionization. Results prove that two calibration curves can be determined and used to correct the online measurements for the charge losses in the ICs for recombination.