Real-Time Particle Radiography by Means of Scintillating Fibers Tracker and Residual Range Detectors
Applications of Optical Fibers for Sensing [Working Title]
In this chapter, a detailed description of the construction and the procedure for the measurement of performances of a charged particle imaging system is given. Such a system can be realized by the combined use of a position sensitive detector and a residual range detector. The position sensitive detector is made up of two superimposed and right-angled planes, each of which subsists of two layers of pre-aligned and juxtaposed scintillating fibers. The selected 500 μm square section fibers are
... ection fibers are optically coupled to two silicon photomultiplier arrays adopting a channel reduction system patented by the Istituto Nazionale di Fisica Nucleare. The residual range detector consists of 60 parallel layers of the same fibers used in the position detector, each of which is optically coupled to a channel of silicon photomultiplier array by means of two wavelength-shifting fibers. The sensitive area of both detectors is 90 × 90 mm 2 . The performance of the prototypes was tested in different facilities with protons and carbon ions at energy up to about 250 MeV and rate up to about 10 9 particles per second. The comparison between simulations and measurements confirms the validity of this system. Based on the results, a future development is a real-time radiography system exploiting high-intensity pencil beams and real-time treatment plan verification. Applications of Optical Fibers for Sensing 2 detector (PSD) and a residual range detector (RRD) (see Figure 1) . The main parts of this system are detectors expressly designed to achieve high-resolution imaging, high-resolution residual range measurement, large sensitive area, and high-rate beam compliance. The QBeRT system performs all these tasks and, advantageously, requires a low number of readout channels, making possible the reduction of the complexity and cost of the electronic data acquisition (DAQ ) chain, by means of a readout channel reduction system patented by Istituto Nazionale di Fisica Nucleare (INFN)  . Both detectors, PSD and RRD, are able to work in imaging conditions, with particle rate up to 10 6 particles per second, and in therapy conditions (up to 10 9 particles per second). In imaging condition, the system is capable to realize a particle radiography and permits a real-time monitoring of the patient position in treatment room. In therapy condition, the PSD acts as a profilometer, detecting the position, the profiles, and the fluence of the beam. The combined use of the information measured by the PSD and the RRD allows to check the treatment plan in real time. The design of both detectors is based on scintillating optical fibers (SciFi) with 500 μm nominal square section. The working principle of the scintillating optical fibers is schematized in Figure 2 . SciFi consist of a polystyrene-based core and a PMMA cladding. The scintillating core is a mix of polystyrene and fluorescent dopants selected to produce the scintillation light when a particle releases energy in it and sets the optical characteristics for light propagation in the fiber. Scintillation light is produced isotropically but only a portion of these photons, in the two opposite directions along the fiber, can propagate by total internal reflection (TIR) mechanism. Multi-clad fibers have a second layer of cladding that has an even lower refractive index and permits TIR at a second boundary. External EMA is an optional external layer used to eliminate optical cross talk. SciFi sizes range from 0.25 to 5 mm square or round cross-sections and available in canes, spools, ribbons, and arrays. The scintillation light is routed by the SciFi in the PSD, by means of wavelengthshifting fibers in the RRD, toward two silicon photomultiplier (SiPM) arrays, which output a proportional electric signal. PSD and RRD employ a DAQ chain divided in two sections. The first section consists of the front-end (FE) boards, which process the electric signal from the light sensor and perform the analog-to-digital conversion. Data from the FE is acquired by a readout (RO) board based on a National Instrument system on module (SoM) for pre-analysis and filtering. The actual readout channel reduction scheme applied to the PSD limits the performances of Figure 1. Schematic of the QBeRT proton tracking system.