Particle scintillating trackers: design and read-out of real-time, large-area, highly segmented detectors

Abstract

This thesis aims to show as technological advances and suitable technical solutions can make scintillation detectors for charged particles applicable in two research areas completely different, but both of great interest for improving the quality of human life: medical diagnostics, in particular before or during a hadrontherapy treatment and muon tomography applied to cargo containers for the identification and localization of potential radioactive, high-Z substances that are illegally transported. The structure of this thesis is illustrated with the description, the characterization and the possible applications of large-area, highly segmented trackers, made of scintillating materials. The OFFSET Research Project aims to realize a detector made of water equivalent material for the real-time imaging and tracking of charged particles to be applied in medical diagnostic. The first prototype of the OFFSET tracker was designed with a single X-Y position plane having a sensitive area 20x20 cm^2. It has the great advantage of a read-out channel reduction system applied to a large-area detector with a high spatial resolution, employing 500 micron Sci-Fis and an appropriate functional architecture. Subsequently a new version of the detector, the OFFSET3 tracker, having two X-Y position planes was constructed in order to reduce the overall size and to improve imaging performances being able to acquire charged particles tracks and by extending the field of view to 28.8x28.8 cm^2. OFFSET3 has the additional advantage of using the reduction channel technique combined with the Time Over Threshold analysis to reduce the number of read-out channels and the ribbons to be employed for the X-Y planes. The tracker has about 1 ns time resolution, it is water equivalent and has about 5 MHz maximum acquisition rate.Both detectors were tested with cosmic rays, beta sources, 62 MeV proton beam at the LNS-INFN. In addition, the OFFSET3 tracker was tested with 226 MeV proton beams and 400 MeV/A carbon beams at the CNAO hadrontherapy center, in Pavia. Complete characterizations were carried out and the results were presented. The use of Silicon photomultiplier replacing a multi-anode Position Sensitive Photo-Multiplier (PSPM) array coupled to the fibers is also under consideration. The technological know-how acquired with the design, construction and characterization of the OFFSET3 detector has been of great assistance for the construction of the Muon Portal detector (PORTAL) for the containers inspection. PORTAL consists of eight 3x6 m^2 layers corresponding to two X-Y trackers separated by 280 cm. In each layer there are six 1x3 m^2 identical modules, made of one hundred scintillating strips (10x10x3002 mm^3) coated with reflective film. The detection principle is based on the determination of the scattering angle of each muon from cosmic ray radiation as a result of crossing a high-Z material (both nuclear material and high-Z shielding materials). Indeed, the angular deflection of the trajectory is very sensitive to the Z of the atomic nuclei. The scattering angle can be reconstructed by the incoming and outgoing tracks with respect to the scanned object. Comparing the inspection techniques exploiting other particle sources to muon tomography, the last presents some advantages. For example the scan is not invasive and the acquisition time required is of a few minutes for a complete 3D tomography. The second part of this thesis reports the results of work on the PORTAL detector to define the technical details and choices connected with the construction phase. Preliminary test results on different strips of a module are ready and additional tests on modules are in progress. The first layer was already placed on the custom-designed mechanical support (an iron steel 6x3x7 m^3 framework). In total, 20 modules were already assembled. The final large-scale prototype will be ready around mid-2015 and it will be used to inspect containers transiting in ports.