Advanced SiC detectors development for time-of-flight particle diagnostics

A suitable instrumentation development is critical for exploring the new challenging boundaries of high energy physics experiments. In particular, laser-accelerated proton (ion) beams raise interest in branch of particle acceleration for the promising perspectives that such a system can offer. Compact Joule-scale laser systems in the sub-100 fs domain are attractive for the operation of high-intensity proton (ion) beam line because of the high repetition rate, potentially reaching the 100 Hz or even the kHz range, and of the compactness that an integrated laser-driven acceleration system could guarantee, improving the performance of common acceleration systems. Starting from the beginning of this century, many independent experiments involving high intensity laser systems (1019-1020 W/cm2) irradiating thin solid targets have provided the evidence that multi-MeV proton bunches are accelerated at the rear side of the target. The most of these experiments can be interpreted through the so-called target normal sheath acceleration (TNSA) model introduced by Wilks et al. (2001). Although some other acceleration mechanism models have been proposed, TNSA has become the reference framework to interpret observations of multi-MeV protons coming from the target rear side. TNSA-generated proton beams are highly laminar and have very low emittance, features that make this mechanism appealing for a number of applications. The unique properties listed before suggest possible application in fields of proton imaging, probing or radiography as well as in creation of warm dense matter. Biomedical application are however those probably more interested in developing laser-accelerated proton irradiation systems. Future hadrontherapy facilities could be based on laser-accelerated ion beams rather than on current accelerator system, with substantial advantages from economical and facility compactness points of view. Radioisotope production represents another potentially interested medical application. In view of such possible applications, a proper diagnostics with stringent requirements in terms of temporal and spatial resolution is required. Diagnostics must be implemented at the operational repetition rate. Furthermore, laser-driven proton detection must be more robust than that of conventional ion accelerators due to high peak current (high bunch charge with duration of few tens of ns), the strong electromagnetic emission during the laser-plasma interaction and the hostile detection environment close to the source (highly radioactive environment). Laser-accelerated ion monitoring is a complex task because of some beam properties emerged during the various experimental campaigns conducted along the years. As already said, accelerated ion bunches are polyenergetic, show scarce repeatability even within the same experimental session with an unvaried setup and are characterized by a high angular divergence, in spite of the reduced beam emittance. Detectors employable in such a beam diagnostics must fulfil stringent requirements to guarantee good performances. Within this high complexity context, INFN (Istituto Nazionale di Fisica Nucleare) projects SiCILIA (Silicon Carbide Detectors for Intense Luminosity Investigations and Applications) and L3IA (Line for Laser Light Ion Acceleration) find their natural placement. SiCILIA aims at the realization of innovative detection systems based on silicon carbide (SiC) for monitoring high-energy nuclear physics experiment. The excellent electronic properties of 4H-SiC epitaxial layers are advisable for the manufacturing of proton and light ion monitors. The most appealing properties are the wide bandgap, the high carrier velocity and the high energy threshold for defect formation. SiC detectors can operate at temperature higher than 300°C and are more resistant to ionizing radiations than Si devices. They can therefore be used in a high-level radioactive ambient, such as nuclear reactor or particle accelerator, where the temperature and the radiation environment preclude the use of conventional microelectronic semiconductors. In particular, a 4H-SiC detector is a perfect candidate for operating in harsh conditions typical of laser-plasma accelerated ion beam diagnostics. SiCILIA project points at using epitaxial layers grown on custom 4H-SiC substrates for the implementation of ΔE and E detectors. Both device types are developed as Schottky diodes and p-n junctions. L3IA has the purpose of establishing a beam-line operation of a laser-plasma source in Italy. It is established at the Intense Laser Irradiation Laboratory of INO-CNR, in Pisa, where the ILIL-PW laser facility features a >100 TW laser system, a beam transport line and a multi-purpose interaction area with radiation shielding. Here, TNSA mechanism is exploited to accelerate proton bunches at several MeV energy. Radiation is monitored through a range of diagnostics, such as radio-chromic films (GAF), Thomson Parabola and Time of Flight (TOF) solid-state detectors. The present thesis work lives on the meeting point between the two introduced projects: the study of SiC detectors applied in TOF diagnostics for laser-accelerated ion measurements. The aim of this analysis is to characterize some 10 μm epitaxial layer SiC-Au Schottky Barrier Diodes (SBD) produced by the CMM (Center for Materials and Microsystems) of FBK (Fondazione Bruno Kessler, Trento, Italy) institute within the project SiCILIA in collaboration with the Electronics Laboratory of the Energy Department of Politecnico di Milano and the LASA (Laboratorio Acceleratori e Superconduttività Applicata) Laboratory of INFN. Electrical and radiation detection characterizations are performed with particular attention towards the layout influence in detector performances. Being the analysed detectors realized with different junction termination extension (JTE) edge structure, one of the main achievement is to comprehend the effect of different JTE in terms of leakage current, breakdown strength and radiation detection energy and time resolution. Then, characterization results are used to interpret the experimental pulses acquired by one of this FBK 10 μm thick SiC device mounted as TOF detector inside L3IA interaction room. The detector working principles are substantial in laser-accelerated pulse interpretation and ion bunch spectrum reconstruction. Analysed experimental pulse are compared with those acquired by other TOF SiC detectors (15 μm thick layer) placed in different positions inside the interaction room. SBD detectors, object of this work, are 10 µm epi-layer contacted with a thin gold layer (20 nm) to form a Schottky contact. Some samples of various size are available: 2.5 × 2.5 mm and 5.0 × 5.0 mm with different border structures. The forward characteristics, measured with high-precision Keithley picoammeters, evidence the good rectifying properties of the diodes. The assessment of detector leakage current through reverse I-V characteristics is a quite delicate measurement, being the proper reverse currents lower than those induced by PCBs insulation, onto which the device is assembled. A measurement performed with a specific probe station confirms the optimal properties of SiC SBD devices in terms of reverse current density with a value of 5 pA/cm2 at an average electric field of 10 V/µm (105 V/cm).All the devices tested support a reverse bias voltage of 500 V without incurring in breakdown or discharge phenomena independently from their JTE structure. C-V measurements verify that diode capacitances scale with device effective surface, namely with the nominal surface (Au-SiC contact) plus that covered by Al field plate deposited on detector periphery. Border effects are less pronounced for 5.0 × 5.0 mm devices, having a field plate surface on overall surface ratio lower than 2.5 × 2.5 mm ones. For what concerns the response to ionizing radiation, a series of measurements with 241Am α particles are presented. α spectra acquired with a classical gaussian shaping electronic chain give reassuring results in terms of particle detection resolution. A 6.3% energy resolution is found. This value is quite higher than those presented in literature because of a non-optimized experimental setup. Peak width is broadened by straggling contributions introduced by the fact that measurement is performed in air and that 241Am α particles are too energetic to be stopped in 10 µm of SiC. Time resolution properties are instead assessed through Transient Current Technique (TCT) measurements, performed with broadband amplifiers. Even when polarized with sufficiently high voltages to reach collected charge saturation velocity, detector pulse is slowed down by an exponential tail due to RC effect that detector high capacitance (of the order of 30-40 pF for 2.5 × 2.5 mm devices) makes with amplifier input impedance. Such an effect is taken into account in modelling detector TOF response. Finally, a series of experimental results obtained using a FBK device to monitor the radiation emitted by plasma generated high-intensity ILIL-PW laser are presented. All the difficulties tied to non-monocromacity, scarce repeatability and high angular divergence of ion bunches are evidenced. To tackle this problem trying to gain a deeper understanding of physics, two analysis tools have been developed. The first simulates detector pulse after irradiation with a bunch of ions, taking into account detector response and dead layers effects in radiation-detector interaction modelling. While the overall shape of the pulse is correctly simulated, the experimental pulse tail does not agree with simulation and is still under investigation. The second tool allows to reconstruct the proton energy spectra that has generated the acquired pulse, taking as always into account detector effect. Spectra can be retrieved into an energy range going from 1 MeV up to proton cut-off energy with an uncertainty of 6% on reconstructed (d^2 N)/dEdΩ for cut-off energies close to 5 MeV.

Lo sviluppo di una strumentazione adatta è fondamentale all’interno della ricerca nel campo della fisica delle alte energie. In particolar modo, la diagnostica di fasci di protoni e ioni leggeri accelerati in seguito a interazione di laser ad elevatissima intensità (1019-1020 W/cm2) con target solidi richiede un’elevata risoluzione spaziale e temporale e continuo sviluppo tanto nell’ambito della pura esplorazione scientifica quanto in vista di un possibile sviluppo applicativo di questa affascinante tecnologia. L’analisi sperimentale condotta in questa tesi si basa su due progetti INFN (Istituto Nazionale di Fisica Nucleare) volti a garantire una spinta sullo sviluppo di diagnostiche di fasci di particelle accelerati sempre più efficienti e che possano garantire una migliore comprensione della fisica e delle possibili vie di applicazione. SiCILIA (Silicon Carbide Detectors for Intense Luminosity Investigations and Applications) punta a sviluppare innovativi rivelatori in carburo di silicio (SiC) per applicazioni nel campo della fisica delle alte energie. Il carburo di silicio è un materiale molto promettente per la rivelazione di protoni, neutroni e ioni leggeri in ambienti ostili, come reattori nucleari o acceleratori di particelle, dove i comuni semiconduttori dell’industria microelettronica (fra tutti il silicio) non possono funzionare adeguatamente. L3IA (Line for Laser Light Ion Acceleration) ha come scopo lo stabilimento di una linea di fascio accelerato da una sorgente laser. La linea è realizzata presso l’Intense Laser Irradiation Laboratory (ILIL) de INO-CNR di Pisa, dove la ILIL-PW laser facility dispone di un laser ad elevatissima intensità (>100 TW), una linea di trasporto di fascio e una camera d’interazione schermata. Qui viene sfruttato il meccanismo di accelerazione TNSA per produrre fasci di ioni ad alcuni MeV di energia. La radiazione è rivelata tramite strumenti diagnostici, come film radiocromici (GAF), una parabola di Thomson e una serie di rivelatori tempo di volo (TOF) allo stato solido. Fra questi ultimi, i migliori risultati sono stati forniti nel corso delle varie campagne sperimentali da rivelatori in SiC, fino a diventare gli unici utilizzati come TOF durante le ultime sessioni. Lo scopo di questa tesi è dunque la caratterizzazione di dispositivi Schottky in SiC realizzati, nell’ambito del progetto SiCILIA, dal centro di ricerca CMM (Center for Materials and Microsystems) dell’istituto FBK (Fondazione Bruno Kessler, Trento, Italia) in collaborazione con il Laboratorio di Elettronica del Dipartimento di Energia del Politecnico di Milano e il laboratorio LASA (Laboratorio Acceleratori e Superconduttività Applicata) dell’INFN. La caratterizzazione è finalizzata alla comprensione e all’analisi degli impulsi sperimentali acquisiti dai rivelatori TOF in carburo di silicio utilizzati come diagnostica del fascio all’interno della camera di interazione di L3IA.