ETD

• Dosimetry
• FLASH Radiotherapy
• Neutron detection
• Semiconductor detector
• Silicon carbide (SiC) detector
• Ultra-high dose rate (UHDR)
• Very high energy electron (VHEE)

FLASH radiotherapy is a preclinical radiotherapy technique that employs ionizing radiation for cancer treatment, delivered at an average dose rate of at least 40 Gy/s. Compared with conventional radiotherapy, FLASH-RT has been shown to widen the therapeutic window by sparing healthy tissue while preserving tumor control, an effect observed only when radiation is delivered in ultra-high dose rate (UHDR) regimes. Before FLASH-RT can be translated into clinical practice, dedicated dosimetric protocols must be established, and this in turn requires either new instrumentation or modified instrumentation.
To help address this gap, STLab, a start-up based in Catania, Italy, and the Catania division of INFN jointly developed a silicon carbide (SiC) detector that has already demonstrated reliable operation in UHDR regimes. This thesis focuses on an encapsulated version of this detector (eSiC) and pursues two objectives: first, to verify that the eSiC responds linearly across different dose-per-pulse regimes and that this behavior is reproducible across multiple units, by characterizing six such instruments; second, to investigate, as a preliminary study, whether the same technology could be employed for neutron detection, with a view to future applications in Very High Energy Electron (VHEE) FLASH radiotherapy, where the collimation of VHEE beams requires dense applicators that can in turn generate neutrons requiring monitoring.
The reproducibility study was carried out at the Centro Pisano Flash Radioterapia (CPFR), using the ElectronFlash LINAC operated at 9 MeV and 100 Hz, with a PTW Advanced Markus ionization chamber and a PTW flashDiamond detector as reference instruments. For each of the six eSiC units, the collected charge was first measured as a function of the applied bias, in order to identify the saturation plateau where all the charge generated is collected. Five of the six instruments exhibited a consistent saturation plateau between 5 V and 20 V; the sixth unit displayed an anomalously large leakage current, and could not be fully characterized in the same range. The linearity of each detector was then assessed by measuring the charge collected per pulse as a function of the dose per pulse (DPP) using two applicators of different diameter (100 mm and 40 mm). All six eSiC units showed a linear response, most uncertain working points were excluded, and with slopes and intercepts consistent to within a few percent across instruments. One detector-applicator combination initially deviated from this trend; repeated measurements, including after neutron irradiation and after an additional 300 Gy delivered by the LINAC, confirmed that the deviation was not due to radiation damage and that the instrument's response remained stable, indicating that the eSiC is radiation hard under the conditions tested.
The feasibility of neutron detection was investigated using one of the characterized eSiC units together with an AmBe source. Since the SiC sensor, being a semiconductor detector, only produces a signal from directly ionizing particles, a plexiglass converter was placed in front of the sensor to generate recoil protons via elastic scattering of neutrons on hydrogen nuclei. With the converter in place, a broad continuum of counts extending well above the 59.5 keV photopeak of 241Am (present in the AmBe source) was observed; this continuum disappeared almost entirely when the converter was removed, confirming that it originated from recoil protons rather than from high-energy photons. A complementary measurement using a lead shield to suppress the gamma-ray background was perfomed, while additional 241Am, 137Cs, and 226Ra sources were used to extend the energy detection capability of the readout chain up to approximately 2.2 MeV.
Overall, these results indicate that the eSiC detector developed by STLab behaves linearly and reproducibly across UHDR regimes, is radiation hard, and shows encouraging, preliminary sensitivity to fast neutrons via recoil-proton detection. Taken together, this supports the potential use of the eSiC as a combined dosimetry and neutron-monitoring instrument for FLASH radiotherapy, including future VHEE applications, although further studies, particularly a quantitative determination of neutron detection efficiency, are needed before firmer conclusions can be drawn.