Performance of SiC Diodes at Very High Doses of Low-Energy Proton Beams Under FLASH Conditions

JOURNAL OF L A T EX CLASS FILES, VOL. 14, NO. 8, AUGUST 2024 1 Performance of SiC diodes at very high doses of low-energy proton beams under FLASH conditions M. Carmen Jim´ enez-Ramos∗†,Carmen Torres-Mu˜ noz∗,Javier Garc´ ıa-L´ opez∗‡,Diego BarrosoMolina∗,Consuelo Guardiola§,Celeste Fleta§ ∗Centro Nacional de Aceleradores (Universidad de Sevilla, CSIC, Junta de Andaluc´ ıa), Sevilla, Spain †Departamento de F´ ısica Aplicada II, Universidad de Sevilla, 41012, Sevilla, Spain ‡Departamento de F´ ısica At´ omica, Molecular y Nuclear, Universidad de Sevilla, 41012, Sevilla, Spain §Centro Nacional de Microelectr´ onica (IMB-CNM, CSIC), Bellaterra, E-08193, Spain Abstract—FLASH therapy has emerged as a promising radiotherapy technique, minimizing damage to healthy tissues while maintaining effective tumor control. Achieving FLASH conditions requires dose rates exceeding 40 Gy/s, but conventional dosimetry systems fail under these conditions. Recently, IMBCNM (CSIC) developed SiC p-n diodes with 30 µm diameter and 3µm thickness, specifically designed for FLASH radiotherapy. This study investigates their response to low-energy UHDR proton beams after high and ultra-high accumulated doses for the first time. Experiments were performed in the 3 MV tandem accelerator at CNA using 1 MeV and 2 MeV protons with a pulsed beam system, achieving mean dose rates of 10 kGy/s, dose-per-pulse of 5.6 Gy, and dose rate within the pulse of 4.6 MGy/s. Ion pulses were characterized using a Faraday Cup and Rutherford Backscattering Spectrometry (RBS). Two SiC diodes were studied: one pre-irradiated with 3.6 MGy for extreme applications and another for early irradiation stages. The pre-irradiated diode showed a sensitivity decrease of -1.34 %/kGy up to 750 kGy, stabilizing within 7 % response variation up to 4.5 MGy. The response remained linear within 10 % at mean dose rate up to 5 kGy/s for 2 MeV protons, demonstrating the feasibility of this technology for FLASH applications. Index Terms—Protontherapy, FLASH, silicon carbide diodes, Ultra-high mean dose rate. I. INTRODUCTION THE main challenge in Radiotherapy (RT) is to deposit a high enough (curative) dose in the tumor while ensuring that nearby risk organs remain at tolerable doses. The deliver of higher doses is limited by radiation-induced toxicities in the healthy tissues surrounding the tumor. One promising approach to address this issue is FLASH radiotherapy [1], which delivers ultra-high dose-rate (UHDR) (≥40Gy/s), i.e., several orders of magnitude faster than the applied dose in traditional RT (∼0.05Gy/s) [1]. FLASH RT aims to reduce the normal tissue complication probability (NTCP), while facilitating a similar or better tumor control probability (TCP) as at conventional mean dose rates, reducing the treatment time and also the organ motion-related issues. Hence, the FLASH effect may significantly extend the therapeutic window of radiotherapy and overcome the dose-limiting radiation toxicity, opening a new paradigm in RT treatments. However, enabling the clinical implementation of FLASH RT is very challenging This work did not involve human subjects or animals in its research. and current dosimeter systems render obsolete most of the available dosimetry equipment [2], [3]. The FLASH phenomenon was demonstrated first with electron linear accelerators [1], [4], [5], and then using proton beams [6], [7]. It has been demonstrated protective effects on normal tissues and similar effectiveness for tumor control of FLASH irradiation in in-vivo preclinical studies, both at the cellular level and at the physiological level (e.g., lung fibrosis [2], neurocognitive impairment [4], skin necrosis [5], and developmental disorder [8]). Recently, the first FLASH proton therapy clinical trial has started at the Cincinnati Children’s Hospital Medical Center (CCHMC), USA [9]. Major advances are being made by clinical accelerators and reference centers [10], [11] towards the design of clinical accelerators operating at FLASH dose-rates. Those machines could be operational to treat humans in the incoming years. Although the first patients have already been treated with FLASH as part of feasibility trials [12], [13], the clinical implementation of FLASH RT is challenging due to the lack of dosimeters able to provide accurate dose measurements. At the present, the current state-of-the-art is mainly to rely on passive dosimeters (e.g. radiochromic films, alanine-EPR dosimeters, TLDs) [14]–[16], but they require extensive postprocessing of hours or days for accurate dose determination, which is a significant drawback. Chemical dosimeters could be considered as suitable for FLASH, but suffer from low sensitivity, complicated handling, and absence of traceability. In contrast, active detection techniques traditionally employed in conventional RT are not a valid option in FLASH therapy, because they can be heavily affected by the mean dose rates and dose per pulse present in UHDR beams [17]. The international reports recommend the use of ion chambers (IC) for the measurement of the absorbed dose [18]. However, at UHDR the electric field of the ICs is affected by the high density of charges generated as well as the recombination of ions in the chamber cavity is significantly enhanced due to the fact that the particles drift time is slow, causing a sharp decrease of the ion collection efficiency for >0.1Gy per pulse [3] and making ion chamber protocols inoperative that are difficult to correct [19]. There are a few works dealing with these issues [3], [19]. In particular, G´ omez et al. [20] have recently created the first ultra-thin parallel (0.25 mm) plate ionization chamber which has been demonstrated to operate effectively This article has been accepted for publication in IEEE Transactions on Radiation and Plasma Medical Sciences. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TRPMS.2025.3591229 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ JOURNAL OF L A T EX CLASS FILES, VOL. 14, NO. 8, AUGUST 2024 2 in UHDR beams with minimal recombination. Di Martino et al., have proposed ICs including gas mixtures and pressures that could decrease the ion recombination [21], but they are still under study. Another alternative is based on plastic scintillators, which have time constants of a few nanoseconds, but they undergo scintillation quenching at the high linear energy transfer (LET) characteristic of proton Bragg peak [22] . Recently, calorimeters have been investigated as a primary standard for determining absolute dose for FLASH proton beam radiotherapy with an uncertainty of 0.9% [23]. In this context, semiconductor-based active dosimeters (silicon and diamond) have been widely used for active dosimetry in radiotherapy. However, they may suffer from mean dose rate dependence and show saturation effects with high mean dose rate beams. In addition, silicon has a limited use in terms of radiation hardness [24]. Although it has recently been found that some diamond prototypes are suitable for dosimetry in FLASH-RT [25]–[27], diamond is an expensive material and is difficult to find in high-quality large wafers, which limits its widespread use. Silicon Carbide (SiC) is another semiconductor material with properties that make it a very interesting option for radiation hard dosimeters. The main characteristic of SiC is its wide band-gap of 3.27 eV that significantly reduces the leakage current and makes it insensitive to visible light. SiC is more radiation resistant than silicon thanks to a larger displacement energy threshold and also has better tissue equivalence. Of particular interest is the signal yield per deposited mGy in SiC (425 pC/(mGy·mm3) for 4H-SiC) that is smaller than for silicon. This makes SiC a good option for dosimetry in ultra-high dose pulsed radiation fields or direct beam monitoring where the instantaneous dose deposition in the semiconductor is large and might saturate traditional silicon diodes. In the last years the Instituto de Microelectr´ onica de Barcelona (IMB-CNM) has developed their own technology for SiC radiation detectors. In collaboration with the ALBA and ESRF synchrotrons, four-quadrant SiC photodiodes fabricated for beam position monitoring demonstrated excellent performance at variable temperature and visible light illumination conditions, in addition to improved radiation hardness for particle detection compared to silicon [28], [29]. Furthermore, 4H-SiC pn junction diodes (PND), especially designed as a plasma diagnostic system for the detection of alpha-born particles in future D-T based fusion devices, did not show any degradation in their spectrometric behaviour against irradiation at room temperature with alpha particles of 3.5 MeV up to fluences of 1011cm−2[30] while maintaining excellent energy resolution (≤2%) up to temperatures of 450 C [31]. Recently, IMB-CNM designed and fabricated a set of SiCbased diodes tailored for UHDR dosimetry in the framework of the EMPIR-UDHPulse project [32]. For this experiment, diodes with very low sensitivity (∼pC/Gy) were chosen due to the extremely high expected dose rates from the tandem accelerator. They are not intended for use at conventional dose rates. For conventional dosimetry, larger diodes from the same production with nC/Gy sensitivity, like the characterized by Fleta et al [33], are more suitable. Their response fulfilled the requirements for relative dosimetry in UHDR pulsed electron Fig. 1: Photograph of a 4x4 array of 30 µm diameter SiC diodes, showing the diodes to the right and the connecting pads to the left. For these tests, only one of the 16 channels was connected. beams up to a dose per pulse of 11 Gy and pulse durations ranging from 0.5 to 3 µs of 20 MeV electrons. In addition, they showed excellent radiation hardness with a long-term sensitivity reduction with an accumulated electron dose of 0.018 % kGy [33]. In this work it has been investigated, for the first time, the response of these novel SiC diodes to ultra-high accumulated doses reaching 4.6 MGy using high-LET proton (up to 61.6keV/µm) pulsed beams in the National Center of Accelerators (CNA, Seville). Likewise, it is discussed the capacity of our laboratory to irradiate in FLASH conditions and the methodology employed to monitor the mean dose rate using a Faraday cup and Rutherford Backscattering Spectrometry (RBS) technique [34]. MATERIAL AND METHODS Detector The SiC diodes used in this study were designed, fabricated and electrically characterized by IMB-CNM. The sensitive semiconductor material is a 3 µm epitaxial layer of 4H-SiC, n-type doped with nitrogen with a nominal concentration of 2×1014cm−3, that was grown on a 350 µm low resistivity n-type SiC substrate. Sequential Al ion implantations followed by a high-temperature annealing process were used to achieve a good quality P+/N rectifying junction. Further details regarding the wafer fabrication and electrical characterization can be found in [33]. The diodes used are circular with a diameter of 30 µm. Each single diode is part of a 4x4 array with 200 µm pitch designed for 2-D dosimetry measurements, although in this case only one channel was connected to the readout system since the development of the corresponding multi-channel readout is still ongoing. Figure 1 shows a photograph of one of these arrays. The SiC chips were encapsulated with a custom-made housing made of a ceramic PCB that was mounted on a cylindrical PMMA support. The diode sensitive face was left uncovered to avoid obstructing the incoming proton beam. Device readout was made in current mode using a coaxial This article has been accepted for publication in IEEE Transactions on Radiation and Plasma Medical Sciences. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TRPMS.2025.3591229 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ JOURNAL OF L A T EX CLASS FILES, VOL. 14, NO. 8, AUGUST 2024 3 Fig. 2: Schematic of the accelerator and beam pulsing system. The upper section shows the 3 ions sources connected to the 3 MV tandem accelerator and the beam line used for this experiment. The lower part illustrates how the pulsed beam system operates. cable plugged to a SunNuclear PC Electrometer. The SiC diodes were operated without external bias voltage. Methodology Experimental conditions: The experiments were carried out in the 3 MV Tandem accelerator of the National Accelerator Center (CNA) in Seville. The SiC detectors were placed in an electrically isolated aluminium sample holder and irradiated in a vacuum chamber located at the 0º beamline of the switching magnet. To compare the influence of LET at two high values on the response of the detectors, Diodes # 1 and # 2 were irradiated, respectively, with monoenergetic proton beams of 1 MeV (LET = 61.6 keV/µm) and 2 MeV (LET = 38.9 keV/µm) respectively. The size of the beam at the target position was defined using a system of 3 mm diameter collimators located at the entrance to the vacuum chamber, which minimizes proton scattering, ensuring that the detector surface is irradiated perpendicular. Although under normal working conditions the accelerator provides a continuous ion beam, for these experiments it has been used a custommade system that allows the beam to be pulsed in a simple and versatile way. This, in turn, has allowed to evaluate the capacity of the CNA facility to carry out low-energy UHDR proton irradiations. The operation of the pulsed system is based on an electrostatic steerer placed at the output of each of the ion sources to deflect the beam by applying an electric field created between its two plane-parallel plates.In this experiment, we employed the Source of Negative Ions by Cesium Sputtering (SNICS). The main advantage of pulsing the beam at this point is that the energy of the ions is small (about 72 keV) and relatively low voltages can be used, typically lower than 1000 volts. The pulsed system (Figure 2) is formed by a solid-state fast switch model FSWP 51-02 from Belhke that allows creating pulses with a minimum width of 50 ns and a maximum frequency of 50 kHz. An ORTEC 556 high voltage power supply is connected to the HV input of the fast switch which, in turn, is governed by the arbitrary functions’ generator AFG Fig. 3: Example of a single proton pulse with a width of 1 µs measured at the target position with a fast plastic scintillation detector. 310000 from Tektronix, allowing us to define the frequency and pulse width. In this work a voltage of 700 V was used at the elestrostatic steerer, sufficient to completely deflect the beam, time pulses between 1 and 20 µs and frequencies from 500 Hz to 10 kHz. As an example, Figure 3 shows a signal of a 1 µs pulse measured with a fast plastic scintillator detector , Eljen EJ-214 [35], attached to a PMT which is connected to an oscilloscope with a bandwidth of 4 GHz. Dose calculation: To determine the dose, immediately before and after each irradiation the intensity of the ion beam was measured on the aluminium sample holder, which was connected to a current integrator (Brookhaven Instrument Corporation (BIC), model 1000-C). In comparison to continuous beams, the current indicated by the BIC integrator for pulsed beams decreased according to the duty cycle. Therefore, under pulsed conditions, an average current is measured. During the different irradiations, whose duration was on the order of 100 s, the beam current remained constant, with fluctuations of about 10 %. Dose =Mean dose rate ×ti=Flux ×∆E d×ρ×ti(1) The irradiated dose was obtained from equation 1, knowing the mean dose rate and the irradiation time, ti. The mean dose rate was calculated from the ion flux (i.e. the number of protons reaching the detector per unit area and time, obtained from the current and beam size), the thickness of the devices (d = 3 microns), the energy deposited by a proton when passing through the detector (∆E) and the density of the SiC material (ρ= 3.22g/cm3) For the calculation of ∆Ethe Stopping Range in Matter (SRIM) software was employed [36]. The energy loss in the dead layers on the surface was included in the simulation. In the case of Diode #1, the methodology consisted in two different steps. First, the sensor was irradiated to ultra-high doses using a continuous 1 MeV proton beam of 10 nA for 140 s, accumulating a dose of 3.6 MGy, to explore the limit of this technology for extreme applications. Then, experiments were conducted to study the linear response for different mean dose rates. In this case, pulses of 20 µs at 10 kHz (i.e. duty This article has been accepted for publication in IEEE Transactions on Radiation and Plasma Medical Sciences. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TRPMS.2025.3591229 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ JOURNAL OF L A T EX CLASS FILES, VOL. 14, NO. 8, AUGUST 2024 4 Fig. 4: Mean dose rate and dose per pulse versus current values for the 1 MeV proton energy. Blue dots mark the experimental values used. cycle = 20 %) were used; with the mean dose rate ranging from 0.7 kGy/s to 9.4 kGy/s. In Figure 4 are shown mean dose rate and dose per pulse as a function of pulsed ion current, blue dots mark experimental values used. A similar methodology was employed in the experiments carried out on Diode #2, using 2 MeV protons, although in this case the early stages of irradiation were investigated using lower accumulated doses. As shown in Figure 5, two different time structures were employed. For the first set of irradiations, pulses of 3 µs at 500 Hz (i.e. duty cycle = 0.15%) were used, with the mean dose rate ranging from 45 Gy/s to 2.7 kGy/s. Subsequent irradiations were carried out with pulses of 1 µs at 1 kHz (i.e. duty cycle = 0.10 %) and with the mean dose rate ranging from 245 Gy/s to 4.6 kGy/s. Rutherford Backscattering Spectrometry (RBS) technique: It is important to highlight that during the irradiation of the devices, as they are electrically isolated from the sample holder, the BIC integrator is unable to measure the beam current. Although, as previously stated, the current observed before and after the irradiations remained constant, fluctuations or temporal drifts in the beam current are not uncommon in experiments with particle accelerators. In such cases, the precision of the calculated dose may be compromised. This becomes particularly important when operating with low currents, in the order of pA, which are more difficult to measure with accuracy due to the limitation of the current electrometer. In order to observe and correct this potential issue, a methodology was developed during the study of Diode #2. The method employs the Rutherford Backscattering Spectrometry (RBS) technique to measure the protons elastically scattered by the target to a 300 mm2silicon PIPS detector, situated at a scattering angle of 160° to maximize the counting rate. Figure 6 shows a simplified scheme of the electronic chain associated with the RBS detector, as well as the electrometers connected to the SiC detector and the sample holder, respectively. Since the number of protons scattered to the RBS detector is proportional to the number of protons incident on the target, this technique allows monitoring the current in the detector in a non-invasive way and in real time. RBS signal was recorded in Pulse Height Analyzer (PHA) and in Multichannel Scaling (MCS) modes simultaneously. To do this, the front (a) Pulses of 3 µs at 500 Hz (b) Pulses of 1 µs at 1000 Hz. Fig. 5: Mean dose rate and dose per pulse versus current values for the 2 MeV experiment. Fig. 6: (Top) Schematic of the RBS detector and its associated electronic chain, including the preamplifier, amplifier, and MCA+ADC. (Bottom) Schematic of the sample holder with the SiC dosimeter connected to the Sun Nuclear Electrometer and the BIC current integrator. and rear output signals of the amplifier (Tennelec TC245) were digitized and sent to two different multichannel analyzers (MCA). In PHA mode (Figure 7 (a) and Figure 8), the measured energy spectra allow to verify that the impact point This article has been accepted for publication in IEEE Transactions on Radiation and Plasma Medical Sciences. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TRPMS.2025.3591229 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ JOURNAL OF L A T EX CLASS FILES, VOL. 14, NO. 8, AUGUST 2024 5 (a) (b) Fig. 7: PHA (a) and MCS (b) mode for measurement at 2 nA with temporal structure of 1 kHz and 1 µs. The MCS spectrum shows that the beam current remains constant during the irradiation time. Fig. 8: Normalized RBS spectra in PHA mode for different beam currents in pulsed mode. does not change from one irradiation to another and that the current density at the target remains homogeneous when the mean dose rate is varied. This was done by changing the intensity of the ion source focus lens. In contrast, in MCS mode (Figure 7 (right)), the MCA records the count rate of pulses over time, logging all measured counts in a channel during a set time interval (dwell time) before moving to the next channel to record the count rate in the subsequent time interval. Thus, the MCS mode was especially useful in this work, as by selecting the dwell time of the MCS system equal to that of the Sun Nuclear electrometer output (t = 0.5 seconds), it was possible to correlate, channel by channel, the response of the SiC detector with the number of incident protons in that time interval. Once the RBS system is calibrated, it is possible to know the intensity of the incident beam from the spectra. This is shown in Figure 9, where the beam current measured directly on the aluminum sample holder using the BIC integrator is compared with the beam current calculated from the RBS spectra (a pulsed beam with 1 nA intensity, pulse width = 1 µs and frequency = 1 kHz was taken as a reference). As observed, there is a very good agreement, demonstrating that the RBS technique can be used to control both cumulative dose and dose rate in real time and non-invasively. With the smallest current used in this work (160 pA), 94 ±7 counts/channel were measured, and therefore, with typically 200 channels Fig. 9: Beam current versus intensity measured using RBS technique in MCS mode. per spectrum, the statistical contribution to the calculation of the irradiated dose is less than 1 %. However, it is important to highlight that in our present experimental conditions, the RBS technique is not well adapted for experiments that require much smaller currents, of the order of pA, since in this case the average number of counts/channel is less than 1, which would obviously make it impossible to follow the incident current and would entail a significant error in the measurement of the total dose. Despite this drawback, which could be improved by increasing the detector area and the solid angle, it is evident that the complementary use of this technique helps to better understand the behaviour of the SiC sensors under irradiation in FLASH mode. RESULTS AND DISCUSSION As an example of the utility of this complementary RBS system, in figure 10 (a) the output of the Sun Nuclear electrometer is compared with the number of normalized counts/channel of the RBS spectrum in MCS mode for the Diode #2. For both devices, the data is synchronized and recorded every 0.5 seconds. In this example, the irradiation began with a cumulative dose of 96 kGy and a rate of 346 Gy/s, i.e., receiving another 51.9 kGy. It is worth noting that the electrometer signal decreases with time, from 350 pA to 300 pA during the 150 seconds that the irradiation lasted, while the MCS spectrum shows that the rate of protons on the target remains constant indicating the degradation process of the sensor. Figure 10 (b) corresponds to an irradiation at higher cumulative doses, starting with 3448 kGy and a mean dose rate of 1528 Gy/s, so during this irradiation another 112 kGy were added. In this case, both the diode signal and the MCS signal are comparable from the qualitative point of view. Figure 11 shows the mean response of the detectors (in pC/Gy) as a function of the accumulated dose. As previously stated, Diode #1 had been irradiated with a continuous proton beam up to a dose of 3.6 MGy, with the objective of studying the limits of the sensor for ultra-high doses. As can be observed, the charge-to-dose ratio in diode #1 presents a plateau at 0.3 pC/Gy up to 7.5 MGy and only exhibits a slight decrease from 8 MGy, remaining constant thereafter up to the highest dose used, i.e., 8.7 MGy. This demonstrates the This article has been accepted for publication in IEEE Transactions on Radiation and Plasma Medical Sciences. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TRPMS.2025.3591229 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ JOURNAL OF L A T EX CLASS FILES, VOL. 14, NO. 8, AUGUST 2024 6 (a) (b) Fig. 10: Detector response and normalized MCS spectrum as a function of time for different accumulated dose between : (a) 96-148 kGy and (b) 3448-3560 kGy. reliability and durability of these SiC detectors for extreme radiation environments. On the other hand, in the case of Diode #2, the accumulated doses during the first irradiations were low enough to be able to observe the degradation of the device. It is important to note that during the process of adjusting the electronic chain and optimizing the position of the RBS detector, the pristine SiC sensor received an approximate dose of 68 kGy. Considering the volume of the detector and the signal yield per deposited mGy in SiC, the maximum sensitivity of the device should have been 0.9 pC/Gy. Surprisingly the initial sensitivity for the first experimental point is close to 2.4 pC/Gy. We hypothesize that this could be due to the carriers created outside the diode, which may reach the diode active volume by diffusion since the carrier diffusion length for lowly-doped pristine n-type 4H-SiC ranges several micrometres (4 and 13 µm in ref [30], [37], respectively), similar to the dimensions of the device. These diffused carriers would increase considerably the measured signal. Sensitivity values for pristine detectors above the theoretical ones have not been reported in similar SiC diodes fabricated with larger sizes [33], where the contribution through lateral diffusion of carriers is comparatively much less important than in this case. Therefore, if micrometric-sized diodes were to be used as dosimeters, it would be advisable to confine their active area by means of a grounded guard ring or other strategies. Fig. 11: Detector response to dose ratio for the two dosimeters irradiated with 1 MeV (Diode #1) and 2 MeV (Diode #2) protons. Fig. 12: Diode #2 response to dose ratio in the first 800 kGy of 2 MeV protons. A more detailed analysis of the initial detector degradation is presented in Figure 12. It shows the relative deviation in percentage of the detector response over the first 750 kGy of accumulated dose. Two degradation trends are observed, initially the decay rate is -1.34 %/kGy, while once accumulated about 400 kGy the degradation rate decreases to -0.04 %/kGy. From 750 kGy, as can be seen in Figure 11, the response of the detector no longer shows a significant degradation, with the response being constant with dose within 7 % up to 4.5 MGy. It is worth noting that the initial rate of -1.34 %/kGy is already better than the values measured with commercial silicon diodes irradiated in clinical proton beams [37]. Irradiation studies of SiC Schottky diodes with 18 MeV and 24 GeV/c protons have shown that the transport properties, such as the lifetime of the minority carriers [38], degrade rapidly with fluence [37], [39], which manifests in a significant deterioration of the Charge Collection Efficiency (CCE) especially at low bias voltages. Displacement damage is the key damage mechanism linked to the non-ionizing energy loss (NIEL), which is a magnitude that depends on the particle type and energy. To our knowledge, there are no experimental measurements of the NIEL in SiC for protons. By using the Web NIEL calculator [40], we estimated that the NIELs ratio referenced to 18 MeV, i.e., N IEL(2M eV ) N IEL(18M eV ), is 5. For the This article has been accepted for publication in IEEE Transactions on Radiation and Plasma Medical Sciences. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TRPMS.2025.3591229 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ JOURNAL OF L A T EX CLASS FILES, VOL. 14, NO. 8, AUGUST 2024 7 Fig. 13: Upper panel: Detector response versus mean dose rate of 1 and 2 MeV protons. Diode #2 had been previously irradiated to a total dose of 720 kGy of 2 MeV protons and Diode #1 with 3785 kGy of 1 MeV protons. Lower panel: relative deviation of the diode signal form linearity. case of 200 MeV protons, an energy in the order of to the maximum used in protontherapy, the ratio N IEL(2M eV ) N IEL(200M eV ), is 20. Furthermore, the NIEL value is orders of magnitude lower for electron irradiation. This means that the low-energy protons used in this experiment are close to the worst-case scenario in terms of radiation damage of the SiC material in radiotherapy environments. From the operational point of view of these sensors as future dosimeters in conventional or UHDR hadrontherapy, it is also important to study their response in a range of mean dose rates as wide as possible, as mean dose rates much than 40 Gy/s, are still being explored for FLASH treatments. Indeed, a recent study revealed that proton dose rates up to 1 kGy/s mitigated delayed adverse effects in normal cells [6]. Given these promising results, it is interesting to explore the efficacy and safety of proton irradiation at even higher mean dose rates. For that, in this study mean dose rates up to 10 kGy/s have been used. Figure 13 shows the response of diodes #1 and #2 versus mean dose rate. As observed, there is a linear trend up to mean dose rates close to 5 kGy/s. Although for Diode #1, the response remains linear for even higher mean dose rates, its slope seems to decrease slightly between 6 and 10 kGy/s, which could indicate that saturation effects begin to appear at such extreme rates. The difference between the slopes of the two sensors is correlated with the different sensitivities shown in Figure 11 and may be due to the fact that they come from different wafers and therefore different productions, which could justify that the two diodes are not identical. II. CONCLUSION The unique properties of SiC and the significant improvements achieved in recent years in detector manufacturing technology make SiC diodes potential candidates for dose measurements in UHDR radiotherapy where classical dosimeters can present problems due to the high intensity and short duration of the ion pulses used in this promising therapy. This study demonstrates the high radiation resistance and reliability in the response of SiC diodes under irradiation with intense pulsed proton beams, which opens the door to carry out dosimetry studies with an excellent spatial resolution of tens of microns. Even after the detectors are irradiated to doses close to 750 kGy, they respond linearly in a wide range of mean dose rates and cumulative doses of several MGy, and all this without the need for any external applied voltage, which would allow their use in clinical facilities with an almost long lifetime, since accumulated doses higher than those achieved in this work will not be reached. It is also important to highlight that FLASH hadrontherapy is a novel technique, in continuous evolution, and the optimal irradiation conditions in terms of pulse length, frequency, instantaneous dose, etc., are still the subject of study. In this regard, it is expected that scientific facilities based on low-energy ion accelerators, such as the CNA, will play a fundamental role both for the versatile production of intense pulsed beams and for the study and calibration of instrumentation capable of measuring this radiation. ACKNOWLEDGMENT This work has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation program under project 18HLT04-UHDpulse. It has also received funding from project NEWDOSI (PID2021-123484OB-I00), financed by MCIN / AEI / 10.13039/501100011033 / FEDER, UE and from the project “Dosimetry monitor for FLASH therapy” financed by LaCaixa foundation (HR23-00718) M. Carmen Jim´ enez-Ramos acknowledges the support of this work through a VI PPIT-US contract. This investigation has been partially funded by the projects refs. “ASTRO21/1.1/1” and “ASTRO21/1.4/4” with financing from the European Union - NextGenerationEU”, the Ministry of Science, Innovation and Universities, Recovery Plan, Transformation and Resilience, the Department of University, Research and Innovation of the Junta de Andaluc´ ıa and the University of Sevilla. 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