Periodic Reporting for period 1 - SiCNeutronFlash (Out-of-field neutron dosimetry for FLASH therapy based on SiC sensors)
Reporting period: 2023-12-01 to 2025-11-30
Cancer represents one of the most significant public health challenges worldwide. A large proportion of patients will require radiotherapy at some point during their treatment, making this modality a cornerstone of modern oncology. In recent years, a novel approach known as ultra-high dose-rate radiotherapy (UHDR), also referred to as FLASH therapy, has emerged as a highly promising innovation. FLASH therapy delivers radiation doses several orders of magnitude faster than conventional radiotherapy, while maintaining equivalent or improved tumor control and significantly reducing damage to surrounding healthy tissues. This is called “FLASH effect” has the potential to overcome current dose-limiting toxicities and to open a new paradigm in cancer treatment.
Following encouraging preclinical results, major technological efforts are underway to translate FLASH therapy into clinical practice. One of the most important is the development of reliable dosimetry and monitoring systems capable of operating accurately under ultra-high dose-rate and ultra-short pulse conditions, where conventional radiation detectors fail or suffer from severe limitations.
An additional and largely unexplored challenge concerns the production of secondary radiation, particularly neutrons, during high-energy radiotherapy treatments. In conventional radiotherapy, secondary neutrons generated by particle interactions can scatter far from the treatment field, delivering unwanted doses to healthy tissues and sensitive organs. In FLASH therapy, the extremely high dose per pulse introduces new uncertainties regarding neutron production, detector response, and dose quantification. At present, the measurement and characterization of out-of-field neutron doses under FLASH conditions remain largely unaddressed, representing a critical knowledge gap for patient safety and risk assessment.
The overall objective of this project is to address this gap by developing and validating a new generation of active neutron detectors specifically designed for use in FLASH radiotherapy and other ultra-high dose-rate radiation environments. The project builds on recent advances in solid-state radiation detector technology based on silicon carbide (SiC), a material with a high radiation hardness, low noise, thermal stability, and improved tissue equivalence compared to conventional silicon. These characteristics make SiC particularly well suited for operation in extreme radiation conditions where traditional detectors are inadequate.
The first main objective of the project is the development of a novel matrix of SiC-based neutron detectors capable of measuring out-of-field neutron fluxes in FLASH facilities and mixed radiation fields. This has been achieved by optimizing SiC diodes combined with tailored neutron conversion/moderation layers, enabling high gamma rejection and enhanced sensitivity to a wide range of neutron energies. The second main objective is the experimental validation of these detectors in representative radiation environments, including reference neutron fields, mixed radiation beams, and pulsed neutron sources, to demonstrate their performance, reliability, and suitability for ultra-high dose-rate applications.
By delivering the first active neutron monitoring system adapted to FLASH radiotherapy conditions, this project is expected to make a significant contribution to the safe clinical implementation of this emerging treatment modality.
Following encouraging preclinical results, major technological efforts are underway to translate FLASH therapy into clinical practice. One of the most important is the development of reliable dosimetry and monitoring systems capable of operating accurately under ultra-high dose-rate and ultra-short pulse conditions, where conventional radiation detectors fail or suffer from severe limitations.
An additional and largely unexplored challenge concerns the production of secondary radiation, particularly neutrons, during high-energy radiotherapy treatments. In conventional radiotherapy, secondary neutrons generated by particle interactions can scatter far from the treatment field, delivering unwanted doses to healthy tissues and sensitive organs. In FLASH therapy, the extremely high dose per pulse introduces new uncertainties regarding neutron production, detector response, and dose quantification. At present, the measurement and characterization of out-of-field neutron doses under FLASH conditions remain largely unaddressed, representing a critical knowledge gap for patient safety and risk assessment.
The overall objective of this project is to address this gap by developing and validating a new generation of active neutron detectors specifically designed for use in FLASH radiotherapy and other ultra-high dose-rate radiation environments. The project builds on recent advances in solid-state radiation detector technology based on silicon carbide (SiC), a material with a high radiation hardness, low noise, thermal stability, and improved tissue equivalence compared to conventional silicon. These characteristics make SiC particularly well suited for operation in extreme radiation conditions where traditional detectors are inadequate.
The first main objective of the project is the development of a novel matrix of SiC-based neutron detectors capable of measuring out-of-field neutron fluxes in FLASH facilities and mixed radiation fields. This has been achieved by optimizing SiC diodes combined with tailored neutron conversion/moderation layers, enabling high gamma rejection and enhanced sensitivity to a wide range of neutron energies. The second main objective is the experimental validation of these detectors in representative radiation environments, including reference neutron fields, mixed radiation beams, and pulsed neutron sources, to demonstrate their performance, reliability, and suitability for ultra-high dose-rate applications.
By delivering the first active neutron monitoring system adapted to FLASH radiotherapy conditions, this project is expected to make a significant contribution to the safe clinical implementation of this emerging treatment modality.
The work carried out within the project focused on the design, fabrication, modelling, and experimental validation of silicon carbide SiC–based neutron detectors for operation in mixed radiation fields relevant to radiotherapy and other harsh environments. The activities were structured around the development of radiation-hard solid-state detectors, the optimization of neutron conversion layers, and their validation under thermal, epithermal, and fast neutron beams in continuous and pulsed irradiation conditions. This validation was carried out both in R&D facilities and in clinical environments, and was complemented by Monte Carlo simulations of LINACs and experimental installations for the verification of the results.
A first achievement was the successful fabrication of an array of SiC neutron detectors based on devices fabricated in the clean-room facility of the Institute of Microelectronics of Barcelona (IMB-CNM). These detectors exhibited low leakage current, excellent thermal stability, and high radiation hardness, making them suitable for neutron detection in environments with intense photon backgrounds. Their active thickness, typically between 10 and 50 µm, enabled an intrinsic suppression of photon-induced signals and provided a high gamma rejection, a key requirement for operation in mixed photon–neutron fields. Low-noise, cost-effective readout electronics for the detector operation were designed and optimized, and a pulse acquisition system was implemented for the acquisition of the data generated by the detectors. The project achieved a comprehensive experimental characterization of these detectors under thermal, epithermal, and fast neutron irradiations:
1- Thermal neutron detection was enabled by coupling the SiC diodes with optimized neutron conversion layers based on enriched lithium and boron compounds. In particular, enriched ⁶LiF layers were synthesized and deposited at IMB-CNM in collaboration with the Institute of Materials Science of Barcelona (ICMAB-CSIC) using dedicated procedures to achieve uniform thickness and good adhesion. Detection efficiencies of approximately 4–6% for thermal neutrons were demonstrated, depending on converter thickness and detector configuration. The detectors showed a linear response over a wide range of neutron fluxes, with no evidence of saturation or dead-time effects.
2- Epithermal neutron detection was carried out using a detector in combination with a polyethylene moderator layer, cadmium shielding, and a LiF conversion layer.
3- Fast neutron detection was addressed by combining SiC diodes with hydrogen-rich conversion layers made of polypropylene, enabling detection via recoil protons generated in neutron–hydrogen interactions. The response to fast neutrons was systematically studied as a function of conversion layer thickness and neutron energy. The results demonstrated a clear and predictable dependence of detection efficiency on converter geometry, confirming the feasibility of SiC-based detectors for fast neutron monitoring.
A key achievement of the project was the validation of the detectors in realistic and operationally relevant environments. Measurements were successfully performed at accelerator-based neutron facilities (National Center of Accelerators, Seville, Spain), in clinical radiotherapy linear accelerators operating at high photon energies (Hospital Josep Trueta, Girona, Spain), and at the RA6 Nuclear Research Reactor, Bariloche, Argentina. In radiotherapy conditions, the detectors were able to measure out-of-field thermal neutron fluences in the presence of an intense pulsed photon background, demonstrating excellent stability and linearity over clinically relevant dose rates. At the research reactor, the detectors showed a robust and reproducible response to thermal neutrons up to high reactor power levels, as well as a well-characterized angular dependence.
In parallel with the experimental work, detailed Monte Carlo simulations were carried out using state-of-the-art particle transport codes. These simulations reproduced the detector geometry, materials, neutron spectra, and experimental configurations with high fidelity. The simulated energy deposition spectra and detection efficiencies were in good agreement with the experimental results, providing a solid physical interpretation of the detector response and validating the design approach. Overall, the project delivered a set of validated SiC-based neutron detectors with demonstrated capability for thermal, epithermal, and fast neutron detection in mixed and pulsed radiation fields.
A first achievement was the successful fabrication of an array of SiC neutron detectors based on devices fabricated in the clean-room facility of the Institute of Microelectronics of Barcelona (IMB-CNM). These detectors exhibited low leakage current, excellent thermal stability, and high radiation hardness, making them suitable for neutron detection in environments with intense photon backgrounds. Their active thickness, typically between 10 and 50 µm, enabled an intrinsic suppression of photon-induced signals and provided a high gamma rejection, a key requirement for operation in mixed photon–neutron fields. Low-noise, cost-effective readout electronics for the detector operation were designed and optimized, and a pulse acquisition system was implemented for the acquisition of the data generated by the detectors. The project achieved a comprehensive experimental characterization of these detectors under thermal, epithermal, and fast neutron irradiations:
1- Thermal neutron detection was enabled by coupling the SiC diodes with optimized neutron conversion layers based on enriched lithium and boron compounds. In particular, enriched ⁶LiF layers were synthesized and deposited at IMB-CNM in collaboration with the Institute of Materials Science of Barcelona (ICMAB-CSIC) using dedicated procedures to achieve uniform thickness and good adhesion. Detection efficiencies of approximately 4–6% for thermal neutrons were demonstrated, depending on converter thickness and detector configuration. The detectors showed a linear response over a wide range of neutron fluxes, with no evidence of saturation or dead-time effects.
2- Epithermal neutron detection was carried out using a detector in combination with a polyethylene moderator layer, cadmium shielding, and a LiF conversion layer.
3- Fast neutron detection was addressed by combining SiC diodes with hydrogen-rich conversion layers made of polypropylene, enabling detection via recoil protons generated in neutron–hydrogen interactions. The response to fast neutrons was systematically studied as a function of conversion layer thickness and neutron energy. The results demonstrated a clear and predictable dependence of detection efficiency on converter geometry, confirming the feasibility of SiC-based detectors for fast neutron monitoring.
A key achievement of the project was the validation of the detectors in realistic and operationally relevant environments. Measurements were successfully performed at accelerator-based neutron facilities (National Center of Accelerators, Seville, Spain), in clinical radiotherapy linear accelerators operating at high photon energies (Hospital Josep Trueta, Girona, Spain), and at the RA6 Nuclear Research Reactor, Bariloche, Argentina. In radiotherapy conditions, the detectors were able to measure out-of-field thermal neutron fluences in the presence of an intense pulsed photon background, demonstrating excellent stability and linearity over clinically relevant dose rates. At the research reactor, the detectors showed a robust and reproducible response to thermal neutrons up to high reactor power levels, as well as a well-characterized angular dependence.
In parallel with the experimental work, detailed Monte Carlo simulations were carried out using state-of-the-art particle transport codes. These simulations reproduced the detector geometry, materials, neutron spectra, and experimental configurations with high fidelity. The simulated energy deposition spectra and detection efficiencies were in good agreement with the experimental results, providing a solid physical interpretation of the detector response and validating the design approach. Overall, the project delivered a set of validated SiC-based neutron detectors with demonstrated capability for thermal, epithermal, and fast neutron detection in mixed and pulsed radiation fields.
The project has delivered several results that advance the state of the art in neutron detection and dosimetry, particularly in the context of radiotherapy and other radiation-hard environments characterized by mixed and pulsed radiation fields.
A key result beyond the current state of the art is the development and experimental validation, for the first time in a P-N junction configuration, of active SiC neutron detectors capable of operating reliably in environments with intense photon backgrounds and pulsed radiation, together with customized, cost-effective readout electronics. The detectors developed in this project enable real-time or near-real-time neutron detection with excellent gamma rejection and without saturation effects. Another major advancement is the implementation of a detector matrix based on SiC P–N junction diodes that can be adapted to different neutron energy ranges. By combining the same detector technology with optimized neutron conversion and moderation layers, the project demonstrated sensitivity to thermal, epithermal, and fast neutrons, including under pulsed beam conditions. The successful operation of a SiC neutron detector matrix in both continuous and pulsed neutron fields goes beyond previously reported capabilities of active neutron detectors. It addresses a limitation for applications in modern radiotherapy and accelerator-based facilities.
The project also achieved the first experimental measurements of out-of-field thermal neutrons in conventional clinical radiotherapy accelerators using silicon carbide detectors. The ability to directly measure secondary thermal neutron fluences in realistic treatment room conditions using radiation-hard SiC devices provides new experimental evidence that can support improved risk assessment, patient safety studies, and the optimisation of radiotherapy techniques.
In terms of potential impact, the results of the project open new possibilities for active neutron monitoring in radiotherapy, including emerging treatment modalities such as ultra-high dose-rate radiotherapy. The demonstrated robustness, radiation hardness, and gamma discrimination of the SiC detectors make them strong candidates for deployment in clinical, research, and industrial environments where neutron doses must be quantified accurately. Beyond medical applications, the results are relevant for nuclear facilities, radiation protection, and scientific infrastructures requiring compact and durable neutron monitoring solutions.
To ensure further uptake and long-term success, several key needs have been identified. Further research is required to complete the analysis and publication of the results obtained with pulsed neutron beams, and to extend the detector matrix concept towards full neutron spectrometry. Additional demonstration activities in clinical and accelerator environments would support technology validation under operational conditions. From a technological perspective, the scaling-up of detector fabrication and the integration of dedicated readout electronics optimised for ultra-high pulse rates will be important steps towards practical deployment. In this context, funding is currently being sought to develop a demonstrator system. In parallel, building on the results of this project and related activities, a Generalitat de Catalunya Indústria del Coneixement 2025 grant has been awarded to support a market study on SiC-based dosimeters in general, from which this project is also expected to benefit.
A key result beyond the current state of the art is the development and experimental validation, for the first time in a P-N junction configuration, of active SiC neutron detectors capable of operating reliably in environments with intense photon backgrounds and pulsed radiation, together with customized, cost-effective readout electronics. The detectors developed in this project enable real-time or near-real-time neutron detection with excellent gamma rejection and without saturation effects. Another major advancement is the implementation of a detector matrix based on SiC P–N junction diodes that can be adapted to different neutron energy ranges. By combining the same detector technology with optimized neutron conversion and moderation layers, the project demonstrated sensitivity to thermal, epithermal, and fast neutrons, including under pulsed beam conditions. The successful operation of a SiC neutron detector matrix in both continuous and pulsed neutron fields goes beyond previously reported capabilities of active neutron detectors. It addresses a limitation for applications in modern radiotherapy and accelerator-based facilities.
The project also achieved the first experimental measurements of out-of-field thermal neutrons in conventional clinical radiotherapy accelerators using silicon carbide detectors. The ability to directly measure secondary thermal neutron fluences in realistic treatment room conditions using radiation-hard SiC devices provides new experimental evidence that can support improved risk assessment, patient safety studies, and the optimisation of radiotherapy techniques.
In terms of potential impact, the results of the project open new possibilities for active neutron monitoring in radiotherapy, including emerging treatment modalities such as ultra-high dose-rate radiotherapy. The demonstrated robustness, radiation hardness, and gamma discrimination of the SiC detectors make them strong candidates for deployment in clinical, research, and industrial environments where neutron doses must be quantified accurately. Beyond medical applications, the results are relevant for nuclear facilities, radiation protection, and scientific infrastructures requiring compact and durable neutron monitoring solutions.
To ensure further uptake and long-term success, several key needs have been identified. Further research is required to complete the analysis and publication of the results obtained with pulsed neutron beams, and to extend the detector matrix concept towards full neutron spectrometry. Additional demonstration activities in clinical and accelerator environments would support technology validation under operational conditions. From a technological perspective, the scaling-up of detector fabrication and the integration of dedicated readout electronics optimised for ultra-high pulse rates will be important steps towards practical deployment. In this context, funding is currently being sought to develop a demonstrator system. In parallel, building on the results of this project and related activities, a Generalitat de Catalunya Indústria del Coneixement 2025 grant has been awarded to support a market study on SiC-based dosimeters in general, from which this project is also expected to benefit.