First Characterization of Novel Silicon Carbide Detectors with Ultra-High Dose Rate Electron Beams for FLASH Radiotherapy

At a Glance

Metadata	Details
Publication Date	2023-02-25
Journal	Applied Sciences
Authors	F. Romanò, G. Milluzzo, Fabio Di Martino, Maria Cristina D’Oca, G. Felici
Institutions	Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Azienda Ospedaliera Universitaria Pisana
Citations	32
Analysis	Full AI Review Included

Technical Documentation & Analysis: MPCVD Diamond for UHDR Dosimetry

Reference Paper: Romano et al., “First Characterization of Novel Silicon Carbide Detectors with Ultra-High Dose Rate Electron Beams for FLASH Radiotherapy,” Appl. Sci. 2023, 13, 2986.

Executive Summary

The characterization of solid-state detectors for Ultra-High Dose Rate (UHDR) FLASH Radiotherapy dosimetry highlights the critical need for materials exhibiting extreme radiation hardness, high speed, and minimal beam perturbation.

Application Validation: The study successfully validates the use of solid-state sensors (SiC) for real-time beam monitoring in FLASH-RT, demonstrating linear response up to 2 Gy/pulse.
Material Comparison & Pivot: While Silicon Carbide (SiC) offers a compromise between Silicon and Diamond, the intrinsic properties of Single Crystal Diamond (SCD) offer superior performance metrics critical for next-generation UHDR systems.
Radiation Hardness: Diamond (43 eV displacement energy) significantly exceeds SiC (30-40 eV), ensuring long-term stability and reliability beyond the 90 kGy cumulative dose tested.
Speed and Mobility: Diamond’s electron mobility (1800 cm²/Vs) is more than double that of SiC (800 cm²/Vs), enabling faster charge collection and minimizing saturation effects observed in the SiC study.
Transparency Requirement: The paper emphasizes the need for ultra-thin, “free-standing membranes” (<20 µm) to minimize beam perturbation. 6CCVD specializes in producing high-purity, ultra-thin SCD wafers ideal for this requirement.
6CCVD Value Proposition: 6CCVD provides the highest quality MPCVD SCD material, offering the ultimate solution for UHDR dosimetry where radiation damage and response speed are paramount design constraints.

Technical Specifications

The following data points were extracted from the characterization of the SiC detector and compared against the intrinsic properties of Diamond, highlighting the performance ceiling achievable with 6CCVD materials.

Parameter	SiC Value (Tested)	Unit	Context / Diamond Comparison
Active Area	1 x 1	cm²	Standard size, easily replicated by 6CCVD.
Active Thickness	10	µm	Ultra-thin layer required for transparency.
Substrate Thickness	370	µm	n+ thick substrate (removal required for “free-standing membrane”).
Operational Voltage	480	V	Chosen to ensure full depletion of the active region.
Linearity Limit	Up to 2	Gy/pulse	SiC response remains linear up to this dose rate.
Cumulative Dose Stability	±0.75	%	Variation in charge per pulse up to 90 kGy.
Electron Mobility (Diamond)	1800	cm²/Vs	Significantly higher than SiC (800 cm²/Vs), enabling faster response.
Energy Gap (Diamond)	5.5	eV	Higher than SiC (3.23 eV), resulting in lower leakage current.
Displacement Energy (Diamond)	43	eV	Highest radiation hardness (SiC: 30-40 eV), crucial for long-term UHDR use.
e-h Pair Creation Energy (Diamond)	13	eV	Higher than SiC (7.6-8.4 eV), indicating lower intrinsic sensitivity but higher signal integrity under extreme fields.

Key Methodologies

The experimental setup utilized advanced techniques to characterize the solid-state detectors under extreme UHDR conditions.

Accelerator System: ElectronFLASH (EF) LINAC (SIT-Sordina, IT) operating in electron mode only.
- Nominal Energies: 5 MeV to 12 MeV (9 MeV used for characterization).
- Dose Rate: 0.01 to 4000 Gy/s and higher.
- Pulse Duration: Fixed at 2 µs.
Detector Structure: Novel SiC PIN junction detectors (p+ highly doped layer on n- low doped layer on n+ thick substrate).
Dose Variation: Dose-per-pulse (D/p) was varied by adjusting the applicator diameter (3.5 cm to 12 cm) and the Applicator-to-Detector Distance (ADD) (0 cm to 111 cm).
Reference Dosimetry: Alanine pellet dosimeters and EBT-XD radiochromic films (RCFs) were used as reference standards for D/p measurement.
Measurement Setup: A Keithley 6517A electrometer was used to supply bias voltage and read the beam charge directly.
Simulation: Monte Carlo Geant4 toolkit was used to simulate energy deposition and produced charge for various SiC thicknesses (2 µm to 20 µm) and energies (7 MeV to 200 MeV).
Radiation Hardness Test: Cumulative dose up to 90 kGy was delivered using 400 pulses at 5 Gy/pulse (35 mm applicator, 5 Hz frequency).

6CCVD Solutions & Capabilities

The research demonstrates the viability of solid-state detectors for UHDR dosimetry but also exposes limitations in SiC related to speed and ultimate radiation tolerance. 6CCVD’s MPCVD diamond materials are engineered to overcome these limitations, providing the optimal platform for clinical translation of FLASH-RT.

Applicable Materials for Next-Generation UHDR Dosimetry

To replicate and extend this research, particularly for Very High Energy Electron (VHEE) beams and long-term clinical use, 6CCVD recommends the following materials:

6CCVD Material	Recommended Specification	Rationale for UHDR/FLASH-RT
Optical Grade Single Crystal Diamond (SCD)	SCD, Type IIa, High Purity, 10 x 10 mm to 125 mm wafers.	Highest intrinsic radiation hardness (43 eV displacement energy) ensures stability far beyond 90 kGy. Superior electron mobility (1800 cm²/Vs) minimizes charge collection time, avoiding saturation issues seen with SiC at high D/p.
Ultra-Thin SCD Membranes	Thickness: 0.1 µm to 20 µm.	Directly addresses the paper’s requirement for “free-standing membranes” (<20 µm) to achieve beam transparency and minimize angular dispersion, especially for low-energy beams.
Boron-Doped Diamond (BDD)	PCD or SCD, customized doping levels.	Can be used for robust, high-sensitivity electrodes or specialized electrochemical sensing applications related to dosimetry.

Customization Potential for Detector Fabrication

The SiC detector utilized a specific geometry (1 x 1 cm² active area, 10 µm active thickness, PIN junction with metal contacts). 6CCVD offers full customization to meet or exceed these requirements using superior diamond material:

Custom Dimensions: 6CCVD can supply SCD plates up to 10 x 10 mm or PCD wafers up to 125 mm diameter, allowing for the fabrication of large-area sensors required for clinical field monitoring.
Precision Thickness Control: We offer SCD active layers from 0.1 µm up to 500 µm, enabling precise control over detector sensitivity and beam perturbation (crucial for the <20 µm membrane design).
Advanced Metalization: The PIN junction design requires high-quality metal contacts. 6CCVD provides in-house metalization services, including Ti/Pt/Au, W, and Cu, ensuring robust, low-resistance contacts necessary for high instantaneous current readout in FLASH regimes.
Surface Finish: SCD polishing to Ra < 1 nm ensures optimal interface quality for metal contacts and minimizes surface defects that could affect charge collection efficiency.

Engineering Support & Call to Action

The challenges associated with UHDR dosimetry—specifically ion recombination in gas chambers and saturation limits in solid-state detectors—require specialized material expertise.

Expert Consultation: 6CCVD’s in-house PhD team specializes in MPCVD growth and material physics for extreme environments. We can assist researchers in selecting the optimal diamond grade, thickness, and metalization scheme to maximize signal-to-noise ratio and minimize saturation effects for similar FLASH Radiotherapy projects.
Addressing SiC Limitations: By leveraging diamond’s higher displacement energy and electron mobility, 6CCVD provides a path to detectors that are inherently more stable and faster than SiC, ensuring reliable secondary standard dosimetry for the clinical transition of FLASH-RT.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Ultra-high dose rate (UHDR) beams for FLASH radiotherapy present significant dosimetric challenges. Although novel approaches for decreasing or correcting ion recombination in ionization chambers are being proposed, applicability of ionimetric dosimetry to UHDR beams is still under investigation. Solid-state sensors have been recently investigated as a valuable alternative for real-time measurements, especially for relative dosimetry and beam monitoring. Among them, Silicon Carbide (SiC) represents a very promising candidate, compromising between the maturity of Silicon and the robustness of diamond. Its features allow for large area sensors and high electric fields, required to avoid ion recombination in UHDR beams. In this study, we present simulations and experimental measurements with the low energy UHDR electron beams accelerated with the ElectronFLASH machine developed by the SIT Sordina company (IT). The response of a newly developed 1 × 1 cm2 SiC sensor in charge as a function of the dose-per-pulse and its radiation hardness up to a total delivered dose of 90 kGy, was investigated during a dedicated experimental campaign, which is, to our knowledge, the first characterization ever done of SiC with UHDR-pulsed beams accelerated by a dedicated ElectronFLASH LINAC. Results are encouraging and show a linear response of the SiC detector up to 2 Gy/pulse and a variation in the charge per pulse measured for a cumulative delivered dose of 90 kGy, within ±0.75%.

Tech Support

Original Source

DOI: https://doi.org/10.3390/app13052986

References

2017 - Faster and safer? FLASH ultra-high dose rate in radiotherapy [Crossref]
2019 - Clinical translation of FLASH radiotherapy: Why and how? [Crossref]
2019 - Re: Differential impact of FLASH versus conventional dose rate irradiation: Spitz et al [Crossref]
2017 - Experimental platform for ultra-high dose rate FLASH irradiation of small animals using a clinical linear accelerator [Crossref]
2014 - Ultrahigh dose-rate FLASH irradiation increases the differential response between normal and tumor tissue in mice [Crossref]
2018 - X-rays can trigger the FLASH effect: Ultra-high dose-rate synchrotron light source prevents normal brain injury after whole brain irradiation in mice [Crossref]
2018 - Experimental set-up for FLASH proton irradiation of small animals using a clinical system [Crossref]
2019 - Feasibility of proton FLASH effect tested by zebrafish embryo irradiation [Crossref]
2022 - Ultra-high dose rate dosimetry: Challenges and opportunities for FLASH radiation therapy [Crossref]
2020 - Beam monitors for tomorrow: The challenges of electron and photon FLASH RT [Crossref]