Monocrystalline diamond detector for online monitoring during synchrotron microbeam radiotherapy

At a Glance

Metadata	Details
Publication Date	2023-10-10
Journal	Journal of Synchrotron Radiation
Authors	Francesca di Franco, Nicolas Rosuel, L. Gallin-Martel, M.-L. Gallin-Martel, Mostafa Ghafooryan-Sangchooli
Institutions	Centre National de la Recherche Scientifique, Institut polytechnique de Grenoble
Citations	11
Analysis	Full AI Review Included

Executive Summary

This analysis focuses on the development and characterization of a monocrystalline diamond striped detector designed for online portal monitoring during Synchrotron Microbeam Radiation Therapy (MRT).

Core Achievement: Demonstrated the feasibility of using a microstriped monocrystalline diamond detector for real-time, high-spatial-resolution dosimetry in ultra-high radiation flux environments typical of MRT.
Performance Linearity: The detector exhibited a linear current response across four orders of magnitude, successfully operating up to 10⁴ Gy s^-1, exceeding typical clinical MRT dose rates (6 x 10³ Gy s^-1).
Detector Design: The prototype is an eight-strip device (550 µm thick) with micrometric precision, enabling simultaneous measurement of both high-dose microbeam peaks and low-dose interbeam valleys.
Efficiency and Stability: Achieved approximately 100% charge collection efficiency (CCE). Transient effects (priming/overshoot) observed at low dose rates (1 Gy min^-1) disappeared completely at high synchrotron flux, confirming stability for MRT applications.
Spatial Resolution: The striped geometry successfully resolved alternating microbeam and interbeam areas, proving its capability for micrometric spatial dose fractionation monitoring.
Phantom Validation: Measurements using RW3 and anthropomorphic head phantoms showed excellent agreement with Monte Carlo simulations, with absolute differences in transmitted beam energy deposit generally less than 2%.
Future Prototype: The next generation will utilize 150 µm thick diamond (optimized to minimize interbeam background noise) and feature 153 strips to cover the entire irradiation field (30 mm wide).

Technical Specifications

Parameter	Value	Unit	Context
Detector Material	Monocrystalline CVD Diamond	N/A	Bulk material
Optimized Thickness (Simulated)	150	µm	Minimizes background noise (interbeam ratio maximized)
Prototype Thickness (Tested)	550	µm	Initial bulk diamond
Strip Count (Prototype)	8	N/A	3 mm high strips
Strip Width	178	µm	Active metallization area
Strip Spacing (Interstrip Gap)	60	µm	Separating active strips
Charge Collection Efficiency (CCE)	~100	%	Measured via spectroscopy tests
Dose Rate Linearity Range	1 to 10⁴	Gy s^-1	Measured using filtered polychromatic beam
Typical MRT Dose Rate	6 x 10³	Gy s^-1	Clinical operational rate
MRT X-ray Energy Range	50-500	keV	Polychromatic spectrum (Average: 121 keV)
Diamond Density (ρ)	3.52	g cm^-3	Material property
Energy Gap (E_g)	5.47	eV	Semiconductor property
Electron-Hole Creation Energy (E_e-h)	13.1	eV	Required energy for charge generation
Mobility (Charge Carrier)	~2000	cm² V^-1 s^-1	Enables fast detector response
Experimental Agreement (RW3)	<2	%	Absolute difference vs. simulation
Bias Voltage (550 µm diamond)	-500	V	Applied during characterization
Bias Voltage (150 µm diamond)	-150	V	Applied during characterization

Key Methodologies

The development and characterization involved material processing, custom electronics design, and extensive testing at the European Synchrotron Radiation Facility (ESRF).

Detector Fabrication:
- Bulk monocrystalline CVD diamond chips (4.5 mm x 4.5 mm surface area) were used.
- The back side was fully metalized (TiAl or Al).
- The front side received thin aluminum strip metallization (178 µm wide, 60 µm spacing) to define the active areas and induce a linear electric field (typically 1 V µm^-1).
- A guard ring was added to the final prototype to maintain electric field uniformity and prevent charge collection outside the active zone.
Readout System Development:
- An eight-channel ASIC circuit was designed for charge integration and digitization.
- A Charge-to-Digital Converter (QDC) was used for primary signal integration (1 ms to 100 ms integration time).
- An Analog-to-Digital Converter (ADC) was incorporated to account for residual charges not measured by the QDC.
Synchrotron Characterization (ESRF ID17):
- Energy Dependence: Monochromatic X-rays (33-130 keV) were used. Detector current was normalized to the dose rate measured by a reference ionization chamber.
- Dose Rate Linearity: Polychromatic microbeams (50-500 keV) were used. Dose rate was varied over four orders of magnitude (1 to 10⁴ Gy s^-1) using PMMA absorbers.
- Spatial Scan: A horizontal scan was performed using a single microbeam (25 µm steps) to verify strip response homogeneity and charge collection efficiency across the interstrip gaps.
Phantom Dosimetry and Validation:
- Homogeneous Phantom: A staircase RW3 water-equivalent phantom (2 to 16 cm thickness) was scanned vertically (5 mm s^-1 constant speed) using five microbeams.
- Non-Homogeneous Phantom: A realistic CIRS human head phantom was scanned to test detector response in complex geometries.
- Simulation: GATE (Geant4) Monte Carlo simulations were performed, incorporating the specific synchrotron spectrum and geometry, and compared directly to experimental results.

Commercial Applications

The monocrystalline diamond detector technology, characterized by its high speed, radiation hardness, and near-tissue equivalence, is critical for advanced radiation monitoring and high-power beam applications.

Advanced Radiotherapy (MRT/FLASH):
- Online portal monitoring and real-time dosimetry during Synchrotron Microbeam Radiation Therapy (MRT).
- Dosimetry for emerging ultra-high dose rate treatments (FLASH therapy), where conventional detectors fail due to saturation or recombination.
High-Energy Physics and Research Facilities:
- Monitoring and diagnostics of high-flux X-ray beams at synchrotrons and Free-Electron Lasers (FELs).
- Radiation monitoring in high-radiation environments (e.g., particle accelerators, fusion experiments).
Nuclear and Space Applications:
- Radiation-hard sensors for environments requiring long-term stability and resistance to high displacement damage thresholds.
Medical Imaging:
- Development of high-resolution, fast-response detectors for specialized medical imaging modalities utilizing high-intensity X-ray sources.

View Original Abstract

Microbeam radiation therapy (MRT) is a radiotherapy technique combining spatial fractionation of the dose distribution on a micrometric scale, X-rays in the 50-500 keV range and dose rates up to 16 × 10 3 Gy s −1 . Nowadays, in vivo dosimetry remains a challenge due to the ultra-high radiation fluxes involved and the need for high-spatial-resolution detectors. The aim here was to develop a striped diamond portal detector enabling online microbeam monitoring during synchrotron MRT treatments. The detector, a 550 µm bulk monocrystalline diamond, is an eight-strip device, of height 3 mm, width 178 µm and with 60 µm spaced strips, surrounded by a guard ring. An eight-channel ASIC circuit for charge integration and digitization has been designed and tested. Characterization tests were performed at the ID17 biomedical beamline of the European Synchrotron Radiation Facility (ESRF). The detector measured direct and attenuated microbeams as well as interbeam fluxes with a precision level of 1%. Tests on phantoms (RW3 and anthropomorphic head phantoms) were performed and compared with simulations. Synchrotron radiation measurements were performed on an RW3 phantom for strips facing a microbeam and for strips facing an interbeam area. A 2% difference between experiments and simulations was found. In more complex geometries, a preliminary study showed that the absolute differences between simulated and recorded transmitted beams were within 2%. Obtained results showed the feasibility of performing MRT portal monitoring using a microstriped diamond detector. Online dosimetric measurements are currently ongoing during clinical veterinary trials at ESRF, and the next 153-strip detector prototype, covering the entire irradiation field, is being finalized at our institution.

Tech Support

Original Source

DOI: https://doi.org/10.1107/s160057752300752x