A Highly Versatile X-ray and Electron Beam Diamond Dosimeter for Radiation Therapy and Protection

At a Glance

Metadata	Details
Publication Date	2023-01-14
Journal	Materials
Authors	Sara Pettinato, M. Girolami, A. Stravato, Valerio Serpente, Daniela Musio
Institutions	Roma Tre University, Institute of Structure of Matter
Citations	23
Analysis	Full AI Review Included

Executive Summary

This study demonstrates a highly versatile single-crystal Chemical Vapor Deposition (CVD) diamond dosimeter suitable for a wide range of radiation therapy (RT) and protection applications.

Versatility Confirmed: The dosimeter exhibits excellent performance across low-energy continuous X-rays (keV range), high-energy pulsed X-rays (MeV range), and high-energy pulsed electron beams (MeV range).
Energy and Nature Independence: Sensitivity values for 6 MeV X-rays (0.299 ± 0.002 µC/Gy) and 6 MeV electrons (0.298 ± 0.004 µC/Gy) are nearly identical, confirming the device’s response is independent of the radiation nature at high energies.
Excellent Linearity: The device showed near-perfect linearity (Δ ≈ 1.0003) with both dose and dose-rate across all tested high-energy regimes, crucial for accurate clinical dosimetry.
Ultra-Low Detection Limit: Under low-energy X-rays, an extremely low Limit of Detection (LoD) of 23 nGy/s (at SNR=3) was achieved, qualifying the device for sensitive radiation protection monitoring.
Fast Response: The intrinsic detector response speed is in the nanosecond range, though system-limited rise time (t_r) during LINAC operation was measured at approximately 100 ms.
Tissue Equivalence: Utilizing electronic-grade CVD diamond ensures high tissue-equivalence, minimizing the need for complex correction factors when quantifying absorbed dose in human tissue.

Technical Specifications

Parameter	Value	Unit	Context
Active Material	Single-crystal CVD Diamond	N/A	Electronic-grade (Element Six Ltd.)
Dimensions	4.5 x 4.5 x 0.5	mm³	Sample size
Active Volume	~4	mm³	MSM structure
Contact Material	Ag (Silver)	N/A	300 nm thickness, sputtered
Bias Voltage (V_bias)	10	V	Photoconductive mode operation
Low-Energy X-ray Range	0.18-189	µGy/s	Continuous X-rays (40 kV tube)
Low-Energy Sensitivity	1.988 ± 0.005	µC/Gy	Uncorrected (LE-X)
High-Energy X-ray Sensitivity	0.299 ± 0.002	µC/Gy	6 MeV, corrected for phantom attenuation
High-Energy Electron Sensitivity	0.298 ± 0.004	µC/Gy	6 MeV, at 10 Gy/min
Limit of Detection (LoD)	23	nGy/s	Low-energy X-rays (SNR=3)
Linearity Coefficient (HE-X)	1.0003	N/A	High-Energy X-rays (R = 0.99994)
Response Time (LE-X)	< 0.5	s	Rise and fall times (system limited)
System Rise Time (HE-X)	~100	ms	Limited by LINAC transient
Angular Dependence (Preliminary)	< 1	% (Std. Dev.)	Within ±10° angle

Key Methodologies

The dosimeter fabrication and characterization involved precise material processing and specialized radiation setups:

Diamond Cleaning Procedure:
- Acid cleaning in a H₂SO₄:HClO₄:HNO₃ (1:1:1) mixture for 30 min at boiling point.
- Ultrasonic bath in hot acetone for 5 min.
- Rinsing in deionized water, followed by drying in pure nitrogen flow.
Contact Fabrication:
- Two 300 nm thick Ag contacts were fabricated on the top and bottom surfaces via sputtering deposition.
- A stainless-steel shadow mask defined the circular contact geometry (1.6 mm radius), creating a Metal-Semiconductor-Metal (MSM) structure.
Device Packaging:
- The sample was fixed into a circular Rexolite® ring using epoxy resin (Epotek® 301).
- Al wires connected contacts to a 3.5 m triaxial cable.
- Encapsulation in a Polymethyl-methacrylate (PMMA) cylindrical housing (7.4 mm diameter, 40 mm length), filled with epoxy, and coated with black acrylic paint for waterproofing and ambient light elimination.
Low-Energy X-ray Characterization (Radiation Protection):
- Source: Coolidge tube (Cu target, 40 kV acceleration voltage).
- Measurement: Current measured using a Keithley 487 electrometer (10 V bias).
- Dose-rate calibration performed using a reference Farmer ionization chamber (NE 2536/3C).
High-Energy X-ray and Electron Characterization (Radiotherapy):
- Source: Medical LINAC (Varian Clinac iX), 6 MeV energy.
- Setup: Dosimeter placed in a water-equivalent Plexiglass® phantom (5 cm depth for X-rays, 3 cm depth for electrons) at the LINAC isocenter (100 cm SSD).
- Measurement: Dedicated front-end electronics/gated integrator used for pulse-by-pulse measurement of photocurrent (current mode) or total charge (charge mode).
- Calibration: Reference dose measurements performed using a calibrated ionization chamber (FC65-G, IBA Dosimetry) according to IAEA TRS398 protocol to determine the actual dose absorbed by the phantom.

Commercial Applications

The demonstrated performance characteristics—especially the high versatility, tissue equivalence, and wide dynamic range—make this CVD diamond dosimeter highly valuable across several specialized fields:

External Beam Radiation Therapy (EBRT):
- Quality Assurance (QA) and calibration of high-energy linear accelerators (LINACs) producing pulsed X-ray and electron beams (4-25 MeV range).
- Dosimetry for complex techniques like Intensity Modulated RT (IMRT) and Volumetric Arc RT (VMAT), requiring high spatial and temporal resolution.
Intra-Operative Radiation Therapy (IORT):
- Accurate, real-time dose monitoring during surgery using mobile LINACs, handling both low-energy X-rays (up to 50 keV) and MeV electron beams.
Radiation Protection Dosimetry:
- Environmental monitoring and personal dosimetry due to the extremely low Limit of Detection (23 nGy/s) achieved under low-energy X-rays.
Transfer Standard Devices:
- The excellent agreement between X-ray and electron sensitivities at 6 MeV qualifies the diamond detector as a robust transfer standard for dose measurement in tissue-equivalent media.
Industrial and Research Applications:
- High-speed monitoring of pulsed radiation sources (e.g., flash X-ray systems, high-power electron beam facilities) where nanosecond response time is critical.

View Original Abstract

Radiotherapy is now recognized as a pillar in the fight against cancer. Two different types are currently used in clinical practice: (1) external beam radiotherapy, using high-energy X-rays or electron beams, both in the MeV-range, and (2) intraoperative radiotherapy, using low-energy X-rays (up to 50 keV) and MeV-range electron beams. Versatile detectors able to measure the radiation dose independently from the radiation nature and energy are therefore extremely appealing to medical physicists. In this work, a dosimeter based on a high-quality single-crystal synthetic diamond sample was designed, fabricated and characterized under low-energy X-rays, as well as under high-energy pulsed X-rays and electron beams, demonstrating excellent linearity with radiation dose and dose-rate. Detector sensitivity was measured to be 0.299 ± 0.002 µC/Gy under 6 MeV X-ray photons, and 0.298 ± 0.004 µC/Gy under 6 MeV electrons, highlighting that the response of the diamond dosimeter is independent of the radiation nature. Moreover, in the case of low-energy X-rays, an extremely low limit of detection (23 nGy/s) was evaluated, pointing out the suitability of the device to radiation protection dosimetry.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma16020824

References

2018 - Future directions of intraoperative radiation therapy: A brief review [Crossref]
2005 - Ion recombination correction for very high dose-per-pulse high-energy electron beams [Crossref]
2017 - Present state and issues in IORT physics [Crossref]
2011 - Volumetric modulated arc therapy: A review of current literature and clinical use in practice [Crossref]
2012 - Diamond detectors for UV and X-ray source imaging [Crossref]
2016 - Diamond device architectures for UV laser monitoring [Crossref]
2022 - Self-powered solar-blind ultrafast UV-C diamond detectors with asymmetric Schottky contacts [Crossref]