A Diamond-Based Dose-per-Pulse X-ray Detector for Radiation Therapy

At a Glance

Metadata	Details
Publication Date	2021-09-10
Journal	Materials
Authors	Sara Pettinato, M. Girolami, Riccardo Olivieri, A. Stravato, C. Caruso
Institutions	Azienda Ospedaliera San Giovanni Addolorata, Institute of Structure of Matter
Citations	23
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond-Based Dose-per-Pulse X-ray Detector

This document analyzes the requirements and achievements detailed in the research article, “A Diamond-Based Dose-per-Pulse X-ray Detector for Radiation Therapy,” and maps them directly to the advanced MPCVD diamond solutions available from 6CCVD.

Executive Summary

The research successfully demonstrates a high-speed, high-fidelity diamond detection system for real-time dose-per-pulse monitoring in dynamic radiation therapy (LINAC, 6-MV X-rays).

Real-Time Dosimetry: Achieved accurate, pulse-by-pulse charge measurement at high repetition rates (up to 360 Hz), crucial for modern Intensity-Modulated Radiation Therapy (IMRT) and Volumetric-Modulated Arc Therapy (VMAT).
High-Purity Material: Utilized optical-grade Single Crystal Diamond (SCD) with ultra-low impurity levels ([N] < 5 ppb, [B] < 0.5 ppb) to ensure fast response time (nanosecond range) and high radiation hardness.
Exceptional Sensitivity: Measured detector sensitivity of 302.2 nC/Gy under 6-MV X-photons, exhibiting nonlinearity < 0.5% across the 0.1-10 Gy dose range.
System Speed & Resolution: The integrated system, featuring a custom gated integrator, achieved a time resolution of 1 µs, enabling signal processing in the millisecond period between pulses.
High Signal Quality: Demonstrated a Signal-to-Noise Ratio (SNR) greater than 60 dB without specialized shielding, confirming the robustness of the SCD detector and front-end electronics.
Validation: Pulse-by-pulse charge measurements (~84.68 pC) showed excellent agreement (< 0.5% difference) with continuous integration measurements performed by a commercial electrometer.

Technical Specifications

Parameter	Value	Unit	Context
Detector Material	Single Crystal CVD Diamond (SCD)	N/A	Optical/Electronic Grade
Detector Dimensions	4 x 4 x 0.5	mm³	Custom SCD plate size
Nitrogen Impurity [N]	< 5	ppb	Required for high-speed response
Boron Impurity [B]	< 0.5	ppb	Required for high-speed response
Contact Structure	Metal-Semiconductor-Metal (MSM)	N/A	Sputter deposited Ag film
Contact Diameter	3.2	mm	Circular contacts
Bias Voltage	10	V	Operating condition
X-ray Energy	6	MV	LINAC X-ray beams
Dose Range Investigated	0.1-10	Gy	Typical radiotherapy range
Detector Sensitivity	302.2 ± 0.1	nC/Gy	Measured at 10 V bias
Nonlinearity	< 0.5	%	Over the investigated dose range
Pulse Repetition Rate	Up to 360	Hz	Corresponds to 6 Gy/min dose rate
Measured Charge/Pulse	~84.68	pC	Average value across all dose rates
System Time Resolution	1	µs	Gated integrator capability
System Response Time	Few tens of	µs	Enables high-rate pulse monitoring
Signal-to-Noise Ratio (SNR)	> 60	dB	Achieved without specialized shielding

Key Methodologies

The successful implementation of the dose-per-pulse detector relies on the synergy between high-quality CVD diamond material and specialized front-end electronics.

Diamond Detector Fabrication:
- Used optical-grade single-crystal CVD diamond (4 x 4 x 0.5 mm³) grown via Microwave Plasma-Assisted CVD (MPCVD).
- Metal-Semiconductor-Metal (MSM) structure was fabricated using sputter deposition of a 300 nm-thick Silver (Ag) film through a shadow mask.
- The detector was encapsulated in a PMMA cylinder and positioned at the LINAC isocenter for 6-MV X-ray testing.
Pulsed X-ray Source Synchronization:
- The X-ray pulses were generated by a Varian Clinac iX LINAC system.
- Measurements were synchronized with the 360 Hz sync signal provided by the LINAC console.
Gated Integration Readout Electronics:
- A custom front-end system based on a high-precision-switched integrator transimpedance amplifier (IVC102) was employed.
- An LPC845 microcontroller timer generated digital control signals (S1/S2) to precisely define the integration window (set to 30 µs total integration time).
- Synchronization accuracy was maintained within ±33 ns relative to the LINAC sync signal.
- The system intrinsically performs a sample-and-hold operation, allowing signal processing during the millisecond interval between pulses.
Calibration and Validation:
- Electronics were calibrated using a Keithley 6221 current source to determine the integration capacitance (C_INT = 88.49 pF).
- Detector sensitivity was preliminarily characterized using a Keithley 6517A electrometer in continuous integration mode (302.2 nC/Gy).
- Final results were validated by comparing the accumulated charge Q(t) from the gated integrator system against the Q₆₅₁₇(t) values from the commercial electrometer, showing agreement within 0.5%.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-purity CVD diamond materials and custom fabrication services required to replicate, scale, and advance this critical radiotherapy dosimetry research.

Applicable Materials

To achieve the nanosecond response times and high radiation hardness required for pulse-by-pulse dosimetry, the following 6CCVD materials are recommended:

6CCVD Material Grade	Description & Application Match
Electronic Grade SCD	Ultra-high purity Single Crystal Diamond (SCD) with guaranteed impurity levels ([N] < 5 ppb, [B] < 0.5 ppb typical). Essential for high charge collection efficiency and fast response in high-energy physics and medical dosimetry.
Optical Grade SCD	Optimized for low absorption and high transparency, suitable for applications where the detector must maintain optical properties while functioning as a dosimeter.
Custom PCD Plates	For large-area beam profiling or QA systems requiring detectors larger than 10 x 10 mm², 6CCVD offers Polycrystalline Diamond (PCD) plates up to 125 mm in diameter.

Customization Potential

The research utilized specific dimensions and metal contacts. 6CCVD’s in-house fabrication capabilities ensure seamless material supply and detector customization:

Custom Dimensions and Thickness: The paper used a 4 x 4 x 0.5 mm³ SCD plate. 6CCVD supplies SCD plates with thicknesses ranging from 0.1 µm up to 500 µm and offers custom laser cutting to meet precise geometric requirements for integration into LINAC phantoms.
Advanced Metalization Services: The paper utilized Ag contacts. 6CCVD offers superior, high-stability metal stacks via internal sputter deposition capabilities, including:
- Ti/Pt/Au: Standard stack for robust adhesion and oxidation resistance.
- W, Cu, Pd: Available for specialized contact requirements or high-temperature applications.
- We can replicate the 3.2 mm circular contact geometry or design custom electrode patterns (e.g., interdigitated electrodes) for enhanced charge collection.
Surface Finish: 6CCVD guarantees ultra-smooth surfaces, with SCD polishing achieving Ra < 1 nm, ensuring optimal interface quality for metal contact deposition and minimizing surface defects that could affect charge transport.

Engineering Support

6CCVD’s in-house PhD team specializes in the physics of CVD diamond detectors and can provide comprehensive support for similar high-speed dosimetry projects:

Material Selection Consulting: Assistance in selecting the optimal diamond grade (SCD vs. PCD, doping levels) to maximize charge collection efficiency and minimize polarization effects in high-dose-rate environments.
Detector Design Optimization: Support for electrode design, contact metal selection, and packaging strategies to maintain the high SNR (> 60 dB) achieved in this study and ensure long-term stability under prolonged radiation exposure.
Global Supply Chain: We offer reliable, global shipping (DDU default, DDP available) to ensure researchers and engineers worldwide receive their custom diamond materials quickly and securely.

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

View Original Abstract

One of the goals of modern dynamic radiotherapy treatments is to deliver high-dose values in the shortest irradiation time possible. In such a context, fast X-ray detectors and reliable front-end readout electronics for beam diagnostics are crucial to meet the necessary quality assurance requirements of care plans. This work describes a diamond-based detection system able to acquire and process the dose delivered by every single pulse sourced by a linear accelerator (LINAC) generating 6-MV X-ray beams. The proposed system is able to measure the intensity of X-ray pulses in a limited integration period around each pulse, thus reducing the inaccuracy induced by unnecessarily long acquisition times. Detector sensitivity under 6-MV X-photons in the 0.1-10 Gy dose range was measured to be 302.2 nC/Gy at a bias voltage of 10 V. Pulse-by-pulse measurements returned a charge-per-pulse value of 84.68 pC, in excellent agreement with the value estimated (but not directly measured) with a commercial electrometer operating in a continuous integration mode. Significantly, by intrinsically holding the acquired signal, the proposed system enables signal processing even in the millisecond period between two consecutive pulses, thus allowing for effective real-time dose-per-pulse monitoring.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma14185203

References

2008 - The radiobiological effect of intra-fraction dose-rate modulation in intensity modulated radiation therapy (IMRT) [Crossref]
2017 - Measuring radiotherapy setup errors in IMRT treated head and neck cancer patients requiring bilateral neck irradiation, NCI-egypt experience [Crossref]
2011 - Commissioning of volumetric modulated arc therapy (VMAT) in a dual-vendor environment [Crossref]
2010 - Clinical applications of volumetric modulated arc therapy [Crossref]
2018 - Dosimetric comparison and feasibility of Simultaneous Integrated Boost (SIB) in treatment of malignant gliomas using Intensity Modulated Radiotherapy (IMRT) or Volumetric Modulated Arc Therapy (VMAT)
2014 - A dosimetric comparison of Volumetric Modulated Arc Therapy (VMAT) and non-coplanar Intensity Modulated Radiotherapy (IMRT) for nasal cavity and paranasal sinus cancer [Crossref]
2011 - Volumetric modulated arc therapy: A review of current literature and clinical use in practice [Crossref]