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	Institute of Structure of Matter, University Niccolò Cusano
Citations	23
Analysis	Full AI Review Included

Executive Summary

This research details the successful development and testing of an ultra-fast, diamond-based detection system designed for real-time, pulse-by-pulse dosimetry in modern dynamic radiation therapy (RT).

Core Value Proposition: Enables real-time Quality Assurance (QA) by measuring the dose delivered by every single X-ray pulse from a Linear Accelerator (LINAC), a capability not feasible with standard electrometers due to their long integration times.
Material and Performance: Utilizes a high-purity, single-crystal CVD diamond detector, leveraging its nanosecond response time and high radiation hardness (>10 MGy).
System Architecture: Employs a compact, custom-designed gated-integrator readout circuit (IVC102 TIA) synchronized precisely (±33 ns) to the LINAC sync signal.
Speed and Efficiency: Achieves pulse-by-pulse monitoring at high repetition rates (up to 360 Hz), with the system ready for a new acquisition within 50 µs.
Accuracy Validation: The measured average charge per pulse (<Q> = 84.68 pC) showed excellent agreement (within 0.1%) with the value estimated using a commercial Keithley 6517A electrometer operating in continuous mode.
Signal Quality: Demonstrated a high Signal-to-Noise Ratio (SNR) greater than 60 dB without specialized shielding, confirming the robustness of the integrated electronics.
Dose Measurement: Successfully measured a dose-per-pulse value of approximately 3 x 10^-4 Gy for 6-MV X-ray photons.

Technical Specifications

Parameter	Value	Unit	Context
Detector Material	Single-crystal CVD Diamond	N/A	Optical-grade (Element Six)
Detector Dimensions	4 x 4 x 0.5	mm³	Active volume
Nitrogen Impurity ([N])	< 5	ppb	High purity
Boron Impurity ([B])	< 0.5	ppb	High purity
Contact Material/Thickness	Ag (300 nm)	N/A	Sputter deposited, Metal-Semiconductor-Metal (MSM)
Bias Voltage (V_bias)	10	V	Operating condition
X-ray Source Energy	6	MV	LINAC (Varian Clinac iX)
Dose Range Tested	0.1 - 10	Gy	Typical radiotherapy range
Detector Sensitivity	302.2 ± 0.1	nC/Gy	Measured under 6-MV photons
Nonlinearity	< 0.5	%	Over the investigated dose range
Average Charge per Pulse (<Q>)	84.68	pC	Measured at 1-6 Gy/min Dose Rate (DR)
Estimated Dose per Pulse	~3 x 10^-4	Gy	Derived from sensitivity
Maximum Pulse Rate Tested	360	Hz	Corresponds to 6 Gy/min DR
System Time Resolution	1	µs	Gated integrator control precision
Integration Time (T_int)	30	µs	Set around the X-ray pulse
System Recovery Time	~50	µs	Time until ready for next pulse acquisition
Signal-to-Noise Ratio (SNR)	> 60	dB	Measured output signal amplitude
Integration Capacitance (C_INT)	88.49	pF	Calibrated value of readout electronics (IVC102)

Key Methodologies

The system relies on a highly synchronized gated-integration technique to isolate and measure the charge generated by each microsecond-duration X-ray pulse.

Detector Fabrication and Setup:
- An optical-grade single-crystal CVD diamond (4 x 4 x 0.5 mm³) was prepared with circular Ag contacts (3.2 mm diameter) via sputter deposition, forming a Metal-Semiconductor-Metal (MSM) structure.
- The detector was biased at 10 V and placed within a Plexiglas® phantom at the LINAC isocenter for 6-MV X-ray testing.
Synchronization and Timing Control:
- The 360 Hz synchronization signal (sync) from the LINAC console was fed into a fast comparator (TL3016).
- The comparator output triggered the embedded timer of an LPC845 microcontroller, which generated precise digital control signals (S1 and S2) for the integrator.
- Synchronization accuracy was maintained within ±33 ns.
Gated Integration Cycle:
- Integration Start: The integration phase (S2 high) was programmed to begin 2 µs after the rising edge of the LINAC sync signal.
- Integration Duration: The integration time was set to 30 µs to fully capture the charge generated by the X-ray pulse (which lasts approximately 4 µs).
- Hold Period: A 20 µs hold period (S1 high) followed the integration, allowing the output voltage to stabilize for measurement (sample-and-hold operation).
- Reset: A reset phase (S1 and S2 low) cleared the integrator before the arrival of the next pulse, ensuring the system was ready for a new acquisition within 50 µs total time.
Charge Measurement and Calibration:
- The output voltage (V₀) from the IVC102 integrator was measured.
- The collected charge (Q) was calculated using the calibrated integration capacitance (C_INT = 88.49 pF) via the formula Q = V₀ * C_INT.
- The system’s sensitivity (302.2 nC/Gy) was confirmed using continuous-mode measurements with a Keithley 6517A electrometer.

Commercial Applications

The unique combination of ultra-fast response time, high radiation hardness, and tissue equivalence makes this diamond-based system critical for advanced medical and industrial applications.

Advanced Radiation Therapy (RT):
- Real-Time QA: Essential for quality assurance of modern dynamic RT techniques (IMRT, VMAT) where dose gradients are delivered rapidly, requiring verification of every pulse.
- LINAC Diagnostics: Monitoring the stability and modulation of X-ray beams at high pulse repetition rates (up to 360 Hz and potentially higher).
Ultra-High Dose Rate Dosimetry:
- FLASH Radiotherapy Research: The system’s ability to measure dose in the microsecond range is crucial for characterizing and monitoring beams used in experimental FLASH RT, which delivers therapeutic doses in extremely short time intervals.
High-Energy Physics and Accelerators:
- Beam Monitoring: Used as a robust, long-term beam profile and intensity monitor in high-radiation environments, such as particle physics experiments (e.g., CERN) where conventional detectors degrade quickly.
Industrial Radiography:
- Fast, high-resolution X-ray detection for non-destructive testing and imaging applications utilizing pulsed sources.

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

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