Diamond Detectors for Radiotherapy X-Ray Small Beam Dosimetry

At a Glance

Metadata	Details
Publication Date	2021-04-16
Journal	Frontiers in Physics
Authors	C. Talamonti, K. Kanxheri, S. Pallotta, L. Servoli
Institutions	Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, University of Florence
Citations	43
Analysis	Full AI Review Included

Diamond Detectors for Radiotherapy X-Ray Small Beam Dosimetry: 6CCVD Technical Analysis

This document analyzes the requirements for high-performance diamond dosimeters in modern radiotherapy (IMRT, VMAT, SRT) and outlines how 6CCVD’s advanced MPCVD diamond materials and fabrication services meet these critical engineering specifications.

Executive Summary

The reviewed research confirms that Chemical Vapor Deposition (CVD) diamond is the optimal material for small field X-ray dosimetry, addressing the limitations of traditional detectors (ionization chambers, silicon diodes).

Material Superiority: Diamond offers near-perfect water equivalence (Z ~ 6/7), high spatial resolution, and energy/dose-rate independence, crucial for high-gradient small fields (< 3 x 3 cm²).
CVD Advantage: Synthetic CVD diamond (Single Crystal SCD and Polycrystalline PCD) provides reproducible quality and lower cost compared to natural or HPHT diamonds, enabling widespread clinical adoption (e.g., PTW MicroDiamond).
SCD vs. PCD: Single Crystal Diamond (SCD) offers superior dosimetric properties but is limited in size (< 1 cm²). Polycrystalline Diamond (PCD) enables large-area devices (up to 25 x 25 mm²) necessary for 1D/2D dose mapping and pretreatment verification.
Advanced Geometry: Next-generation detectors utilize complex geometries, including planar (sandwich) and 3D columnar electrode structures, requiring high-precision material processing and metalization.
Critical Parameters: Detector performance hinges on optimizing Charge Collection Distance (CCD), ensuring stable Ohmic or Schottky contacts (e.g., Cr/Au, Ti/Pt/Au), and minimizing polarization effects through high-quality substrate growth.
6CCVD Value Proposition: 6CCVD specializes in providing custom, high-purity SCD and large-area PCD substrates, along with integrated metalization and polishing services, essential for replicating and advancing the prototype devices discussed (e.g., DIAPIX, 3DOSE).

Technical Specifications

The following table summarizes key performance metrics and material requirements extracted from the analysis of diamond dosimeters.

Parameter	Value	Unit	Context
Small Field Size Threshold	< 3 x 3	cm²	Field size where standard dosimetry fails.
SCD Maximum Achievable Area	< 1	cm²	Limitation of single-crystal growth on HPHT seeds.
PCD Maximum Achievable Area	25 x 25	mm²	Achievable size for large-area polycrystalline films.
CVD Growth Temperature	~700	°C	Standard MPCVD growth temperature.
HPHT Growth Temperature	> 1300	°C	High-pressure, high-temperature synthesis.
SCD CCD (Detector Grade)	~300	µm	Charge Collection Distance (CCD) for high-performance SCD.
PTW-SCDD Active Volume	3.79 mm² x 1	µm	Example of a commercial single-crystal dosimeter.
3DOSE Pixel Dimensions	70 x 114 x 500	µm	Dimensions of 3D columnar electrodes in prototype PCD.
3DOSE Bias Voltage	10	V	Low bias operation for 3D columnar PCD.
3DOSE Sensitivity Range	25 to 95	nC Gy^-1	Sensitivity range across pixels in the 3DOSE array.
3DOSE Rise Time	0.5	s	Response time for following beam intensity changes.
Required Linearity Deviation	< 0.5	%	Deviation from linearity required for clinical use (0.04-50 Gy range).

Key Methodologies

Successful fabrication of high-performance diamond dosimeters relies on precise control of the MPCVD growth process and subsequent material engineering steps.

CVD Growth Modality Selection:
- Homo-epitaxial CVD (SCD): Uses HPHT diamond seeds to grow high-quality single crystals, optimizing charge collection efficiency (CCE) and minimizing defects.
- Hetero-epitaxial CVD (PCD/DOI): Uses non-diamond initial matrices (e.g., Iridium) or nano-crystal seeds to achieve large-area polycrystalline films (up to 125mm).
Growth Optimization:
- Temperature Control: Maintaining low growth temperatures (~700 °C) for CVD to ensure reproducible substrate quality.
- Gas Mixture Control: Introducing oxygen (O₂) into the gas mixture to control film thickness uniformity (e.g., 5% uncertainty over wide areas) and improve quality.
Substrate Characterization:
- Raman Spectroscopy: Used to verify crystal purity (peak at 1,332.8 cm^-1) and detect contaminants (graphite, sp² carbon) before metalization.
- Charge Collection Distance (CCD) Measurement: Quantifying charge collection efficiency using alpha-emitting radio-nuclides (e.g., ²⁴¹Am) to ensure detector-grade material (CCD typically ~300 µm).
Electrode Geometry and Fabrication:
- Planar (Sandwich) Configuration: Electrodes deposited on the front and back surfaces, often operated in photoconduction (biased) or photovoltaic (zero-bias Schottky).
- 3D Columnar/Interdigitated Geometry: Electrodes fabricated directly inside the diamond bulk using pulsed laser techniques to create small, highly segmented sensitive volumes (e.g., 80 x 120 µm² cells).
Metalization and Contact Engineering:
- Surface Preparation: H-termination for Ohmic contacts (less stable) or O-termination for stable Schottky contacts.
- Contact Materials: Deposition of high work function noble metals (Pt, Au) often layered with adhesion promoters (Ti, Cr, W) or Diamond-Like Carbon (DLC) films to ensure low contact resistance and stability.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the specialized MPCVD diamond materials and custom fabrication services required to replicate, optimize, and scale the next generation of radiotherapy dosimeters described in this research.

Applicable Materials

Application Requirement (from Paper)	6CCVD Material Solution	Key Specification Match
High-Resolution Point Dosimetry (e.g., PTW-SCDD)	Optical Grade Single Crystal Diamond (SCD)	SCD thickness available from 0.1 µm up to 500 µm. Polishing to Ra < 1nm for optimal surface quality and contact adhesion.
Large-Area Dose Mapping (e.g., DIAPIX, 3DOSE)	Polycrystalline Diamond (PCD) Wafers	Custom dimensions up to 125mm diameter, enabling large 2D pixel matrices for pretreatment verification. Substrate thickness up to 10mm.
Conductive Layers/Doping (for Ohmic contacts)	Boron-Doped Diamond (BDD)	Available for creating highly conductive layers or specialized electrodes to minimize polarization effects and improve signal stability.

Customization Potential

The development of advanced dosimeters, particularly those utilizing 3D columnar structures and specialized contacts, requires precise material engineering beyond standard wafer supply. 6CCVD offers comprehensive in-house services to meet these needs:

Custom Dimensions and Shapes: We provide plates and wafers cut to specific geometries required for detector housing and array integration, including the small, precise dimensions necessary for small field dosimetry.
Advanced Metalization Services: The paper emphasizes the critical role of metal contacts (Ti/Pt/Au, Cr/Au, DLC/Pt/Au). 6CCVD offers internal metalization capabilities using Au, Pt, Pd, Ti, W, and Cu to create stable Ohmic or Schottky contacts essential for reliable detector operation (both biased and zero-bias photovoltaic modes).
High-Precision Polishing: SCD substrates are polished to Ra < 1nm, and inch-size PCD to Ra < 5nm, ensuring optimal surface preparation for subsequent electrode deposition and micro-machining (e.g., laser pulsing for 3D structures).

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD growth optimization and diamond material physics. We can assist researchers and engineers with material selection for similar High Conformal Radiotherapy Dosimetry projects, focusing on achieving the required Charge Collection Distance (CCD) and minimizing structural defects that lead to polarization and slow response times.

Call to Action

To accelerate your research in next-generation diamond detectors, leverage 6CCVD’s expertise in high-quality, reproducible MPCVD diamond. We offer the custom dimensions, specialized metalization, and material purity necessary for high-performance SCD and large-area PCD dosimeters.

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

View Original Abstract

Many new X-Ray treatment machines using small and/or non-standard radiation fields, e.g., Tomotherapy, Cyber-knife, and linear accelerators equipped with high-resolution multi-leaf collimators and on-board imaging system, have been introduced in the radiotherapy clinical routine within the last few years. The introduction of these new treatment modalities has led to the development of high conformal radiotherapy treatment techniques like Intensity Modulated photon Radiation Therapy, Volumetric Modulated Arc Therapy, and stereotactic radiotherapy. When using these treatment techniques, patients are exposed to non-uniform radiation fields, high dose gradients, time and space variation of dose rates, and beam energy spectrum. This makes reaching the required degree of accuracy in clinical dosimetry even more demanding. Continuing to use standard field procedures and detectors in fields smaller than 3 × 3 cm 2 , will generate a reduced accuracy of clinical dosimetry, running the risk to overshadowing the progress made so far in radiotherapy applications. These dosimetric issues represent a new challenge for medical physicists. To choose the most appropriate detector for small field dosimetry, different features must be considered. Short- and long-term stability, linear response to the absorbed dose and dose rate, no energy and angular dependence, are all needed but not sufficient. The two most sought-after attributes for small field dosimetry are water equivalence and small highly sensitive (high sensitivity) volumes. Both these requirements aim at minimizing perturbations of charged particle fluence approaching the Charged Particle Equilibrium condition as much as possible, while maintaining high spatial resolution by reducing the averaging effect for non-uniform radiation fields. A compromise between different features is necessary because no dosimeter currently fulfills all requirements, but diamond properties seem promising and could lead to a marked improvement. Diamonds have long been used as materials for dosimeters, but natural diamonds were only first used for medical applications in the 80 s. The availability of reproducible synthetic diamonds at a lower cost compared to natural ones made the diffusion of diamonds in dosimetry possible. This paper aims to review the use of synthetic poly and single-crystal diamond dosimeters in radiotherapy, focusing on their performance under MegaVoltage photon beams. Both commercial and prototype diamond dosimeters behaviour are described and analyzed. Moreover, this paper will report the main related results in literature, considering diamond development issues like growth modalities, electrical contacts, packaging, readout electronics, and how do they affect all the dosimetric parameters of interest such as signal linearity, energy dependence, dose-rate dependence, reproducibility, rise and decay times.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2021.632299

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