Microdosimetry for hadron therapy - A state of the art of detection technology

At a Glance

Metadata	Details
Publication Date	2022-11-24
Journal	Frontiers in Physics
Authors	Gabriele Parisi, F. Romanò, Giuseppe Schettino
Institutions	Istituto Nazionale di Fisica Nucleare, Sezione di Catania, University of Surrey
Citations	20
Analysis	Full AI Review Included

Technical Documentation & Analysis: MPCVD Diamond for Hadron Therapy Microdosimetry

Executive Summary

This review confirms that diamond-based Solid-State Detectors (SSDs) are a critical, high-performance technology for microdosimetry in advanced radiation fields, particularly hadron therapy.

Superior Material Performance: Single Crystal Diamond (SCD) is highlighted as the most promising solid-state material due to its high radiation hardness (up to 5/10 times more resistant than silicon in clinical energy ranges), excellent thermal stability, and near tissue-equivalence.
High Fluence Rate Capability: Unlike conventional Tissue Equivalent Proportional Counters (TEPCs), SCD detectors operate reliably at clinical fluence rates (up to 10⁷ cm^-2 s^-1) without saturation or significant pile-up distortion, crucial for Quality Assurance (QA).
Precision Layering Requirement: SCD microdosimeters rely on highly controlled p-i-n junction structures, requiring the intrinsic (i) sensitive volume (SV) layer to be grown with thicknesses ranging from 0.1 µm to tens of µm.
6CCVD Core Value: 6CCVD specializes in providing the high-purity, homoepitaxially grown MPCVD SCD material necessary for these complex layered structures, ensuring precise thickness control (0.1 µm to 500 µm).
Customization for Research: Successful SCD microdosimeter fabrication requires advanced post-growth processing, including precise polishing (Ra &lt; 1nm) and custom metalization (e.g., Al, Cr, Ag, Ti/Pt/Au), all available through 6CCVD’s internal capabilities.

Technical Specifications

The following hard data points extracted from the review emphasize the stringent requirements for solid-state microdosimeters, particularly those based on diamond.

Parameter	Value	Unit	Context
SCD Sensitive Volume (SV) Thickness	0.1 to tens	µm	Required thickness for intrinsic layer (p-i-n junction).
Silicon SV Thickness (3D Cylindrical)	5 to 20	µm	Typical thickness for high-resolution silicon devices.
Clinical Fluence Rate (Hadron Therapy)	~10⁷	cm^-2 s^-1	Rate at which SSDs must operate without saturation.
SCD Radiation Hardness (Protons)	2/3 to 5/10 times more	N/A	Resistance compared to silicon in clinical energy range.
SCD CCE Drop (Post-Treatment)	~1	%	Expected drop after 2000 full hadron treatments (excellent stability).
SCD Low Energy Cut-off (Reported)	~80	keV	Achieved in Verona et al. p-i-n structure.
SCD Polishing Requirement	Ra &lt; 1	nm	Necessary for high-quality homoepitaxial growth interfaces.

Key Methodologies

The fabrication of high-performance diamond microdosimeters relies heavily on advanced MPCVD growth and precise post-processing techniques, which 6CCVD supports.

Homoepitaxial MPCVD Growth: High-quality artificial Single Crystal Diamond (SCD) is grown using Microwave Plasma Enhanced Chemical Vapor Deposition (MWPECVD) to create the necessary multi-layered structures (e.g., p-type BDD layer followed by intrinsic SCD layer).
Precise Thickness Control: The intrinsic SCD layer, which serves as the Sensitive Volume (SV), must be grown or thinned (via slicing/polishing) to highly accurate thicknesses, typically 1 µm to 50 µm, to optimize operational range and spatial resolution.
Junction Formation: p-i-n or p-n junction structures are created using selective CVD techniques or ion implantation (e.g., Boron implantation) to define the electrical field and charge collection region.
Micro-structuring and Etching: Deep Ar/O₂ plasma etching or laser ablation is used to define the micrometric SV geometry (e.g., cylindrical or mushroom shapes) and create isolation trenches.
Metalization and Contacting: Thin metal electrodes (e.g., Al, Cr, Ag, Ti/Pt/Au) are deposited via thermal evaporation or sputtering to form ohmic contacts and define the sensitive area, often requiring complex patterning for guard-ring (GR) structures.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the foundational MPCVD diamond materials and specialized processing required to replicate and advance the microdosimetry research described in this review.

Applicable Materials

To achieve the high performance, radiation hardness, and precise SV definition required for hadron therapy microdosimetry, researchers need specific, high-quality diamond materials:

Optical Grade Single Crystal Diamond (SCD): Essential for the intrinsic (i) layer of p-i-n structures. 6CCVD provides high-purity SCD with exceptional crystalline quality, minimizing defects that could compromise Charge Collection Efficiency (CCE).
Custom Boron-Doped Diamond (BDD): Required for the conductive p-type layer (p+) used as the backing contact or electrode. 6CCVD offers precise control over Boron doping concentration (e.g., 0.5 * 10²⁰ cm^-3 mentioned in the text) to ensure optimal conductivity and junction performance.

Customization Potential

The success of solid-state microdosimeters hinges on precise dimensional and material control. 6CCVD’s custom capabilities directly address the fabrication challenges identified in the research:

Research Requirement	6CCVD Custom Capability	Benefit to Researcher
Ultra-Thin SV Layers (1 µm to 50 µm)	SCD Thickness Control: SCD layers available from 0.1 µm up to 500 µm.	Guarantees the precise SV thickness needed for optimal stopping power and energy deposition measurement.
High-Quality Interfaces	SCD Polishing: Achievable surface roughness Ra &lt; 1nm.	Critical for homoepitaxial growth and minimizing charge collection defects at the p-i interface.
Custom Wafer Size	Large Area PCD/SCD: Plates/wafers up to 125mm (PCD) and custom SCD sizes.	Supports scaling up detector arrays (e.g., CMRP’s 4,248 SV array) and multi-element designs.
Electrode Fabrication (Al, Cr, Ag, Ti/Pt/Au)	Internal Metalization: Standard offerings include Au, Pt, Pd, Ti, W, Cu, with capability for custom metal stacks (e.g., Cr/Au or Ti/Pt/Au).	Enables rapid prototyping and fabrication of complex electrode patterns, guard rings, and contacts required for 3D and telescope structures.
Telescope Detectors (ΔE-E stages)	Substrate Supply: SCD substrates available up to 10mm thickness.	Provides the robust, thick E-stage material necessary for residual energy measurement in ΔE-E telescope designs.

Engineering Support

The review highlights that the choice of detector material and geometry significantly impacts the required tissue-equivalence corrections and overall measurement uncertainty.

6CCVD’s in-house PhD team can assist researchers and engineers with material selection and design optimization for similar Hadron Therapy Microdosimetry projects, focusing on:

Optimizing SV Thickness: Balancing low lineal energy cut-off (requiring thinner SVs) against particle stopping power (requiring thicker E-stages).
Radiation Hardness Selection: Advising on SCD specifications to ensure long-term stability under high-dose clinical carbon and proton beams.
Metal Stack Design: Developing custom metalization schemes to minimize contact resistance and support complex readout electronics.

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

View Original Abstract

The interest in hadron therapy is growing fast thanks to the latest technological advances in accelerators and delivery technologies, to the development of more and more efficient and comprehensive treatment planning tools, and due to its increasing clinical adoption proving its efficacy. A precise and reliable beam quality assessment and an accurate and effective inclusion of the biological effectiveness of different radiation qualities are fundamental to exploit at best its advantages with respect to conventional radiotherapy. Currently, in clinical practice, the quality assurance (QA) is carried out by means of conventional dosimetry, while the biological effectiveness of the radiation is taken into account considering the Relative Biological Effectiveness (RBE). The RBE is considered a constant value for protons and it is estimated as a function of the absorbed dose in case of carbon ions. In this framework, microdosimetry could bring a significant improvement to both QA and RBE estimation. By measuring the energy deposited by the radiation into cellular or sub-cellular volumes, microdosimetry could provide a unique characterisation of the beam quality on one hand, and a direct link to radiobiology on the other. Different detectors have been developed for microdosimetry, from the more conventional tissue equivalent proportional counter (TEPC), silicon-based and diamond-based solid-state detectors, to Δ E -E telescope detectors, gas electrons multiplier (GEM), hybrid microdosimeters and a micro-bolometer based on Superconducting QUantum Interference Device (SQUID) technology. However, because of their different advantages and drawbacks, a standard device and an accredited experimental methodology have not been unequivocally identified yet. The establishment of accepted microdosimetry standard protocols and code of practice is needed before the technique could be employed in clinical practice. Hoping to help creating a solid ground on which future research, development and collaborations could be planned and inspired, a comprehensive state of the art of the detector technologies developed for microdosimetry is presented in this review, discussing their use in clinical hadron therapy conditions and considering their advantages and drawbacks.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2022.1035956

References

1983 - ICRU report 36: Microdosimetry [Crossref]
2010 - Radiation detection and measurement
2017 - Microdosimetry [Crossref]
1955 - A device for the measurement of dose as a function of specific ionization [Crossref]
1972 - Wall-less detectors in microdosimetry [Crossref]
1996 - Microdosimetry and its applications [Crossref]
2015 - Experimental microdosimetry: History, applications and recent technical advances [Crossref]
1968 - A high resolution spherical proportional counter [Crossref]
1986 - An investigation of the characteristics of a spherical single-wire proportional counter used for experimental microdosimetry [Crossref]
2010 - Replacement tissue-equivalent proportional counter for the international space station [Crossref]