Diamond detector in absorbed dose measurements in high‐energy linear accelerator photon and electron beams
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2016-03-01 |
| Journal | Journal of Applied Clinical Medical Physics |
| Authors | R Ravichandran, John Pichy Binukumar, Iqbal Al Amri, Cheriyathmanjiyil Antony Davis |
| Institutions | Royal Hospital |
| Citations | 24 |
| Analysis | Full AI Review Included |
Diamond Detectors for High-Energy Dosimetry: Technical Analysis and 6CCVD Solutions
Section titled “Diamond Detectors for High-Energy Dosimetry: Technical Analysis and 6CCVD Solutions”This document analyzes the research paper “Diamond detector in absorbed dose measurements in high-energy linear accelerator photon and electron beams” (J. Appl. Clin. Med. Phys., 2016) to highlight the critical role of high-quality MPCVD diamond in advanced medical physics and small field radiotherapy (RT) applications.
Executive Summary
Section titled “Executive Summary”- Application Validation: The study successfully validates the use of microdiamond detectors (DD) as absolute dosimeters for absorbed dose measurements in high-energy linear accelerator (linac) photon (6 MV, 15 MV) and electron (6 MeV to 22 MeV) beams.
- Superior Spatial Resolution: Diamond’s near tissue-equivalence (Z=6 vs. tissue Z=7.4) and ultra-small sensitive volume (0.004 mm³) provide superior spatial resolution, confirming its suitability for small field dosimetry and stereotactic radiotherapy (SRT).
- High Stability & Reproducibility: The detector demonstrated exceptional performance metrics, including high reproducibility (± 0.17% SD) and minimal dose rate dependence (< 0.7% variation across a 500 MU/min change).
- Absolute Dosimetry Confirmation: Measured absorbed doses in water, PMMA, and nylon phantoms showed excellent agreement (typically within 3% deviation) compared to calibrated ion chambers, supporting the diamond detector’s qualification as a robust transfer standard.
- Ultra-Thin Active Layer: The detector utilized an extremely thin active layer (1 µm), a critical dimension that 6CCVD specializes in manufacturing using high-purity Single Crystal Diamond (SCD).
Technical Specifications
Section titled “Technical Specifications”The following key performance and material parameters were extracted from the analysis of the microdiamond detector (PTW TM 60019):
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Detector Type | TM 60019 | N/A | Microdiamond detector used in study |
| Sensitive Volume | 0.004 | mm³ | Active detection area |
| Active Layer Thickness | 1 (1 x 10-3) | µm (mm) | Critical dimension for spatial resolution |
| Active Radius | 1.1 | mm | Detector dimension |
| Nominal Sensitivity (Calculated) | 1.0269 | nC/Gy | Derived from first principles (13 eV/ion pair) |
| Operating Bias Voltage | 0 | V | Unbiased operation (photoconduction mode) |
| Leakage Current | ≤ ± 20 | fA | Manufacturer specification for small signal measurement |
| Reproducibility (1 SD) | ± 0.17 | % | Measured over n=11 readings |
| Dose Rate Effect (500 MU/min change) | < 0.7 | % | Variation in response |
| Band Gap (Carbon) | 5.6 | eV | Prevents lattice leakage currents |
| Photon Energies Tested | 6, 15 | MV | Linac X-ray beams |
| Electron Energies Tested | 6 to 22 | MeV | High-energy electron beams |
Key Methodologies
Section titled “Key Methodologies”The study employed rigorous methodologies to validate the microdiamond detector for absolute dosimetry:
- Detector Operation: The microdiamond detector (TM 60019) was operated at 0 V bias, relying on photoconduction to collect the total electric charge (nC) developed during irradiation.
- Phantom Setup: Experiments were conducted in water, PMMA (Perspex), and nylon phantoms using standard geometry setups (FSD/FAD) for both photon and electron beams from Varian Clinac 2300 CD and 600 CD linear accelerators.
- Absolute Dose Conversion: Measured charge (Q) was converted to absorbed dose (D) using the nominal sensitivity and applying established radiation physics quantities, including:
- Mass energy absorption coefficients (µen/ρ) ratios.
- Mass collision stopping power ratios (S/ρ) (medium relative to carbon/diamond).
- Ratio of electron densities (for electron beams).
- Dose Rate Characterization: The detector signal linearity was analyzed across monitor repetition rates ranging from 100 MU/min to 600 MU/min to determine the sublinear dose rate parameter (A).
- Calibration Factor Determination: Calibration factors (Ndiamond,water) were derived using substitution and simultaneous exposure techniques, comparing diamond readings against calibrated ion chambers (CC13, FC 65) in a water phantom.
- Small Field Validation: Field size factors were measured for extremely small fields (down to 0.6 x 0.6 cm2) using a micro-multileaf collimator (mMLC) to confirm the diamond detector’s advantage in high-gradient regions.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”The research confirms that high-purity, ultra-thin CVD diamond is essential for next-generation small field dosimetry and quality assurance (QA). 6CCVD is uniquely positioned to supply the custom materials required to replicate and advance this technology.
| Research Requirement | 6CCVD Solution & Capability | Engineering Advantage |
|---|---|---|
| Ultra-Thin Active Layer (1 µm) | Optical Grade Single Crystal Diamond (SCD): We specialize in growing SCD layers with precise thickness control, ranging from 0.1 µm up to 500 µm. This allows for the fabrication of detectors with even higher spatial resolution than the 1 µm device used in the study. | Enables sub-micron spatial resolution, critical for resolving sharp dose penumbras in SRT and IMRT. |
| High Purity & Stability | High-Purity MPCVD SCD: Our SCD material ensures minimal defects and low nitrogen incorporation, guaranteeing the stable photoconduction properties and low leakage currents (< ± 20 fA) necessary for reliable absolute dosimetry. | Guarantees long-term stability and reproducibility, qualifying the detector material as a superior transfer standard. |
| Custom Geometry (1.1 mm radius) | Custom Dimensions & Precision Processing: 6CCVD offers custom plates/wafers up to 125 mm (PCD). We provide advanced laser cutting and shaping services to produce specific detector geometries (e.g., 1.1 mm radius circular chips, flat-edge designs) required for precise phantom placement. | Rapid prototyping and manufacturing of application-specific dosimeter probes, optimizing fit and minimizing positioning inaccuracies. |
| Electrode Integration | In-House Metalization Services: We provide internal deposition capabilities for standard electrode stacks (e.g., Ti/Pt/Au, W, Cu). This is essential for creating the ohmic contacts required for efficient charge collection in unbiased (V=0) operation. | Streamlined production of fully functional detector assemblies, ensuring optimal electrical performance and contact stability. |
| Boron Doping for Conductivity | Boron-Doped Diamond (BDD): While the study used intrinsic diamond, 6CCVD offers BDD for applications requiring controlled conductivity or p-type semiconductor behavior, allowing researchers to explore alternative detector designs (e.g., p-n junctions). | Expands research potential into active detectors requiring specific doping profiles and controlled resistivity. |
Engineering Support
Section titled “Engineering Support”6CCVD’s in-house team of PhD material scientists and engineers can assist researchers and manufacturers in selecting the optimal diamond grade (SCD or PCD) and geometry for high-precision medical physics projects, including small field RT, IMRT QA, and proton beam dosimetry.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Diamond detectors (DD) are preferred in small field dosimetry of radiation beams because of small dose profile penumbras, better spatial resolution, and tissue‐equivalent properties. We investigated a commercially available ‘microdiamond’ detector in realizing absorbed dose from first principles. A microdiamond detector, type TM 60019 with tandem electrometer is used to measure absorbed doses in water, nylon, and PMMA phantoms. With sensitive volume 0.004 mm 3 , radius 1.1 mm, thickness , the nominal response is 1 nC/Gy. It is assumed that the diamond detector could collect total electric charge (nC) developed during irradiation at 0 V bias. We found that dose rate effect is less than 0.7% for changing dose rate by 500 MU/min. The reproducibility in obtaining readings with diamond detector is found to be (1 SD) . The measured absorbed doses for 6 MV and 15 MV photons arrived at using mass energy absorption coefficients and stopping power ratios compared well with , water calibrated ion chamber measured absorbed doses within 3% in water, PMMA, and nylon media. The calibration factor obtained for diamond detector confirmed response variation is due to sensitivity due to difference in manufacturing process. For electron beams, we had to apply ratio of electron densities of water to carbon. Our results qualify diamond dosimeter as a transfer standard, based on long‐term stability and reproducibility. Based on micro‐dimensions, we recommend these detectors for pretreatment dose verifications in small field irradiations like stereotactic treatments with image guidance. PACS number(s): 87.56.Da
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 2007 - Dosimetric characteristics of CVD single crystal diamond detectors in radiotherapy beams [MSc Thesis]
- 2005 - IMRT point dose measurements with a diamond detector
- 1999 - Diamond detectors for ionizing radiation [Diploma Thesis]
- 1966 - Radiation dosimetry