Reduction of γ-ray-induced Noise of Diamond Detector Elements and Estimation of Neutron Detection Efficiency for the Development of a Criticality Proximity Monitoring System for the Decommissioning of the Fukushima Daiichi Nuclear Power Plant
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-05-22 |
| Journal | Sensors and Materials |
| Authors | Kengo Oda, Junichi H. Kaneko, Yusuke Kobayakawa, Kenichi Watanabe, Y. Fujita |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: High-Radiation SCD Diamond Detectors
Section titled “Technical Documentation & Analysis: High-Radiation SCD Diamond Detectors”Executive Summary
Section titled “Executive Summary”This document analyzes the development of radiation-hardened Single Crystal Diamond (SCD) detectors for criticality proximity monitoring in extreme $\gamma$-ray environments (up to 1.5 kGy/h), specifically for the Fukushima Daiichi Decommissioning Project.
- Extreme Radiation Hardness: Diamond detectors demonstrated stable operation and maintained signal integrity at $\gamma$-ray dose rates up to 1.5 kGy/h, significantly exceeding the 1 kGy/h operational requirement.
- Noise Reduction Techniques: $\gamma$-ray induced noise was effectively mitigated by two key material processing steps: Ion Beam Etching (IBE) to remove the defect-rich lift-off separation surface, and the subsequent deposition of a p+ Boron-Doped Diamond (BDD) layer.
- High Signal-to-Noise (S/N): The system achieved a $\gamma$-ray noise count rate of 0.0004 cps at 1 kGy/h (at a 1 MeV threshold), successfully securing the S/N ratio of $\ge 1$ required for the Feynman-$\alpha$ criticality evaluation method.
- Neutron Sensitivity: A prototype combining the SCD detector (2.53 mm2 sensitive area) with a 180 µm thick $^6$LiF converter yielded a neutron detection efficiency of 3.0 x 10-4 cps/nv (using a 252Cf source).
- Future Scalability: The proposed final system, utilizing 1024 SCD elements (6 mm square), is projected to achieve a high neutron detection efficiency of 1.9 cps/nv, requiring high-yield mass production of thin (approx. 50 µm) membranes.
- 6CCVD Value Proposition: 6CCVD is uniquely positioned to supply the high-purity SCD membranes, custom BDD layers, precise thinning, and advanced metalization required for the mass production and deployment of this next-generation monitoring system.
Technical Specifications
Section titled “Technical Specifications”The following hard data points were extracted from the research paper detailing the detector performance and fabrication parameters.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum Stable Operating $\gamma$-ray Dose Rate | 1.5 | kGy/h | Demonstrated stability during testing. |
| Required $\gamma$-ray Dose Rate (Target) | $\ge 1$ | kGy/h | Minimum requirement for criticality monitoring. |
| $\gamma$-ray Noise Count Rate (1 kGy/h) | 0.0004 | cps | Measured at 1 MeV threshold, satisfying S/N $\ge 1$. |
| Neutron Detection Efficiency (Prototype) | 3.0 x 10-4 | cps/nv | Measured using 252Cf source (1 MeV threshold). |
| Thermal Neutron Detection Efficiency | 3.7 x 10-4 | cps/nv | Measured using calibrated neutron gas detector. |
| Future System Target Efficiency | 1.9 | cps/nv | Expected using 1024 SCD elements. |
| SCD Membrane Thickness (Tested Range) | 46 - 80 | µm | Thicknesses of various detectors (D#1-D#7). |
| Target SCD Membrane Thickness (Future) | $\approx 50$ | µm | Required for capacitance reduction and high yield. |
| Neutron Converter Material | $^6$LiF | Sintered body | Used for neutron-to-charged-particle conversion. |
| Neutron Converter Thickness | 180 | µm | Mechanically polished thickness. |
| Detector Sensitive Area (Prototype) | 2.53 | mm2 | Overlapping area of $^6$LiF and Al electrode. |
| CVD Growth Temperature | 900 | °C | Typical synthesis condition. |
| CVD Growth Rate | 0.48 | µm/h | Typical synthesis condition. |
| Front-End Circuit Shaping Time | 0.1 | µs | KEK ASIC design to reduce $\gamma$-ray noise effects. |
Key Methodologies
Section titled “Key Methodologies”The fabrication and testing of the SCD diamond detectors involved specialized CVD growth, post-processing, and advanced integration techniques:
- SCD Homoepitaxial Growth: Single-crystal diamond layers were grown via the CVD method on high-pressure/high-temperature (HP/HT) Type IIa (001) substrates, tilted 3° in the <110> direction.
- CVD Recipe Parameters:
- Gas Ratio: CH4/(H2+CH4) ratio of 0.2%.
- Pressure: 110 Torr.
- Temperature: 900 °C.
- Plasma Power: 700-1100 W.
- Membrane Fabrication: Freestanding diamond membranes were obtained using the lift-off method, involving ion implantation layers followed by electrochemical etching.
- Defect Removal (IBE): Ion Beam Etching (IBE) was applied to the lift-off separation surface (the early growth layer, which contains higher defects) at depths of 10 µm or 20 µm to reduce charge capture levels responsible for $\gamma$-ray noise.
- Charge Collection Enhancement (BDD): A 2 µm thick boron-doped p+ diamond layer was deposited via CVD onto the IBE-processed surface (Detector #7) to enable faster charge collection and further suppress $\gamma$-ray signals.
- Metalization Scheme:
- Schottky Electrode: Aluminum (Al), deposited by resistive heating evaporation.
- Ohmic Electrode: Titanium Carbide/Gold (TiC/Au), deposited by electron beam evaporation, followed by annealing at 400 °C for 30 min to form the TiC layer.
- Neutron Converter Integration: A $^6$LiF sintered body (150-200 µm thick, sintered at 700 °C) was mechanically polished and installed on the Al electrode to convert neutrons into charged $\alpha$-particles and tritons.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”The research highlights the critical need for high-quality, precisely processed SCD membranes and advanced integration techniques to achieve radiation hardness and high neutron sensitivity. 6CCVD’s specialized MPCVD capabilities directly address the material and manufacturing challenges identified in this paper, enabling the successful replication and scaling of this technology.
Applicable Materials
Section titled “Applicable Materials”To replicate or extend the performance of Detector #7 (IBE + p+ layer), 6CCVD recommends the following materials:
| 6CCVD Material | Specification | Application in Research |
|---|---|---|
| Electronics Grade SCD | High Purity, Low Defect Density | Essential for minimizing intrinsic charge capture levels that cause pseudo-$\gamma$-ray signals (as seen in Detector #3). Our SCD offers Ra < 1 nm polishing. |
| Heavy Boron Doped Diamond (BDD) | p+ Doping, CVD Grown | Required for the 2 µm p+ layer (Detector #7) to improve charge collection speed and reduce $\gamma$-ray noise effects. |
| Custom Substrates | SCD Substrates up to 10 mm thick | Ideal for use as robust, high-purity starting material for subsequent thin membrane growth and processing. |
Customization Potential
Section titled “Customization Potential”The paper identifies two major manufacturing hurdles: achieving high-yield production of 50 µm thin membranes and integrating complex metalization schemes. 6CCVD provides turnkey solutions for both:
- Precision Thinning and Dimensions: The future system requires 1024 elements, each 6 mm square, thinned to approximately 50 µm.
- 6CCVD Capability: We routinely manufacture SCD and PCD membranes with thicknesses ranging from 0.1 µm up to 500 µm. Our advanced polishing and thinning processes ensure high yield and minimal breakage, directly solving the manufacturing challenge noted in the paper.
- Custom Dimensions: We offer laser cutting services to achieve the precise 6 mm square dimensions required for array integration.
- Advanced Metalization Schemes: The detector structure relies on a specific Al Schottky contact and a TiC/Au ohmic contact.
- 6CCVD Capability: We offer in-house custom metalization using materials including Au, Pt, Pd, Ti, W, and Cu. We can replicate the required Ti/Au ohmic contacts and Al Schottky contacts, ensuring optimal electrical performance and adhesion for high-radiation environments.
- Large Area Production: While the paper notes current production limits of 16 membranes per synthesis, 6CCVD offers PCD plates/wafers up to 125 mm in diameter, providing a scalable platform for future large-area SCD growth or high-density PCD detector arrays for similar applications.
Engineering Support
Section titled “Engineering Support”6CCVD’s in-house team of PhD material scientists and application engineers specializes in optimizing diamond properties for extreme environments. We offer comprehensive support for:
- Material Selection: Assisting researchers in selecting the optimal SCD grade and BDD doping concentration to maximize Charge Collection Efficiency (CCE) and radiation hardness.
- Process Optimization: Consulting on post-growth processing, including surface preparation and metalization stack design, critical for achieving the low noise floor demonstrated in this Criticality Proximity Monitoring project.
- Global Logistics: Ensuring reliable, DDU (Delivery Duty Unpaid) or DDP (Delivery Duty Paid) global shipping for sensitive, high-value diamond components.
Call to Action: For custom specifications, high-yield thin membrane manufacturing, or material consultation on similar high-radiation detection projects, visit 6ccvd.com or contact our engineering team directly.