Development of Synthetic Diamond Radiation Detector for Neutron Plasma Diagnostics

At a Glance

Metadata	Details
Publication Date	2017-01-01
Journal	hamon
Authors	Junichi H. Kaneko
Institutions	Hokkaido University
Analysis	Full AI Review Included

Technical Documentation & Analysis: MPCVD Diamond for Neutron Plasma Diagnostics

Reference Paper: Development of Synthetic Diamond Radiation Detector for Neutron Plasma Diagnostics (Kaneko, J. H., Hokkaido University)

Executive Summary

This research successfully demonstrates the fabrication and performance of a high-quality, self-standing Single-Crystal Diamond (SCD) detector, achieving world-class energy resolution for 14 MeV neutrons critical for fusion plasma diagnostics.

Record Performance: Achieved an energy resolution of 0.78% for 14 MeV neutrons, surpassing previous records held by natural diamond detectors.
Material Purity: High-quality SCD was synthesized using a low-methane concentration (0.2%) MPCVD process within a full metal seal chamber to minimize nitrogen and structural defects.
Near-Perfect Charge Collection: Demonstrated exceptional charge collection efficiencies (CCE) of 100.1% for holes and 99.8% for electrons, indicating extremely low charge trapping.
Advanced Fabrication: Utilized a “lift-off” or “direct wafer process” involving carbon ion implantation and subsequent electrolysis etching, allowing for the reuse of expensive HP/HT IIa substrates.
Application Validation: Successfully applied the detector for Time-of-Flight (ToF) measurements of DD neutrons in implosion experiments (Gekiko XII laser facility), validating its use in high-flux, complex fusion environments.
6CCVD Value Proposition: 6CCVD specializes in producing the necessary high-purity, Detector Grade SCD wafers (up to 500 µm thick) and offers custom metalization (Pt-Ti/Au) required to replicate and scale this breakthrough technology.

Technical Specifications

The following hard data points were extracted from the analysis of the high-performance SCD detector:

Parameter	Value	Unit	Context
Neutron Energy Measured	14	MeV	DT Fusion Plasma Diagnostics
Energy Resolution (14 MeV Neutrons)	0.78	%	Total system resolution
Intrinsic Detector Resolution	0.8	%	Excluding scattering and circuit noise
Charge Collection Efficiency (Holes)	100.1	%	Measured using 5.486 MeV alpha particles
Charge Collection Efficiency (Electrons)	99.8	%	Measured using 5.486 MeV alpha particles
Intrinsic Energy Resolution (Alpha/Beta)	0.38	%	For both holes and electrons
Detector Thickness	70	µm	Optimized for neutron detection
Detector Area	5 x 5	mm²	Sample dimensions
Methane Concentration (Growth)	0.2	%	Used to reduce nitrogen impurities
Substrate Temperature (Growth)	850	°C	Standard CVD growth temperature
Gas Pressure (Growth)	110	Torr	Standard CVD growth pressure
Substrate Off-Angle	3	°	Tilted <110> direction from (001) plane
Contact Metalization	Pt-Ti/Au	N/A	Used for ohmic contacts

Key Methodologies

The high-performance SCD detector was fabricated using a combination of advanced substrate preparation and specialized MPCVD growth techniques:

Substrate Preparation: High-purity HP/HT IIa single-crystal diamond substrates were used. To suppress abnormal growth and minimize residual stress, the (001) surface was controlled with a 3° off-angle tilt toward the <110> direction.
Lift-Off Layer Creation: Carbon ions (Cion) were implanted into the substrate at a dose of approximately 2 x 10¹⁶ ions/cm², stopping at a depth of 1.6 µm below the surface. This implanted layer serves as the sacrificial layer for the lift-off process.
Homoeptaxial CVD Growth: The diamond detection layer was grown using Microwave Plasma CVD (MPCVD) under highly controlled conditions:
- Methane Concentration: Extremely low concentration of 0.2% CH₄ in H₂ plasma.
- Temperature/Pressure: Substrate temperature of 850 °C and gas pressure of 110 Torr.
Self-Standing Film Release: After growth, the implanted carbon layer was decomposed using electrolysis etching in a strong acid solution, releasing the grown diamond film (the detector) and allowing the substrate to be reused.
Metalization: Ohmic contacts (Pt-Ti/Au) were deposited onto the self-standing SCD film to complete the detector structure.

6CCVD Solutions & Capabilities

The successful development of this high-resolution neutron detector relies entirely on the availability of ultra-high-purity, precisely controlled SCD material and advanced fabrication techniques—core competencies of 6CCVD.

Applicable Materials

To replicate or extend this research, 6CCVD recommends the following materials, optimized for radiation detection and high CCE:

Detector Grade Single Crystal Diamond (SCD):
- Purity: Ultra-low nitrogen content (equivalent to IIa type) necessary to minimize charge trapping centers and achieve the reported 100% CCE.
- Surface Quality: Polishing to Ra < 1 nm is available, ensuring optimal surface preparation for subsequent metalization and minimizing surface leakage current.
Custom Thickness SCD:
- The paper utilized a 70 µm thick detector. 6CCVD routinely supplies SCD wafers in the required range of 0.1 µm up to 500 µm, allowing researchers to optimize thickness for specific neutron energies (e.g., 14 MeV DT neutrons vs. lower energy DD neutrons).

Customization Potential

6CCVD’s in-house engineering and fabrication capabilities directly address the specific requirements of this advanced detector design:

Research Requirement	6CCVD Capability	Technical Advantage
Custom Dimensions	Plates/wafers up to 125 mm (PCD) and large-area SCD are available.	Enables scaling from 5x5 mm² prototypes to large-area arrays for multi-channel diagnostics (e.g., ITER).
Precise Thickness Control	SCD thickness control from 0.1 µm to 500 µm.	Allows precise tuning of the active volume (e.g., 70 µm used here) for optimal energy deposition and resolution.
Custom Metalization	Internal capability for Au, Pt, Pd, Ti, W, Cu deposition.	We can replicate the required Pt-Ti/Au contact scheme or explore alternative metal stacks for enhanced thermal stability or specific contact resistance.
Substrate Reuse (Lift-Off)	6CCVD provides the necessary high-quality, off-angle controlled SCD substrates required for the direct wafer/lift-off process.	Facilitates cost-effective, high-volume production of self-standing films by maximizing substrate lifespan.

Engineering Support

6CCVD’s in-house PhD team provides expert consultation to accelerate research in high-energy physics and fusion diagnostics:

Material Selection: Assistance in selecting the optimal diamond grade (SCD vs. PCD) and thickness for specific applications, such as high-flux neutron spectrometry or Time-of-Flight (ToF) measurements.
Process Optimization: Support for integrating diamond detectors into complex environments like magnetic confinement (ITER) or inertial confinement (Gekiko XII) fusion facilities, including advice on metalization schemes for radiation hardness.
Global Logistics: Reliable global shipping (DDU default, DDP available) ensures prompt delivery of custom materials to international research facilities.

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

View Original Abstract

A self-standing single-crystal CVD diamond was fabricated using a lift-off method. The reduction of charge trapping factors such as structural defects, point defects, and nitrogen impurities, was attempted using 0.2% of low-methane concentration growth and using a full metal seal chamber. A high-quality self-standing diamond with strong free-exciton recombination emission was obtained. Charge collection efficiencies were 100.1% for holes and 99.8% for electrons, provided that εdiamond = 13.1 eV and εSi = 3.62 eV. Energy resolutions were 0.38% for both holes and electrons. Finally, energy resolution of 0.78% for 14 MeV neutrons was achieved. Moreover, ToF measurement for DD neutrons of implosion experiment at Gekiko XII laser was succeeded.

Tech Support

Original Source

DOI: https://doi.org/10.5611/hamon.27.1_20