Thermal Neutron Measurement Capability of a Single Crystal CVD Diamond Detector near the Reactor Core Region of UTR-KINKI

At a Glance

Metadata	Details
Publication Date	2022-06-06
Journal	Plasma and Fusion Research
Authors	Makoto I. KOBAYASHI, Sachiko Yoshihashi, Hirokuni Yamanishi, S. Sangaroon, K. Ogawa
Institutions	Kindai University, The Graduate University for Advanced Studies, SOKENDAI
Citations	6
Analysis	Full AI Review Included

Technical Documentation & Analysis: Thermal Neutron Measurement Capability using SCD Detectors

This document analyzes the research paper “Thermal Neutron Measurement Capability of a Single Crystal CVD Diamond Detector near the Reactor Core Region of UTR-KINKI” to highlight the critical role of high-purity CVD diamond materials and demonstrate 6CCVD’s capability to supply and customize materials for advanced nuclear and fusion diagnostics.

Executive Summary

The research successfully validated the use of Single Crystal Diamond (SCD) detectors for precise thermal and fast neutron flux measurement in complex, mixed-radiation environments, crucial for fusion reactor development (e.g., Tritium Breeding Ratio, TBR).

Core Achievement: Established a reliable procedure for thermal neutron flux evaluation using an SCD detector coupled with a ⁶LiF thermal neutron converter.
Methodology: An advanced Pulse Shape Discrimination (PSD) method, combining rectangularity (R) and pulse width (FW1/4PH), was developed to effectively isolate energetic charged particle events (alpha/triton) while rejecting gamma-ray background.
Material Specification: The detector utilized was a 500 µm thick SCD wafer with 100 nm Titanium (Ti) metalization, demonstrating the need for highly customized diamond substrates.
High Resolution: The PSD technique successfully resolved the energy deposition spectra, clearly identifying the triton peak (2.8 MeV) and the alpha particle peak (1.2 MeV) resulting from the ⁶Li(n,α)³H reaction.
Performance Validation: Demonstrated excellent linearity in count rate proportional to reactor power (0.01 W to 1 W).
Quantitative Result: Thermal neutron flux near the UTR-KINKI reactor core was evaluated at $7.6 \times 10^{6}$ n cm^-2 s^-1 W^-1.

Technical Specifications

The following hard data points were extracted from the experimental setup and results:

Parameter	Value	Unit	Context
Detector Material	Single Crystal Diamond (SCD)	N/A	High-purity CVD diamond
SCD Dimensions	4.5 x 4.5	mm	Detector area
SCD Thickness	500	µm	Detector depth
Metalization Layer	100	nm	Titanium (Ti) electrodes on both surfaces
Bias Voltage	+250	V	Applied to the detector
Thermal Neutron Converter	1.9	µm	⁶LiF (95% ⁶Li enrichment)
DAQ Sampling Rate	1	GHz	Data acquisition speed
DAQ Resolution	14	bit	Analog-to-Digital Conversion
Tritium Recoil Energy Peak	2.8	MeV	Peak observed after PSD processing
Alpha Particle Energy Peak	1.2	MeV	Peak observed after PSD processing
Thermal Neutron Flux (Max)	7.6 x 10⁶	n cm^-2 s^-1 W^-1	Evaluated at 1 W reactor power

Key Methodologies

The experiment relied on precise material engineering and advanced signal processing to achieve reliable neutron flux measurement in a mixed field:

Material Preparation: A 500 µm thick SCD wafer was prepared with 100 nm Titanium (Ti) metalization on both surfaces to serve as electrodes.
Thermal Neutron Conversion: A 1.9 µm thick ⁶LiF layer was applied to the SCD surface. Thermal neutrons react with ⁶Li, producing energetic alpha particles and tritons (⁶Li(n,α)³H), which are then detected by the SCD.
Signal Acquisition: The detector was operated under a +250 V bias. Signals were amplified and digitized using a high-speed DAQ system (1 GHz sampling rate, 14 bit resolution).
Pulse Shape Discrimination (PSD): A two-parameter PSD method was implemented to distinguish pulse shapes:
- Rectangularity (R): Measures how close the pulse shape is to a perfect rectangle (characteristic of energetic charged particles hitting the surface).
- Full Width at One-Fourth Peak Height (FW1/4PH): Used in combination with R to specifically isolate rectangular pulses in the 11.5-14.5 ns range.
Gamma-Ray Rejection: The PSD criteria successfully rejected triangular pulses induced by gamma-rays and two-step pulses induced by fast neutrons interacting in the bulk, focusing only on surface events (alpha/triton/fast neutron surface hits).
Thermal Flux Isolation: Measurements were performed both with and without the ⁶LiF converter. Subtraction of the resulting spectra isolated the count rate attributable solely to thermal neutron capture products.

6CCVD Solutions & Capabilities

This research demonstrates the critical need for high-quality, customized CVD diamond materials in nuclear and fusion diagnostics. 6CCVD is uniquely positioned to supply the exact specifications required to replicate or advance this work.

Applicable Materials for Neutron Detection

To achieve the high charge collection efficiency (CCE) and radiation hardness required for this application, 6CCVD recommends the following materials:

Optical Grade Single Crystal Diamond (SCD): Required for high-resolution spectroscopy and low-noise performance, matching the material used in the study. Our SCD material ensures minimal defects, maximizing CCE.
Polycrystalline Diamond (PCD): For large-area detectors (up to 125 mm) where high flux monitoring is needed, 6CCVD PCD offers superior radiation hardness and scalability compared to traditional silicon detectors.
Boron-Doped Diamond (BDD): While not used in this specific study, BDD can be utilized as a solid-state neutron converter (via ¹⁰B doping) or as a robust electrode material in high-temperature environments.

Customization Potential for Advanced Diagnostics

The success of this experiment hinges on precise material dimensions and electrode quality. 6CCVD offers full customization capabilities that directly address these engineering requirements:

Research Requirement	6CCVD Capability	Technical Advantage for the Client
Specific Dimensions (4.5 x 4.5 mm)	Custom Dimensions & Precision Cutting: We supply plates/wafers up to 125 mm and offer precision laser cutting services to achieve exact, small-scale geometries required for reactor core insertion.	Eliminates post-processing risk and ensures dimensional accuracy for detector arrays.
Thickness (500 µm)	Thickness Control: SCD and PCD wafers are available from 0.1 µm up to 500 µm, and substrates up to 10 mm. We guarantee tight thickness tolerances for consistent detector performance.	Ensures optimal stopping power for energetic charged particles (alpha/triton) while minimizing bulk interaction noise.
Metalization (100 nm Ti)	In-House Custom Metalization: 6CCVD provides internal deposition of Ti, Pt, Au, Pd, W, and Cu. We can precisely control the 100 nm Ti thickness and adhesion for stable electrode performance under high bias (+250 V).	Guarantees low-resistance ohmic contacts essential for fast signal rise times and high-speed PSD analysis (1 GHz DAQ).
Surface Quality for Converter	Ultra-Smooth Polishing: SCD surfaces are polished to Ra < 1 nm.	Critical for uniform deposition of the ⁶LiF converter layer, ensuring consistent detection efficiency across the active area.

Engineering Support

6CCVD’s in-house PhD team can assist with material selection and optimization for similar Fusion Reactor Diagnostics projects, including:

TBR Measurement: Consulting on optimal SCD thickness and converter material integration (e.g., ⁶LiF or ¹⁰B) to maximize thermal neutron detection efficiency.
High-Flux Environments: Designing radiation-hardened diamond detectors capable of operating reliably in the high-temperature, high-flux fields characteristic of fusion devices like ITER or DEMO.
Fast Neutron Spectroscopy: Providing materials optimized for fast neutron detection via elastic scattering, complementing the thermal neutron capability demonstrated here.

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

View Original Abstract

Thermal neutron flux evaluation using a single crystal diamond detector (SDD) was carried out in the core region of the UTR-KINKI reactor where a mixed radiation field by thermal and fast neutrons and gamma-ray exists. The pulse shape discrimination method to extract pulses with a rectangular shape as well as a wide pulse-width was established to exclude pulses induced by gamma-rays. The SDD, using a 6LiF thermal neutron converter, is able to detect pulse events caused not only by fast neutrons but also by thermal neutrons through energy depositions into the diamond by energetic alpha and triton particles induced by thermal neutrons. Additionally, the SDD without the thermal neutron converter was used for the measurement of the energy deposition events only by fast neutrons. A comparison of the pulse counts of the SDD with or without the thermal neutron convertor deduced the energy deposition spectra by thermal neutrons. The thermal neutron flux in the core region of the UTR-KINKI reactor was evaluated to be 7.6×106 n cm−2 s−1 W−1 up to a reactor power of 1 W.

Tech Support

Original Source

DOI: https://doi.org/10.1585/pfr.17.2405045