Thermal Neutron Measurement Capability of a Single Crystal CVD Diamond Detector near the Reactor Core Region of UTR-KINKI
At a Glance
Section titled âAt a Glanceâ| 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 | National Institutes of Natural Sciences, National Institute for Fusion Science |
| Citations | 6 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research establishes a robust methodology for measuring thermal neutron flux using a Single Crystal CVD Diamond Detector (SDD) in complex, mixed-radiation environments typical of fusion reactors.
- Core Achievement: Successful evaluation of thermal neutron flux (7.6 x 106 n cm-2 s-1 W-1) in the core region of the UTR-KINKI reactor, demonstrating excellent linearity with reactor power.
- Key Innovation (PSD): Developed an advanced Pulse Shape Discrimination (PSD) method combining pulse rectangularity (R) and pulse width (FW1/4PH) to effectively reject background signals.
- Background Rejection: The PSD method successfully isolates rectangular pulses induced by energetic charged particles (alpha/triton from 6Li(n,α)3H reaction) while eliminating pulses caused by distributed gamma-ray interactions.
- Detector Configuration: The SDD was used in two modes: with a 1.9 ”m thick 6LiF thermal neutron converter (detects thermal and fast neutrons) and without the converter (detects fast neutrons only).
- Signal Isolation: Subtraction of the fast neutron spectrum (SDD without converter) from the mixed spectrum (SDD with converter) allowed for the precise deduction of the energy deposition spectrum caused solely by thermal neutrons.
- Performance: The PSD technique achieved high efficiency, with triton peak loss measured at less than 3%.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Detector Type | Single Crystal CVD Diamond (SDD) | N/A | CIVIDEC B6-C compact detector |
| SDD Dimensions | 4.5 x 4.5 | mm2 | Active area |
| SDD Thickness | 500 | ”m | Detector depth |
| Electrode Material | Titanium (Ti) | N/A | Layer thickness: 100 nm |
| Bias Voltage | +250 | V | Applied during all measurements |
| Thermal Neutron Converter | 6LiF | N/A | 95% 6Li enrichment |
| Converter Thickness | 1.9 | ”m | Used for thermal neutron detection |
| Measured Thermal Neutron Flux | 7.6 x 106 | n cm-2 s-1 W-1 | Evaluated at 1 W reactor power |
| Data Acquisition Rate | 1 | GHz | Sampling rate of fast processing ADC |
| ADC Resolution | 14 | bit | Data acquisition system (APV8102) |
| Triton Peak Energy (Recoil) | 2.8 | MeV | Observed peak after PSD |
| Alpha Peak Energy (Observed) | 1.2 | MeV | Observed peak after PSD |
| Gamma-ray Rejection | Sufficiently eliminated | N/A | Achieved using combined R and FW1/4PH PSD |
Key Methodologies
Section titled âKey MethodologiesâThe thermal neutron flux was evaluated using a combination of hardware configuration and advanced Pulse Shape Discrimination (PSD) techniques.
- Detector Setup and Operation: A 500 ”m thick SDD with 100 nm Ti electrodes was operated under a +250 V bias. Measurements were conducted sequentially using the SDD with a 1.9 ”m thick 6LiF converter and without the converter.
- Signal Acquisition: Signals were amplified and digitized using a high-speed DAQ system (1 GHz sampling rate, 14 bit resolution) to capture detailed pulse shapes.
- PSD Calibration: Benchmark tests using 241Am (alpha-rays) and 60Co (gamma-rays) defined the optimal PSD parameters for separating rectangular pulses (charged particles) from triangular/distributed pulses (gamma-rays).
- Advanced PSD Filtering: The PSD method utilized two indices to extract rectangular pulses:
- Rectangularity (R): Ratio of charge exceeding one-fourth peak height to the area of a theoretical rectangular pulse (R should be close to unity for rectangular pulses).
- Pulse Width (FW1/4PH): Full Width at one-fourth of the Peak Height (specifically filtered for 11.5-14.5 ns).
- In-Reactor Measurement: The detector was inserted into the core region of the UTR-KINKI reactor, and pulse counting was performed at reactor powers of 0.01 W, 0.1 W, and 1 W.
- Thermal Neutron Isolation: The count rate spectrum from the SDD without the 6LiF converter (fast neutrons only) was subtracted from the spectrum with the converter (thermal neutron induced charged particles + fast neutrons) to isolate the signal generated exclusively by thermal neutrons.
- Flux Evaluation: The isolated count rate of the energetic charged particles (alpha and triton peaks) was confirmed to be linearly proportional to the reactor power, allowing for the calculation of the thermal neutron flux (7.6 x 106 n cm-2 s-1 W-1 at 1 W).
Commercial Applications
Section titled âCommercial ApplicationsâThis technology is critical for monitoring neutron fields in high-radiation environments, particularly within the fusion energy sector.
- Fusion Reactor Diagnostics: Essential for real-time monitoring of both fast (D-T reaction) and thermal neutron fluxes in future fusion devices (e.g., ITER, DEMO).
- Tritium Breeding Ratio (TBR) Measurement: The ability to accurately measure thermal neutron flux within the blanket region is vital for validating and controlling tritium production performance in fusion reactors.
- Nuclear Reactor Safety and Control: High-resolution neutron flux monitoring in research and power reactors, offering superior gamma-ray rejection compared to conventional detectors.
- Radiation Hard Sensing: Utilizing the intrinsic radiation hardness of CVD diamond for long-term operation in high-dose environments where silicon-based detectors fail.
- Neutron Spectroscopy: Application in accelerator facilities and high-energy physics experiments requiring robust detectors capable of discriminating between various radiation types (neutrons, charged particles, gamma-rays).
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 7.6Ă106 n cmâ2 sâ1 Wâ1 up to a reactor power of 1 W.