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

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

Executive Summary

This research details the development of radiation-hard diamond detector elements for a Criticality Proximity Monitoring System (CPMS) crucial for the decommissioning of the Fukushima Daiichi Nuclear Power Plant (FDNPP).

Core Challenge: The system must operate reliably in extremely high gamma-ray dose rates (target: 1 kGy/h or more) with strict payload constraints preventing heavy lead shielding.
Noise Reduction Success: Gamma-ray induced noise was effectively minimized by optimizing the diamond membrane structure. This involved removing the defective lift-off separation surface via Ion Beam Etching (IBE) and incorporating a boron-doped p+ diamond layer.
S/N Ratio Achievement: The estimated gamma-ray noise level at 1 kGy/h was measured to be 0.0004 cps (at a 1 MeV threshold), successfully securing the Signal-to-Noise (S/N) ratio of ≥1 required for criticality evaluation using the Feynman-α method.
System Integration: The diamond detector was successfully combined with a KEK-designed, radiation-resistant Si semiconductor ASIC (65 nm CMOS) featuring a short shaping time (0.1 µs), which further reduced gamma noise effects to approximately 1 MeV.
Neutron Sensitivity: A prototype detector, combined with a 180 µm thick ⁶LiF sintered converter, achieved a neutron detection efficiency of 3.0 x 10^-4 cps/nv when tested with a ²⁵²Cf source.
Future Scaling: The planned CPMS, utilizing 1024 diamond elements, is projected to achieve a total neutron detection efficiency of 1.9 cps/nv.
Operational Stability: The detector demonstrated stable operation even at dose rates up to 1.5 kGy/h.

Technical Specifications

Parameter	Value	Unit	Context
Target Gamma Dose Rate	1	kGy/h	Required operational minimum
Maximum Stable Operation	1.5	kGy/h	Demonstrated stability limit
Required S/N Ratio (Feynman-α)	≥1	Ratio	Neutron signal / Gamma noise
Gamma Noise Count Rate (1 kGy/h)	0.0004	cps	Measured at 1 MeV threshold
Estimated Noise Energy (0.001 cps)	0.915	MeV	Extrapolated threshold at 1 kGy/h
Prototype Neutron Efficiency	3.0 x 10^-4	cps/nv	Measured with ²⁵²Cf source
Target System Neutron Efficiency	1.9	cps/nv	Projected for 1024 elements
Diamond Membrane Thickness	60-80	µm	Typical substrate thickness
⁶LiF Converter Thickness	150-200	µm	Sintered neutron converter
Detector Sensitive Area	2.53	mm²	Overlapping area with ⁶LiF
ASIC Technology	65	nm	CMOS, KEK-designed front-end
ASIC Radiation Resistance	>1	MGy	Demonstrated resistance
ASIC Shaping Time	0.1	µs	Used to reduce gamma noise
Electron-Hole Pair Energy (Diamond)	13.1	eV	ε_Diamond
Alpha Particle Energy (Test Source)	5.486	MeV	Used for CCE measurement (²⁴¹Am)

Key Methodologies

The development involved specialized diamond growth, surface processing, and integration with custom electronics and neutron converters.

Diamond Growth:
- Homoepitaxial single-crystal diamond layers were grown via the Chemical Vapor Deposition (CVD) method on high-pressure/high-temperature (HP/HT) Type IIa substrates.
- Typical CVD conditions included a CH₄/(H₂+CH₄) ratio of 0.2%, 110 Torr pressure, 900 °C substrate temperature, and a growth rate of 0.48 µm/h.
- Freestanding diamond membranes were obtained using the lift-off method followed by electrochemical etching.
Noise Reduction Processing:
- Ion Beam Etching (IBE): The lift-off separation surface (the early growth layer, which contains more defects) was removed by IBE to depths of 10 µm or 20 µm to reduce charge capture levels responsible for gamma noise.
- p+ Layer Addition: For Detector #7, 10 µm was removed by IBE, followed by the deposition of a 2 µm boron-doped p+ diamond layer via CVD to improve charge collection speed.
Electrode Fabrication:
- The detector used a simple electrode-diamond-electrode configuration: Al Schottky electrode and TiC/Au ohmic electrode.
- TiC/Au electrodes were formed by depositing Ti, annealing at 400 °C for 30 min to form TiC, followed by Au deposition.
Neutron Converter Fabrication:
- A ⁶LiF sintered body (95%-enriched ⁶Li) was used as the neutron-to-charged-particle converter.
- The ⁶LiF was sintered in air at 700 °C for 5 hours and mechanically polished to a thickness of 150-200 µm.
Signal Processing Integration:
- The diamond detector was coupled with a custom front-end integrated circuit designed by KEK using radiation-resistant 65 nm CMOS technology.
- The ASIC utilized a short integration time of 0.1 µs to minimize the effect of noise signals generated by gamma rays.
Performance Evaluation:
- Charge Collection Efficiency (CCE) was measured using 5.486 MeV α-particles from a ²⁴¹Am source.
- Gamma irradiation tests were performed using ⁶⁰Co sources up to 1.5 kGy/h to evaluate stability and noise levels.
- Neutron sensitivity was measured using a ²⁵²Cf neutron source, with the neutron flux calibrated by a gas detector.

Commercial Applications

The demonstrated high radiation hardness, low gamma sensitivity, and efficient neutron detection capability of these diamond detectors make them suitable for several demanding engineering applications:

Nuclear Decommissioning and Reactor Monitoring:
- Criticality Proximity Monitoring Systems (CPMS) in highly contaminated environments (e.g., FDNPP fuel debris removal).
- In-core neutron flux monitoring in advanced or damaged reactors where traditional detectors fail due to high temperature or radiation damage.
High-Energy Physics and Accelerators:
- Beam monitoring and particle detection in high-radiation areas of particle accelerators, where silicon detectors quickly degrade.
- Neutron spectroscopy in environments with intense gamma backgrounds.
Space and Defense:
- Radiation detection and dosimetry for satellites and deep-space probes, leveraging diamond’s inherent radiation hardness and low mass.
- Neutron detection for non-proliferation and safeguards applications, requiring high sensitivity and immunity to gamma interference.
Industrial Process Control:
- Monitoring neutron sources used in industrial radiography or material analysis under harsh operating conditions (high temperature, high radiation).

Tech Support

Original Source

DOI: https://doi.org/10.18494/sam5597