Properties of Diamond-Based Neutron Detectors Operated in Harsh Environments

At a Glance

Metadata	Details
Publication Date	2021-10-28
Journal	Journal of Nuclear Engineering
Authors	M. Angelone, C. Verona
Institutions	National Agency for New Technologies, Energy and Sustainable Economic Development, University of Rome Tor Vergata
Citations	40
Analysis	Full AI Review Included

Executive Summary

The review highlights the unique suitability of synthetic single-crystal diamond (SCD) detectors, primarily grown via MWPECVD, for radiation detection in harsh environments characterized by high temperature (HT) and intense radiation fluxes.

Superior Material Properties: Diamond possesses a wide band gap (5.47 eV), the highest thermal conductivity (20 Wcm^-1K^-1), and high displacement energy (43.3 eV), resulting in extremely low dark current, high speed, and excellent intrinsic radiation hardness compared to silicon or germanium.
Fast Neutron Spectrometry: SCD detectors utilize the ¹²C(n,α)⁹Be reaction (threshold 5.7 MeV) to produce a sharp, isolated peak in the pulse height spectrum, enabling high-resolution spectrometry (FWHM typically 1-2%).
Thermal Neutron Capability: Sensitivity to thermal neutrons, which is intrinsically low for carbon, is achieved by coating the diamond surface with converter layers (e.g., ⁶LiF or ¹⁰B), enabling high-efficiency detection in both layered and “sandwich” configurations.
Radiation Hardness and Thickness Dependence: The reduction in Charge Collection Efficiency (CCE) due to neutron damage is inversely dependent on detector thickness (L). Thinner detectors (e.g., 50 µm) show stable response up to fluences of approximately 10¹⁵ n/cm² (14 MeV neutrons).
High-Temperature Operation: Stable spectrometric operation has been demonstrated up to 425 °C (725 K) for thin (65 µm) SCD detectors, although performance limits are often dictated by polarization effects and the degradation of electrical contacts (e.g., Ag contacts peeling after 4.7 MGy gamma dose).
Fast Timing: Diamond detectors exhibit extremely fast response times (pulse rise times in the hundreds of picoseconds), making them ideal for Time-of-Flight (ToF) measurements and high-count-rate applications in pulsed neutron sources (e.g., ISIS).

Technical Specifications

Parameter	Value	Unit	Context
Band Gap (E_g)	5.470 ± 0.05	eV	At 300 K
Thermal Conductivity (σ_T)	20	Wcm^-1K^-1	Highest known material
Electron Mobility (µ_e)	1800-2200	cm²V^-1s^-1	At 300 K, low field
Hole Mobility (µ_h)	1200-1600	cm²V^-1s^-1	At 300 K, low field
Saturation Velocity (v_sat)	2.7 x 10⁷	cm s^-1
Energy per EHP (ε_p)	13	eV	Energy to produce one electron-hole pair
Resistivity (ρ)	>10¹³	Ω cm	Intrinsic material
Breakdown Voltage	>10⁷	Vcm^-1	High dielectric strength
Displacement Energy (E_d)	37.5-47.6	eV	Energy to displace a carbon atom
Max Operating Temp (T_Max)	425	°C (725 K)	65 µm thick SCD detector (highest reported)
Alpha Energy Resolution	~0.4	% FWHM	Comparable to silicon detectors
Fast Neutron Energy Resolution	1-2	% FWHM	¹²C(n,α)⁹Be peak
14 MeV Neutron Fluence Tolerance	~1.0 x 10¹⁵	n/cm²	50 µm thick detector (70% CCE retention)
Thermal Neutron Cross-Section (¹⁰B)	~3600	b	Converter material
Thermal Neutron Cross-Section (⁶Li)	~900	b	Converter material
Alpha Range (5.5 MeV ²⁴¹Am)	~14	µm	Penetration depth in diamond

Key Methodologies

The fabrication and characterization of electronic-grade diamond detectors rely heavily on controlled growth and specialized electrical contact deposition, followed by rigorous testing using advanced techniques.

Diamond Growth (MWPECVD):
- Technique: Microwave Plasma Enhanced Chemical Vapor Deposition (MWPECVD) is the primary method for producing high-quality polycrystalline (pCVD) and single-crystal diamond (SCD) films.
- Gas Mixture: High-purity H₂ (99.9999%) and CH₄ (typically 1-2 sccm CH₄ per 100 sccm H₂) are dissociated by microwaves to form active radicals.
- Substrate: Low-cost High-Temperature, High-Pressure (HTHP) synthetic diamond plates are typically used as substrates for homoepitaxial SCD growth.
- Surface Control: Hydrogen is fundamental during growth to terminate dangling bonds, ensuring a stable sp³ lattice. As-grown CVD diamond surfaces are typically hydrogen-terminated (p-type surface conductivity).
Electrical Contact Fabrication:
- Schottky Contacts (Rectifying): Formed by depositing high work function metals (e.g., Pt, Au, Ag) at room temperature. These contacts provide low dark current and high response speed, useful for dosimetry and ToF measurements.
- Ohmic Contacts (Non-Rectifying):
  - Carbide Formation: Achieved by annealing metals (e.g., Al, Ti, Cr) post-deposition, causing them to react with carbon to form a carbide layer.
  - Layered Diamond Detector (LDD): Created by depositing a highly boron-doped SCD layer (p⁺) on top of the intrinsic SCD film, followed by a standard metal contact (e.g., Al).
Performance Characterization:
- Charge Transport Analysis: The Transient Current Technique (TCT) is used to measure drift velocity, mobility, and trapping/detrapping times by analyzing the shape and timing of induced current pulses generated by ionizing radiation (e.g., ²⁴¹Am alphas).
- Polarization Mitigation: Low-quality or damaged detectors often require “priming” (pre-irradiation with intense sources like electrons or UV) to fill deep traps and reduce internal space charge build-up (polarization).
- Radiation Damage Quantification: Damage is quantified by measuring the reduction in the Charge Collection Efficiency (CCE) as a function of neutron fluence, often normalized using the 1 MeV equivalent flux or Displacement Per Atom (DPA) model.
Neutron Detection Methods:
- Fast Neutron Detection: Intrinsic detection relies on the ¹²C(n,α)⁹Be reaction.
- Thermal Neutron Detection: Achieved by depositing thin converter layers (e.g., ⁶LiF or ¹⁰B) onto the diamond surface (layered configuration) or sandwiching the converter between two diamond films (sandwich configuration) to maximize energy absorption.

Commercial Applications

Diamond detectors are uniquely positioned for applications requiring extreme performance in radiation tolerance, temperature stability, and timing resolution, spanning nuclear, medical, and advanced electronics sectors.

Nuclear Fusion and Fission Reactors:
- 14 MeV neutron flux monitoring and spectrometry in fusion tokamaks (e.g., JET, ITER).
- Measurement of tritium production in fusion breeding blanket mock-ups.
- Neutron flux monitoring in fission reactors (using ²³⁵U or ¹⁰B converters).
High-Energy Physics and Accelerators:
- Minimum Ionizing Particle (MIP) detection and tracking (CERN RD42 collaboration).
- Beam monitoring and beam loss monitoring in high-luminosity experiments (LHC).
- Time-of-Flight (ToF) spectroscopy for fast neutrons at pulsed spallation sources (e.g., ISIS).
Medical Physics and Dosimetry:
- High-resolution radiation dosimetry for X-ray, gamma-ray, proton, and carbon ion therapy.
- Microdosimetry for characterizing complex radiation fields in hadron therapy.
- Boron Neutron Capture Therapy (BNCT) dose evaluation (using ¹⁰B converters).
Advanced Electronics and Sensing:
- High-power and high-frequency Field Effect Transistors (FETs) and diodes.
- High-speed, low-noise electronic devices for next-generation supercomputers.
- Extreme environment electronics (e.g., avionics, aerospace) due to high T_Max.
- UV and extreme UV photodetectors (astrophysics, plasma diagnostics).
- Biosensors.

View Original Abstract

Diamond is widely studied and used for the detection of direct and indirect ionizing particles because of its many physical and electrical outstanding properties, which make this material very attractive as a fast-response, high-radiation-hardness and low-noise radiation detector. Diamond detectors are suited for detecting almost all types of ionizing radiation (e.g., neutrons, ions, UV, and X-ray) and are used in a wide range of applications including ones requiring the capability to withstand harsh environments (e.g., high temperature, high radiation fluxes, or strong chemical conditions). After reviewing the basic properties of the diamond detector and its working principle detailing the physics aspects, the paper discusses the diamond as a neutron detector and reviews its performances in harsh environments.

Tech Support

Original Source

DOI: https://doi.org/10.3390/jne2040032

References

1949 - Crystal counters
2003 - Electronic properties of diamond [Crossref]
1974 - Diamond nuclear radiation detectors [Crossref]
1975 - Preparation and Characteristics of Natural Diamond Nuclear Radiation Detectors [Crossref]
1979 - Transport Properties of Natural Diamond Used as Nuclear Particle Detector for a Wide Temperatue Range [Crossref]