Upgrade of the compact neutron spectrometer for high flux environments

At a Glance

Metadata	Details
Publication Date	2017-11-20
Journal	Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
Authors	M. Osipenko, A. Bellucci, V. Ceriale, D. Corsini, G. Gariano
Institutions	National Agency for New Technologies, Energy and Sustainable Economic Development, Institute of Structure of Matter
Citations	7
Analysis	Full AI Review Included

Technical Documentation and Analysis: High Flux Neutron Spectrometer Upgrade

6CCVD Analysis Reference: arXiv:1707.05780v1 (Osipenko et al.) Application Focus: Compact, radiation-hard neutron spectroscopy for high-flux fission and fusion environments.

Executive Summary

This research successfully demonstrates a significant performance upgrade for a compact ⁶Li-based neutron spectrometer utilizing electronic grade Single Crystal CVD (SCD) diamonds. The key innovation lies in the material selection and advanced ohmic contact deposition, directly addressing issues of space charge build-up and limited timing resolution endemic to previous prototypes.

Material Selection: Use of high-quality electronic grade SCD diamond resulted in superior charge collection and enhanced radiation hardness, eliminating space charge effects observed in previous detectors.
Ohmic Contact Design: A specialized metallization stack (3 nm Diamond-Like Carbon / 100 nm Gold) was employed to ensure stable, ohmic contacts, crucial for operation in high-flux environments.
Exceptional Timing Resolution: The spectrometer achieved a best-in-class coincidence timing resolution of 68 ps (RMS), improved by an order of magnitude compared to earlier versions.
High Energy Resolution: Demonstrated an energy resolution of 72 keV (RMS) for thermal neutrons (Q-value peak at 4.7 MeV).
Compact Integration: The final detector package was highly compact, fitting into a tube design of 1 cm diameter and 13 cm length, suitable for small reactor channels.
Radiation Hardness Verified: Charge Collection Efficiency (CCE) remained stable up to accumulated doses of approximately 100 Gy, confirming the suitability of the SCD material for extreme environments.

Technical Specifications

The following parameters define the core materials and performance metrics achieved using the electronic grade SCD diamond detectors.

Parameter	Value	Unit	Context
Diamond Material	Single Crystal CVD (SCD)	N/A	Electronic Grade, High Purity
Crystal Thickness	300	µm	Optimized charge collection depth
Crystal Area	3 x 3	mm²	Sensor physical size
Ohmic Contact Stack	DLC / Au	N/A	Interfacial layer/Top Contact
DLC Thickness	3	nm	Diamond-Like Carbon (Amorphization Layer)
Au Contact Thickness	100	nm	Deposited via RF magnetron sputtering
Converter Film	⁶LiF (96% enriched)	N/A	Thermal neutron conversion layer
Converter Thickness	100	nm	Thermally evaporated film
Energy Resolution (RMS)	72	keV	For 4.7 MeV thermal neutron peak (Q-value)
Timing Resolution (RMS)	68	ps	Coincidence measurement (α-t pairs)
Operating Voltage (Nominal)	~300	V	Applied detector bias
Nominal Resistance (R_CVD)	10¹⁰ - 10¹¹	Ω	Characteristic of Electronic Grade SCD
Tested Neutron Flux (Thermal)	10⁸	n/cm²/s	TRIGA Reactor Test Environment

Key Methodologies

The robust performance and suppressed space charge effects rely on a precise fabrication methodology focused on surface preparation and customized metallization.

Diamond Selection: Commercial electronic grade SCD crystals (300 µm thick, 3 x 3 mm² area) were chosen for superior charge collection and intrinsic radiation hardness.
Aggressive Cleaning Procedure: Samples were subjected to a rigorous, multi-stage cleaning to remove organic, metallic, and non-diamond impurities:
- Boiling Oxidizing Solution (H₂SO₄:HCIO₄:HNO₃, 1:1:1 ratio, 15 minutes).
- Rinsing in Boiling Aqua Regia (HCl:HNO₃, 3:1 ratio, 5 minutes).
- Final Ultrasound Sonication.
Ohmic DLC Layer Formation: A 3 nm thick Diamond-Like Carbon (DLC) layer was formed using energetic (700 eV) Ar⁺ ion bombardment. This layer induces surface amorphization, which is critical for improving contact ohmicity and stability under high-flux irradiation.
Gold (Au) Deposition: A 100 nm thick Au layer was grown in situ using RF magnetron sputtering.
- RF Power: 200 W.
- Base Pressure: 10^-6 mbar.
- Ar⁺ Pressure: 2.3 x 10^-2 mbar.
- Contact dimensions (2.8 x 2.8 mm²) were defined using stainless steel shadow masks.
Converter Deposition: A 100 nm thick ⁶LiF film (96% enriched) was deposited onto one diamond by thermal evaporation in a chamber evacuated to 10^-6 mbar. Thickness was controlled using a quarz microbalance.
Assembly and Readout: The two sensors were configured in a sandwich geometry (300 µm air gap) on a 250 µm thick double-face PCB using conductive glue (E-solder 3025). Passive preamplification was implemented using an RF transformer (Mini-Circuits T14-1-KK81) to minimize signal loss over long cables in high radiation areas.

6CCVD Solutions & Capabilities: Precision Diamond for Nuclear Spectroscopy

The successful development of this advanced neutron spectrometer highlights the necessity of ultra-high purity, tightly dimensioned SCD diamond wafers combined with precise, multi-layer metalization to achieve stable, high-resolution performance in extreme radiation environments.

6CCVD is uniquely positioned to supply and enhance the materials required to replicate or advance this research, offering control over every critical parameter demonstrated in the paper.

Applicable Materials & Performance Enhancement

To replicate the performance achieved in this paper, 6CCVD recommends:

Optical/Electronic Grade Single Crystal Diamond (SCD): We offer SCD material optimized for charge carrier mobility and minimum defects, ensuring superior Charge Collection Efficiency (CCE) and intrinsic radiation hardness matching or exceeding commercial electronic grades tested.
- Direct Benefit: Enhanced signal-to-noise ratio and guaranteed CCE stability in high-flux, high-dose applications (e.g., fusion diagnostics, reactor monitoring).
Custom Thickness Control: The detector relies on a specific 300 µm SCD thickness. 6CCVD offers SCD production capabilities from 0.1 µm up to 500 µm, allowing for optimization of thickness to balance timing resolution, detection efficiency, and particle stopping power for different neutron spectra (thermal, DD, DT).

Customization Potential: Achieving Ohmic Contact Uniformity

The researchers noted that excessive energy loss tails were likely related to imperfections and non-uniformity in the gold metallization film, particularly at the contact borders. 6CCVD’s in-house capabilities directly solve this challenge:

Feature Requirement (Paper)	6CCVD Capability	Value Proposition
Custom Dimensions	Plates/wafers up to 125mm. Custom laser cutting service for dimensions like 3 x 3 mm² or 2.8 x 2.8 mm² contact areas.	Precise Integration: Ensures perfect fit for compact spectrometer designs (1 cm tube diameter).
Complex Metallization	Internal deposition capabilities for Au, Ti, Pt, Pd, W, Cu. We can deposit the critical DLC/Au ohmic stack (3 nm DLC, 100 nm Au) with high uniformity.	Space Charge Suppression: Guarantees repeatable ohmic contacts, suppressing the space charge build-up critical for high flux operation.
Ultra-High Polishing	Ra < 1 nm (SCD).	Improved Resolution: Minimizing surface roughness reduces scattering and energy loss fluctuations, directly mitigating the asymmetric energy loss tail observed in the paper.
Converter Handling	Expertise in handling sensitive converter materials and custom layering techniques, including thermal evaporation methods used for the ⁶LiF film.	Optimized Efficiency: Ensures uniform and stable deposition of the converter layer onto the metal contact surface.

Engineering Support

6CCVD’s in-house team of PhD material scientists and technical engineers can assist researchers in optimizing material specifications for advanced nuclear and high-energy physics projects.

We offer consultation on:

Material Specification: Selecting the optimal SCD grade and thickness for specific neutron energies or charged particle detection schemes.
Contact Engineering: Customizing complex ohmic contact recipes (e.g., DLC interfaces, high-stability alloys) to maximize detector stability and lifetime in high-radiation, high-temperature environments.
Detector Geometry: Providing laser cutting and dicing services to meet the strict dimensional requirements of compact detector assemblies like this compact neutron spectrometer.

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

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.nima.2017.11.040

References

2013 - Diamond neutron detectors as He-3 alternative
2015 - Diamond as a solid state micro-fission chamber for thermal neutron detection at the VR-1 research reactor
2014 - Diagnostic of fusion neutrons on JET tokamak using diamond detector
2015 - Test of a prototype neutron spectrometer based on diamond detectors in a fast reactor