Response of a diamond detector sandwich to 14 MeV neutrons

At a Glance

Metadata	Details
Publication Date	2016-02-08
Journal	Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
Authors	M. Osipenko, M. Ripani, G. Ricco, B. Caiffi, F. Pompili
Institutions	National Agency for New Technologies, Energy and Sustainable Economic Development, Istituto Nazionale di Fisica Nucleare, Roma Tor Vergata
Citations	21
Analysis	Full AI Review Included

Technical Documentation & Analysis: Thin SCD Detectors for Fast Neutron Diagnostics

Reference Paper: Osipenko et al., “Response of diamond detector sandwich to 14 MeV neutrons” (arXiv:1510.05415v3)

Executive Summary

This research validates the use of ultra-thin Single Crystal Diamond (SCD) detectors in a sandwich configuration for high-flux, high-energy neutron diagnostics, specifically addressing the extreme environmental demands of fusion reactors like ITER.

Radiation Hardness: The use of 50 µm thick SCD crystals is projected to increase radiation hardness significantly, expected to withstand neutron fluences exceeding 10¹⁶ n/cm², satisfying ITER Radial Neutron Camera (RNC) requirements.
Detection Mechanism: The detector successfully measured 14 MeV neutrons via the ¹²C(n, α)⁹Be reaction, utilizing coincidence detection between two stacked SCD layers to suppress physical background noise.
High Resolution: An intrinsic energy resolution of 240 keV FWHM was achieved, representing only 10% of the expected 14 MeV neutron energy spread (500 keV FWHM) at ITER.
Complex Structure: The detector design required precise multi-layer fabrication, including HPHT substrates, p-type Boron-Doped Diamond (BDD) ohmic contacts, intrinsic SCD layers, custom Cr/Au metalization, and a ⁶LiF converter layer.
Performance Metric: The measured 14 MeV neutron detection sensitivity was 5 x 10^-7 counts cm²/n for the 3 x 3 mm² active area.

Technical Specifications

The following hard data points were extracted from the detector design and performance measurements:

Parameter	Value	Unit	Context
Neutron Energy Measured	14	MeV	DT Fusion Source
Target Fluence (50 µm SCD)	> 10¹⁶	n/cm²	Expected radiation hardness limit
SCD Intrinsic Layer Thickness (Range)	49 - 54	µm	Active volume of the two SCDs
P-doped Layer Thickness (Range)	15 - 26	µm	Boron-doped ohmic contact layer
Metallic Contact Material	Cr / Au	nm	Cr (40 nm) as sticking layer; Au (80 nm) strips
LiF Converter Layer Thickness	50	nm	Used for thermal neutron calibration
Detector Active Area	3 x 3	mm²	Size of the metallic contact
Bias Voltage Applied	-80	V	Operational voltage
Electric Field Strength	1.6	V/µm	Calculated field across the intrinsic layer
Intrinsic Energy Resolution (FWHM)	240	keV	Resolution attributed to detector design
Total Measured Resolution (FWHM)	870	keV	Limited primarily by environmental EM noise
Detection Sensitivity (14 MeV)	5 x 10^-7	counts cm²/n	Measured for the 3 x 3 mm² detector
Coincidence Trigger Window	64	ns	Hardware trigger interval

Key Methodologies

The experiment relied on precise material engineering and a specialized coincidence detection setup to isolate the ¹²C(n, α)⁹Be reaction signal from background noise.

Material Selection and Thinning: Electronic grade Single Crystal Diamond (SCD) was grown via MPCVD. The crystals were thinned to approximately 50 µm to maximize radiation hardness against fast neutrons, a critical requirement for long-term fusion reactor operation.
Multi-Layer Fabrication: Each SCD detector was constructed with five distinct layers: HPHT substrate, a p-type Boron-Doped Diamond (BDD) ohmic contact layer, the intrinsic SCD active layer, a metallic anode, and a LiF converter layer.
Custom Metalization: The metallic anode was applied by evaporating a 40 nm Chromium (Cr) layer, followed by 80 nm Gold (Au) strips (0.4 mm x 3 mm) used for grounding and electrical connection.
Sandwich Configuration: Two SCD detectors were stacked with their metallic contacts facing each other to form a “sandwich” structure, enabling coincidence measurements of charged particles (α particles) produced by neutron interactions in the first crystal.
Neutron Source and Calibration: 14 MeV neutrons were generated using a DT fusion generator (FNG). Energy calibration was performed using the highly exothermic ⁶Li(n, t)α reaction in the interposed 50 nm LiF layer.
High-Speed Data Acquisition (DAQ): Signals were processed using custom fast charge amplifiers and a 10-bit digitizer operating at 5 Gs/s. Coincidence events were filtered offline by requiring a maximum 6 ns time difference between the two detector signals to reject EM noise and accidental background.

6CCVD Solutions & Capabilities

The successful replication and extension of this high-performance, radiation-hard detector requires specialized MPCVD diamond materials and advanced fabrication capabilities. 6CCVD is uniquely positioned to supply the necessary components for fusion diagnostics and high-energy physics research.

Research Requirement	6CCVD Solution & Capability	Technical Advantage for Replication/Extension
Ultra-Thin SCD Layers (50 µm)	Custom Thickness SCD: We provide electronic-grade Single Crystal Diamond (SCD) wafers with precise thickness control from 0.1 µm up to 500 µm.	Enables the production of thin detectors necessary to achieve the required radiation hardness (> 10¹⁶ n/cm²) for long-duration fusion experiments.
P-type Ohmic Contact Layer	Boron-Doped Diamond (BDD): 6CCVD offers custom-doped BDD layers (p-type) for stable ohmic contacts, replicating the 15 µm to 26 µm layers used in the study.	Ensures reliable charge collection and low leakage current, critical for high-temperature operation (up to 100 °C in ITER).
Custom Metalization (Cr/Au/LiF)	Advanced Metalization Services: We offer in-house deposition of Cr, Au, Ti, Pt, Pd, W, and Cu. We can precisely replicate the 40 nm Cr / 80 nm Au electrode structure and assist in integrating converter layers like LiF.	Guarantees high-quality, low-loss electrical contacts and facilitates complex multi-layer detector assembly (e.g., sandwich structures).
High Surface Quality	Precision Polishing (Ra < 1 nm): Our SCD wafers are polished to an atomic scale finish (Ra < 1 nm).	Minimizes surface defects that can contribute to leakage current and ensures optimal interface quality for subsequent metalization and layer deposition.
Custom Dimensions & Arrays	Large Format & Dicing: While the paper used 3 x 3 mm² active areas, 6CCVD supplies plates/wafers up to 125 mm (PCD) and provides precision laser cutting and dicing services.	Supports scaling up the detector design into large-area arrays (e.g., 4x4 matrices) required for full-scale RNC systems.

Engineering Support

6CCVD’s in-house PhD team specializes in optimizing diamond material properties (thickness, doping, surface termination) for extreme environments. We can assist researchers and engineers in selecting the optimal Electronic Grade SCD or BDD material to meet the specific flux, temperature, and resolution requirements for similar Fast Neutron Diagnostics projects.

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.2016.02.008