Unfolding Of Neutron Spectra With An Experimentally Determined Diamond Detector Response Function

At a Glance

Metadata	Details
Publication Date	2016-10-26
Authors	A. Zimbal, F. Gagnon-Moisan, Marcel Reginatto, M. Zbořil
Institutions	Paul Scherrer Institute, Physikalisch-Technische Bundesanstalt
Analysis	Full AI Review Included

Technical Documentation and Analysis: CVD Diamond for High-Resolution Neutron Spectrometry

6CCVD specializes in the production of high-purity, custom MPCVD diamond materials essential for extreme-environment scientific applications, such as high-flux neutron diagnostics in fusion research.

Executive Summary

This research validates the use of high-purity Single Crystal CVD (SCD) diamond detectors for extremely high-resolution neutron spectrometry by establishing an experimentally derived response matrix, overcoming limitations inherent in simulation codes.

Core Application: High-resolution neutron spectrometry in high neutron flux environments, specifically targeted toward fusion diagnostics (e.g., analyzing the 14 MeV DT fusion peak).
Material Success: Confirms the superior performance and radiation hardness of artificial SCD diamond detectors (typical size 5 x 5 x 0.5 mm³) for resolving complex neutron spectra.
Key Achievement: Successful experimental determination and validation of the neutron response matrix (R_kl) over the crucial energy range of 10.0 MeV < E_n < 16.0 MeV.
Methodology: Utilized a sophisticated Bayesian approach incorporating thin plate spline (TPS) radial basis function interpolation and the GRAVEL unfolding code to extract precise neutron energy spectra ($\Phi_{E}$).
Validation: Unfolded spectra for monoenergetic 14.0 MeV and 14.8 MeV neutrons showed excellent agreement with numerical TARGET code simulations, confirming the quality of the derived response function.
Value Proposition: This methodology allows engineers to use SCD detectors to measure unknown neutron fields with high fidelity, providing essential information on plasma conditions (shape and width of the neutron peak).

Technical Specifications

The following hard data points detail the material requirements and experimental parameters necessary for high-fidelity neutron response function determination and spectrum unfolding.

Parameter	Value	Unit	Context
Detector Material	Single Crystal Diamond	SCD (CVD)	Selected for superior radiation hardness and low gamma sensitivity
Nominal Detector Dimensions	5 x 5 x 0.5	mm³	Typical size for compact neutron detectors
Measured Detector Dimensions	4.6 x 4.6 x 0.5	mm³	Physical size of the SCD plate used in measurements
Neutron Energy Range (Response Matrix)	10.0 to 16.0	MeV	Key range for high-energy neutron spectrometry
Response Function Normalization Uncertainty	10	%	Estimated uncertainty due to incomplete absolute PHS normalization
Neutron Production Reaction (Response Fx)	D(d,n)³He	N/A	Used with Time of Flight methods to ensure monoenergetic quality
Neutron Production Reaction (Unfolding Test)	T(d,n)⁴He	N/A	Used to generate 14.0 MeV and 14.8 MeV test beams
Deposited Energy Range (PHS Unfolding)	2.0 to 10.0	MeV	Range utilized for pulse height spectrum analysis
Target-Detector Distance	25	cm	Distance used during T(d,n)⁴He unfolding measurements
SCD Electrode Geometry	Circular / Square	N/A	Two different detectors were tested, showing electrode geometry difference did not “strongly affect the response”

Key Methodologies

The study relied on high-quality SCD detectors and advanced numerical techniques to derive and validate the neutron response matrix.

Detector Preparation: High-purity single-crystal CVD diamond plates (thickness 0.5 mm) were used, featuring both circular and square electrode geometries (materialization details are not provided but are implicit).
Neutron Beam Generation:
- Response Matrix Determination: Used the PTB ion accelerator facility (PIAF). Monoenergetic neutrons (E_n: 10.08 MeV to 16.01 MeV) were generated via the D(d,n)³He reaction.
- Unfolding Validation: Used the T(d,n)⁴He reaction (Deuteron beam E_d = 215 keV, current I ~ 10 µA) to produce monoenergetic test neutrons at 14.0 MeV (98° emission) and 14.8 MeV (0° emission).
Data Acquisition: Standard analog electronics were used to collect Pulse Height Spectra (PHS) for each monoenergetic neutron beam.
Response Function Derivation (R_kl): A Bayesian approach was applied to model the PHS data, separating it into a smooth background and a fine-structure signal (peaks).
Interpolation: Thin Plate Spline (TPS) Radial Basis Function (RBF) interpolation was used to smoothly map the background component across the energy range.
Spectrum Unfolding: The iterative GRAVEL code (part of the UMG software package) was used, starting from a constant default spectrum, to mathematically select the fluence vector ($\Phi_{l}$) that matches the measured PHS through the response matrix.
Validation: Unfolded spectra were compared directly against independently calculated neutron energy spectrum simulations performed using the TARGET code.

6CCVD Solutions & Capabilities

6CCVD provides the custom CVD diamond materials and advanced engineering services required to replicate this foundational research or scale diamond detector technology for operational deployment in fusion facilities (e.g., ITER, DEMO).

Applicable Materials

To achieve the superior resolution and radiation hardness demonstrated in this work, high-quality, Electronic Grade SCD material is necessary.

Required Material: Optical/Electronic Grade Single Crystal Diamond (SCD)
- Why 6CCVD SCD: Our SCD material ensures extremely low nitrogen concentration and defects, maximizing carrier mobility and collection length. This high intrinsic purity is essential for high Charge Collection Efficiency (CCE) and resolving the fine structure peaks (e.g., (n,$\alpha$), (n,d), (n,p) peaks) observed in the PHS.
- Thickness: SCD plates are available from 0.1 µm up to 500 µm, perfectly matching the 0.5 mm (500 µm) thickness used in this study.

Customization Potential

The flexibility of CVD diamond dimensions and electrode design is critical for optimizing detector response. The paper explicitly mentions the use of specific dimensions (4.6 x 4.6 mm) and varying electrode geometries (circular vs. square).

Research Requirement	6CCVD Customization Service	Value Proposition
Custom Geometry/Size (e.g., 4.6 x 4.6 mm)	Precision Laser Cutting: We provide precise custom dimensions for both SCD and PCD wafers (up to 125mm).	Enables replication of established detector designs and optimization of active area/volume ratios for desired efficiency.
Electrode Variation (Circular/Square)	Advanced Metalization Services: In-house capability for deposition of Au, Pt, Pd, Ti, W, and Cu contacts.	Allows engineers to tailor electrode design (e.g., circular, guard rings, Schottky contacts) for improved electric field uniformity and stability, minimizing signal distortion.
Surface Finish (High-Resolution PHS)	Ultra-Smooth Polishing: Achievable surface roughness down to R_a < 1 nm (SCD).	Crucial for minimizing surface leakage current and improving detector noise characteristics, directly impacting the quality of the pulse height spectrum used for unfolding.

Engineering Support

This research involves highly specific unfolding algorithms and complex reaction physics. 6CCVD provides support for application integration.

6CCVD’s in-house PhD team can assist researchers and engineers in selecting the optimal SCD material specifications (purity, thickness, crystal orientation) required for high-flux Fusion Diagnostic projects.
We offer consultation on post-processing techniques, including metalization recipes and polishing specifications, to ensure the finished detector plate meets the stringent performance metrics necessary for high-fidelity spectral unfolding.

Call to Action: For custom specifications, high-purity SCD material, or expert engineering consultation on advanced neutron spectrometry or radiation hardness projects, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available).

View Original Abstract

Radiation detectors made from artificial chemical vapor deposition (CVD) single crystal diamond have shown great potential for neutron spectrometry. The detectors are small, typically about (5 × 5 × 0.5) mm3, they are not very sensitive to gamma radiation, and they have good radiation hardness properties. They are, therefore, very promising candidates for applications where high resolution neutron spectrometry in very high neutron fluxes is required, such as in fusion research.<br />A measurement with a single crystal CVD diamond detector results in a pulse height spectrum which contains information about the energy spectrum of the incident neutrons. Unfolding methods can be used to extract this information, but this requires a response matrix. Current particle transport codes, while able to provide important information, are of limited use because they cannot simulate neutron responses of CVD diamond detectors that are of high enough quality to be used for unfolding. Consequently, we have determined the neutron response matrix from measurements.<br />The response matrix covers the energy range 10.0 MeV < En < 16.0 MeV. It is based on six measurements of monoenergetic neutron beams produced at the Physikalisch-Technische Bundesanstalt (PTB) ion accelerator facility (PIAF). A Bayesian approach that incorporates signal-background separation techniques and thin plate spline radial basis function interpolation was used to get the full response matrix from this rather limited amount of data. To test the quality of the response matrix, we have done unfoldings of additional measurements with monoenergetic neutron spectra. These were also made at PIAF. The unfolded spectra are in good agreement with numerical simulations of these spectra.

Tech Support

Original Source

DOI: https://doi.org/10.22323/1.240.0097