X-ray Spectrum Reconstruction by Diamond Detectors with Linear Response to Dose Rate

At a Glance

Metadata	Details
Publication Date	2021-10-18
Journal	Crystals
Authors	D.M. Trucchi, P. Ascarelli
Institutions	Institute of Structure of Matter
Citations	2
Analysis	Full AI Review Included

Technical Documentation & Analysis: X-ray Spectrum Reconstruction by Diamond Detectors

This document analyzes the research paper “X-ray Spectrum Reconstruction by Diamond Detectors with Linear Response to Dose Rate” (Trucchi et al., Crystals 2021, 11, 1258) to highlight the critical role of high-quality MPCVD diamond and to propose specific material solutions available through 6CCVD.

Executive Summary

This research successfully validates the use of MPCVD Polycrystalline Diamond (PCD) thin films for high-accuracy X-ray spectrometry in current mode, a critical requirement for high-flux applications.

Core Achievement: Demonstrated accurate reconstruction of the X-ray bremsstrahlung spectrum using a revisited absorbers method.
Material Validation: Confirmed the suitability of 50 µm thick MPCVD PCD detectors for high-flux ionizing radiation detection.
Key Operational Condition: Achieved near-ideal linear response to the radiation dose rate (linearity coefficient of 0.997 ± 0.003).
Charge Collection Efficiency (CCE): Linearity was strictly dependent on applying a high bias voltage (≥90 V), corresponding to an electric field of ≥1.8 x 10⁴ V/cm, ensuring complete collection of photogenerated charges.
Device Architecture: Utilized a simple metal-diamond-metal (Ag/PCD/Ag) sandwich configuration, leveraging the low leakage current and high damage resistance inherent to CVD diamond.
Future Extension: The authors propose using variable-thickness Nickel absorbers to accurately disentangle the Cu Kα and Kβ peaks, requiring precise material control.

Technical Specifications

The following hard data points were extracted from the experimental setup and results:

Parameter	Value	Unit	Context
Detector Material Type	Polycrystalline Diamond (PCD)	N/A	Free-standing thin film
Detector Thickness	50	µm	Active volume
Detector Lateral Size	8 x 8	mm²	Used for perpendicular incidence
Electrode Material	Silver (Ag)	N/A	Thermally evaporated, sandwich configuration
Electrode Thickness	200	nm	Minimized radiation absorption perturbation
Minimum Linear Bias Voltage (V_b)	≥90	V	Required for signal saturation/complete charge collection
Minimum Electric Field (E)	≥1.8 x 10⁴	V/cm	Across the 50 µm film
Dose Rate Linearity Coefficient (Δ)	0.997 ± 0.003	N/A	Achieved at V_b ≥ 90 V (Ideal dosimeter = 1)
X-ray Tube Accelerating Voltage (V_acc)	30, 40	kV	Used for Al and Nichrome absorbers, respectively
X-ray Tube Cathode Current	30	mA	Constant operational parameter
Diamond Ionization Energy (w)	13.1	eV	Energy required to generate one electron-hole pair

Key Methodologies

The experiment relied on precise MPCVD growth and meticulous detector preparation to ensure the necessary electronic properties for linear response.

Material Growth: Polycrystalline diamond film was grown via Microwave Plasma Chemical Vapor Deposition (MPCVD) heteroepitaxy on a <100> oriented p-type silicon substrate.
Growth Recipe: Utilized a 0.5% methane/hydrogen concentration at a substrate temperature of 700 °C.
Film Preparation: The silicon substrate was chemically etched away. Non-diamond phases were subsequently removed using a hot acid solution (HNO₃:H₂SO₄:HClO₄).
Electrode Fabrication: Two 200 nm thick, 7 x 7 mm² Silver (Ag) contacts were thermally evaporated onto opposite sides of the 50 µm film (transversal/sandwich configuration).
Detector Priming: The detector was subjected to prolonged irradiation (approximately 10 Gy total dose) to saturate deep level traps, achieving a stable, reproducible response.
Spectrometry Method: The absorbers method was applied, interposing filters (Nichrome or Aluminum) of varying thickness to achieve energy-selective attenuation of the X-ray beam.
Operational Mode: Detector operated in current mode (not pulsed mode) due to the high photon fluxes, requiring strict linearity to dose rate for accurate spectrum reconstruction.

6CCVD Solutions & Capabilities

The success of this X-ray spectrometry technique hinges on the availability of high-quality, dimensionally precise MPCVD diamond films and custom fabrication services. 6CCVD is uniquely positioned to supply materials and services that meet or exceed the requirements of this research, enabling scaling and optimization for industrial or clinical applications.

Applicable Materials

The research utilized high-quality PCD for its large-area capability and robust performance. 6CCVD recommends:

Optical Grade Polycrystalline Diamond (PCD): Ideal for replicating this work. Our PCD offers high purity and large area availability (up to 125 mm), suitable for developing large-area dosimeters or high-density mosaic arrays required in medical physics and synchrotron applications.
Single Crystal Diamond (SCD): For applications requiring the absolute highest charge carrier mobility and collection efficiency (approaching 100%), 6CCVD SCD films (up to 500 µm thick) offer superior electronic properties, potentially reducing the required bias voltage for full charge collection.

Customization Potential

6CCVD’s in-house manufacturing capabilities directly address the specific material and fabrication requirements demonstrated in this paper:

Requirement in Paper	6CCVD Solution & Capability	Technical Advantage for Researchers
PCD Film Thickness (50 µm)	SCD/PCD thickness range: 0.1 µm to 500 µm	Precise control over active volume for optimizing sensitivity and CCE across specific X-ray energy spectra (e.g., 30-40 keV).
Detector Size (8 x 8 mm²)	Custom dimensions up to 125 mm (PCD)	Enables scaling to large radiation fields or fabrication of complex, inch-size detector arrays for high-resolution imaging.
Silver (Ag) Electrodes	Internal Custom Metalization services: Au, Pt, Pd, Ti, W, Cu	Allows researchers to optimize contact architecture (Schottky vs. Ohmic) and minimize electrode thickness (e.g., 200 nm used here) to prevent spectral perturbation.
Surface Quality	Polishing capability: Ra < 5 nm (Inch-size PCD)	Ensures optimal surface preparation for electrode deposition and minimizes surface defect states that can affect linearity and depletion region size.
Shipping & Logistics	Global shipping (DDU default, DDP available)	Reliable, secure delivery of sensitive materials worldwide.

Engineering Support

The successful application of the absorbers method relies on achieving and maintaining a linear response, which is highly dependent on material quality, defect density, and electrode configuration.

6CCVD’s in-house PhD engineering team specializes in diamond electronic properties and can provide consultation on:

Material Selection: Choosing the optimal PCD or SCD grade and thickness for specific X-ray energy ranges (e.g., low-energy mammography vs. high-energy radiotherapy).
Device Architecture: Designing custom metalization schemes (e.g., Ti/Pt/Au stacks) to achieve stable ohmic or Schottky contacts, crucial for maximizing charge collection efficiency (CCE) at low bias voltages.
Replication and Extension: Assisting researchers in replicating the high-linearity performance (Δ ≈ 1) demonstrated in this X-ray spectrometry project for similar dosimetry or imaging applications.

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

View Original Abstract

The absorbers method is here applied by interposing filters of variable thickness between the X-ray source and a detector so to attenuate the radiation intensity by using the attenuation coefficient as a selective photon energy operator. The analysis of the signal provided by a polycrystalline diamond thin film detector exposed to the energy-selectively-attenuated X-ray beam was used for the reconstruction of the radiation spectrum. The 50 μm thick diamond detector achieves conditions of linear response to the dose rate of the incident radiation (linearity coefficient of 0.997 ± 0.003) for a bias voltage ≥90 V, corresponding to an electric field ≥1.8 × 104 V/cm. Once the absorbers method is applied, only the detector signal linearity to dose rate allows reconstructing the source X-ray bremsstrahlung spectrum with sufficiently high accuracy.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst11101258

References

2020 - Enhanced Responsivity of Diamond UV Detector Based on Regrown Lens Structure [Crossref]
2010 - Extreme UV single crystal diamond Schottky photodiode in planar and transverse configuration [Crossref]
2018 - Impact of UV spot position on forward and reverse photocurrent symmetry in a gold-diamond-gold detector [Crossref]
2020 - A single-crystal diamond X-ray pixel detector with embedded graphitic electrodes [Crossref]
2017 - Properties comparison between nanosecond X-ray detectors of polycrystalline and single-crystal diamond [Crossref]
2012 - X-ray beam monitor made by thin-film CVD single-crystal diamond [Crossref]
2018 - Energy response of diamond sensor to beta radiation [Crossref]
2021 - Behavioral Contrast of Electron and Hole Transport in High-Resolution Diamond Detectors: A Biparametric Correlation Study [Crossref]
2019 - A study of the radiation tolerance of poly-crystalline and single-crystalline CVD diamond to 800 MeV and 24 GeV protons [Crossref]