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

Executive Summary

This research demonstrates the successful reconstruction of high-flux X-ray bremsstrahlung spectra using a polycrystalline CVD diamond detector, enabled by achieving a highly linear response to the radiation dose rate.

Core Achievement: Accurate reconstruction of the X-ray source spectrum (bremsstrahlung and Kα line) using the classic absorbers method, refined with a modern analytical model.
Critical Enabling Condition: The 50 µm polycrystalline diamond detector achieved near-ideal linearity to the radiation dose rate, characterized by a linearity coefficient (Δ) of 0.997 ± 0.003.
Operational Requirement: This linear response was only achieved when the detector was biased at ≥90 V, corresponding to a high internal electric field (≥1.8 x 10⁴ V/cm), ensuring complete collection of photogenerated charge carriers.
Material Advantage: The use of relatively cheap, large-area polycrystalline CVD diamond films is validated for high-flux spectrometry applications where current mode operation (not pulsed mode) is necessary.
Methodology: The absorbers method utilized variable thicknesses of Nichrome and Aluminum foils to selectively attenuate the X-ray beam based on energy-dependent attenuation coefficients.
Future Potential: The technique, once optimized with variable thickness Nickel filters, promises accurate disentanglement of closely spaced characteristic lines, such as the Copper Kα and Kβ peaks.

Technical Specifications

Parameter	Value	Unit	Context
Detector Material	Polycrystalline CVD Diamond	N/A	Free-standing film
Film Thickness	50	µm	Active volume dimension
Electrode Material	Silver (Ag)	N/A	Thermally evaporated contacts
Electrode Thickness	200	nm	Minimized to reduce absorption perturbation
Electrode Size	7 x 7	mm²	Sandwich configuration
Bias Voltage (Linearity Saturation)	≥90	V	Required for linear dose rate response
Electric Field (Linearity)	≥1.8 x 10⁴	V/cm	Calculated from 90 V bias / 50 µm thickness
Linearity Coefficient (Δ)	0.997 ± 0.003	N/A	Response linearity to dose rate (Ideal = 1)
Diamond Bandgap (E_g)	5.47	eV	Intrinsic material property
Mean Ionization Energy (w)	13.1	eV	Energy to generate one electron-hole pair
X-ray Source Anode	Copper (Cu)	N/A	Philips microfocus tube
X-ray Accelerating Voltage (V_acc)	30, 40	kV	Used for absorber tests
Fixed Filter Thickness	20	µm	Nickel (Ni) foil to suppress Cu Kβ line
Copper Kα Energy	8.05	keV	Primary characteristic line
Absorber Materials	Aluminum (Al), Nichrome (80% Ni - 20% Cr)	N/A	Used for attenuation measurements

Key Methodologies

Film Synthesis and Preparation:
- Growth: Polycrystalline diamond film was grown via Microwave CVD (MWCVD) heteroepitaxy on a <100> p-type silicon substrate.
- Growth Parameters: 0.5% methane/hydrogen concentration at 700 °C.
- Substrate Removal: Silicon was chemically etched using a standard HNO₃:HF diluted solution.
- Cleaning: Non-diamond phases were removed using a 1:1:1 HNO₃:H₂SO₄:HClO₄ solution.
Device Fabrication:
- Electrodes: 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).
- Electrical Behavior: The resulting contacts exhibited a nearly ohmic behavior at low bias voltages (<10 V) and a non-linear exponential behavior at higher voltages.
Detector Stabilization (Priming):
- The film was exposed to prolonged irradiation (approximately 10 Gy total dose) to saturate deep level traps, ensuring a stable, reproducible response.
Dose Rate Linearity Measurement:
- The photogenerated current (I_ph) was measured as a function of the radiation dose rate (Ď) at various bias voltages (V_b).
- Data was fitted using Fowler’s equation (I_ph ∝ Ď^Δ) to determine the linearity coefficient (Δ).
Spectrum Reconstruction (Absorbers Method):
- The X-ray beam was pre-filtered using a fixed 20 µm Nickel foil.
- Absorbers (Al or Nichrome) of variable thickness (x) were placed in the beam path to attenuate the radiation intensity.
- The detector signal f(x) was measured and modeled as the sum of the Kα contribution (modeled by a Dirac delta function) and the continuous bremsstrahlung contribution.
- The bremsstrahlung component was fitted using an approximated solution similar to a diffusion integral, allowing the reconstruction of the original spectrum F_bremm(E).

Commercial Applications

The demonstrated ability to achieve linear dose rate response in polycrystalline diamond detectors under high-flux conditions opens applications in several engineering and medical fields:

Medical Dosimetry: Monitoring radiation beams in advanced radiation therapy (e.g., Intensity Modulated Radiation Therapy—IMRT, Volumetric Modulated Arc Therapy—VMAT) and mammography, where high spatial resolution and fast response are critical.
Synchrotron and Accelerator Facilities: Used as beam monitors and dosimeters in high-energy physics experiments and synchrotron beamlines characterized by high photon fluxes (current mode operation).
Non-Destructive Testing (NDT): Integration into industrial X-ray systems requiring accurate spectral information for quality control and materials characterization.
Nuclear and Fusion Energy: Deployment as radiation-hard sensors for monitoring ionizing radiation and fast neutrons in fusion reactors (e.g., ITER, JET) and nuclear facilities.
Radiological Protection: Development of robust, radiation-resistant dosimeters for personnel and environmental monitoring in harsh radiation environments.

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]