A Study of the Radiation Tolerance and Timing Properties of 3D Diamond Detectors

At a Glance

Metadata	Details
Publication Date	2022-11-11
Journal	Sensors
Authors	L. Anderlini, Marco Bellini, V. Cindro, C. Corsi, K. Kanxheri
Institutions	University of Florence, University of Urbino
Citations	3
Analysis	Full AI Review Included

Executive Summary

This study validates the superior radiation tolerance and timing performance of 3D monocrystalline Chemical Vapor Deposited (CVD) diamond detectors fabricated via femtosecond laser engineering for high-luminosity particle physics applications.

Enhanced Radiation Hardness: The 3D architecture exhibits significantly higher radiation tolerance compared to standard planar (2D) diamond sensors, maintaining functionality after neutron fluences up to 1.0 x 10¹⁶ n/cm² (1 MeV equivalent).
Bias Voltage Increase Post-Irradiation: High fluence irradiation (>5 x 10¹⁵ n/cm²) dramatically increases the maximum applicable bias voltage (from 70 V to 640 V), likely due to defect passivation, allowing the sensor to operate efficiently in highly damaged states.
Superior Signal Collection: At 10¹⁶ n/cm² fluence, the 3D sensor collected >8000 electrons at 640 V, whereas comparable 2D sensors yielded <700 electrons.
Optimized Timing Resolution: Through optimization of the laser fabrication process, the electrode resistance was reduced to ~30 kΩ, achieving a time resolution of 82 ± 2 ps in beam tests using 180 GeV pions.
Predictive Modeling: A single damage constant, k = (1.5 ± 0.15) x 10^-6 cm² s^-1, successfully fits the Charge Collection Efficiency (CCE) decay across all tested 3D and polycrystalline geometries.
Future Performance Projection: Simulations predict that reducing the columnar electrode resistance further (from 30 kΩ to 1 kΩ) will yield substantial improvements in timing resolution, approaching the performance of 3D silicon detectors.

Technical Specifications

Parameter	Value	Unit	Context
Base Material	Monocrystalline CVD Diamond	N/A	Electronic grade (Element Six Ltd.)
Nominal Thickness	500	µm	Sample dimension
Impurity Concentration	<1	ppb	Substitutional/aggregate impurities
Neutron Fluence Tested	Up to 1.0 x 10¹⁶	n/cm²	1 MeV equivalent
Unirradiated CCE (Planar)	~95	%	At 200 V bias
Unirradiated Max Bias (3D)	70	V	Before electrical breakdown
Irradiated Max Bias (3D)	640	V	At 1.0 x 10¹⁶ n/cm² fluence
Collected Charge (Irradiated 3D)	>8000	electrons	At 1.0 x 10¹⁶ n/cm², 640 V
Damage Constant (k)	(1.5 ± 0.15) x 10^-6	cm² s^-1	Used for CCE decay modeling
Carrier Mobility (µ)	2000	cm²/(V·s)	Assumed for both carriers in modeling
Saturation Velocity (v_s)	2 x 10⁷	cm/s	Assumed for both carriers in modeling
Timing Unit Cell Size	55 x 55	µm²	Used for timing measurements
Optimized Electrode Resistance	~30	kΩ	Columnar electrodes (10 µm diameter)
Beam Test Time Resolution (Strip)	82 ± 2	ps	180 GeV pions
Beam Test S/N Ratio (Strip)	18	N/A	Signal-to-Noise ratio
Modified Material Resistivity	0.4-0.5	Ωcm	Current value achieved by laser modification

Key Methodologies

The 3D diamond detectors were fabricated using a two-step laser engineering process followed by comprehensive electrical and radiation testing.

1. Detector Fabrication (Laser Engineering)

Bulk Electrode Writing (3D Columns):
- Laser System: Ti: Sa laser (800 nm wavelength, mode-locked).
- Pulse Duration: Approximately 50 fs (femtosecond regime).
- Mechanism: Multi-photon excitation induces a phase transition from sp³ (diamond) to a weakly connected sp³/sp² mixture (conductive material) along the optical axis (z).
- Structure: Two interpenetrating matrices of columns were written, terminating ~50 µm from the opposite face to prevent surface discharge.
Surface Graphitization (Contact Interconnection):
- Laser System: Q-switched Nd: YAG laser (1064 nm wavelength).
- Pulse Duration: 8 ns (nanosecond regime).
- Purpose: Connects the columnar electrodes on the surface using graphitic paths to form test structures (strips or combs).

2. Radiation Hardness Testing

Irradiation Source: Fast neutrons (>100 keV) from the Triga reactor (Jozef Stefan Institute, Ljubljana).
Fluence Range: Up to 1.0 x 10¹⁶ n/cm² (1 MeV equivalent).
Measurement Setup: Sr-90 beta source and a Charge Collection Efficiency (CCE) station (developed by NIKHEF).
Procedure: Pulse height spectra were acquired before and after each irradiation step. CCE was calculated as the ratio of collected charge to the calculated generated charge (43.1 e/µm for MIPs).
Priming: Samples were primed (irradiated with a small dose at zero bias) before measurement to fill deep traps and minimize polarization effects.

3. Timing Measurements

Laboratory Setup: Sr-90 beta source used for excitation; MCP-PMT (20 ps resolution) used as the reference trigger. Waveforms recorded by a 6 GHz oscilloscope.
Beam Test Setup: 180 GeV SPS-H8 98 pion beam at CERN. Trigger system used a PHOTONIS MCP-PMT and a 55 x 55 µm Timespot silicon pixel sensor.
Analysis: Time markers were evaluated offline using a Constant Fraction Discrimination (CFD) algorithm. Time resolution was determined by the width of the delay distribution (time difference between the diamond sensor and the reference sensor).

4. Simulation and Modeling

Charge Generation: Geant4 simulation used to model MIP passage and ionization charge generation within a 3x3 pixel matrix.
Charge Transport: ROOT-based KDetSim simulator used to model carrier drift in the non-uniform electric field (pre-determined by Synopsys Sentaurus TCAD).
Signal Output: Induced signal convolved with the response function of the readout line and board; real noise data added for accurate comparison with experimental waveforms.

Commercial Applications

The technology developed for 3D diamond detectors is primarily aimed at extreme environments requiring both high radiation tolerance and excellent temporal resolution.

High Energy Physics (HEP) Experiments:
- Application: Vertex and tracking detectors in high-luminosity colliders (e.g., HL-LHC).
- Value Proposition: Diamond’s inherent radiation hardness and high thermal conductivity allow operation in high-flux regions where silicon detectors degrade rapidly.
4D Tracking Systems:
- Application: Development of next-generation tracking devices (like the Timespot project) that provide both high spatial resolution and picosecond-level timing resolution.
Clinical Dosimetry:
- Application: High-precision measurement of X-ray and particle beams in radiotherapy, leveraging diamond’s tissue equivalence and stability.
Neutron Detection:
- Application: Sensors capable of operating in harsh neutron environments, such as nuclear reactors or fusion experiments.
Deep UV Sensing:
- Application: Due to diamond’s wide bandgap, these materials are suitable for deep UV photodetectors, though this study focuses on ionizing radiation.

View Original Abstract

We present a study on the radiation tolerance and timing properties of 3D diamond detectors fabricated by laser engineering on synthetic Chemical Vapor Deposited (CVD) plates. We evaluated the radiation hardness of the sensors using Charge Collection Efficiency (CCE) measurements after neutron fluences up to 1016 n/cm2 (1 MeV equivalent.) The radiation tolerance is significantly higher when moving from standard planar architecture to 3D architecture and increases with the increasing density of the columnar electrodes. Also, the maximum applicable bias voltage before electric breakdown increases significantly after high fluence irradiation, possibly due to the passivation of defects. The experimental analysis allowed us to predict the performance of the devices at higher fluence levels, well in the range of 1016 n/cm2. We summarize the recent results on the time resolution measurements of our test sensors after optimization of the laser fabrication process and outline future activity in developing pixel tracking systems for high luminosity particle physics experiments.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s22228722

References

2022 - Recent progress in diamond radiation detectors [Crossref]
2021 - Diamond Detectors for Radiotherapy X-Ray Small Beam Dosimetry [Crossref]
2021 - Properties of diamond-based neutron detectors operated in harsh environments [Crossref]
2021 - Progress in semiconductor diamond photodetectors and MEMS sensors [Crossref]
2019 - Recent progress in solar-blind deep-ultraviolet photodetectors based on inorganic ultrawide bandgap semiconductors [Crossref]
2021 - Charge transport in single crystal CVD diamond studied at high temperatures [Crossref]