Diamond Detectors for Timing Measurements in High Energy Physics

At a Glance

Metadata	Details
Publication Date	2020-07-14
Journal	Frontiers in Physics
Authors	E. Bossini, N. Minafra
Institutions	University of Kansas, European Organization for Nuclear Research
Citations	29
Analysis	Full AI Review Included

Executive Summary

This review details the engineering and performance of single-crystal Chemical Vapor Deposition (scCVD) diamond detectors optimized for fast timing measurements in High Energy Physics (HEP).

Superior Material Properties: Diamond is preferred over silicon for fast timing in harsh environments due to its wide band-gap (5.47 eV), high carrier mobility (up to 4,551 cm²/Vs), and exceptional radiation hardness (stable up to ~10¹⁶ protons/cm²).
Fast Signal Generation: The high internal electric fields achievable in diamond allow carriers to reach saturation velocity quickly (~10⁷ cm/s), resulting in transit times on the order of a few nanoseconds and fast signal rise times (~1.7 ns).
Timing Precision Achieved: The technology demonstrates excellent time resolution on Minimum Ionizing Particles (MIPs), achieving precision better than 100 ps (HADES START) and reaching a state-of-the-art ~50 ps using the Double Diamond (DD) architecture (CMS/TOTEM PPS).
Front-End Optimization Strategy: Optimal timing performance relies on minimizing electronic noise (jitter) by using high input impedance amplifiers (few kΩ) placed millimeters from the sensor to drastically reduce parasitic capacitance.
Key HEP Deployments: Diamond detectors are operational in the HADES START detector (GSI) for Time Of Flight (TOF) measurements and the CMS/TOTEM Proton Precision Spectrometer (PPS) at CERN for 4D tracking and pile-up discrimination.

Technical Specifications

Parameter	Value	Unit	Context
Band-gap (E_g)	5.47	eV	Diamond (at 300 K)
Breakdown Field (E_b)	> 10⁷	V/cm	Diamond
Relative Permittivity (ε_r)	5.7	-	Diamond
Resistivity (ρ)	> 10¹⁵	Ω cm	Diamond
Density (ρ)	3.52	g/cm³	Diamond
e-h Pair Creation Energy (E_eh)	13	eV	Diamond
MIP Charge Generated (N/µm)	36	e-h pairs/µm	Diamond (vs 89 for Silicon)
Electron Mobility (µ_e)	4,551	cm²/Vs	High-purity scCVD
Hole Saturation Velocity (V_sat,h)	~1.6 · 10⁷	cm/s	Diamond
Displacement Threshold Energy (T_d)	37.5-47.6	eV	Diamond (High radiation resistance)
Typical Sensor Thickness (d)	300 to 500	µm	scCVD used in HEP
HADES Timing Resolution (SD)	91	ps	2.95 GeV Protons
CMS/TOTEM Timing Resolution (DD)	~50	ps	Single plane, after deconvolution
Radiation Tolerance (Tested)	~10¹⁶	protons/cm²	800 MeV and 24 GeV protons
Target RC Time Constant	~1	ns	Front-end optimization rule of thumb

Key Methodologies

1. Crystal Production (scCVD)

Technique: Chemical Vapor Deposition (CVD) is used to grow high-purity synthetic diamonds.
Conditions: Low temperature (less than 1,000 °C) and low pressure (~0.1 bar).
Precursors: Mixture of methane (CH₄), molecular hydrogen (H₂), and optionally oxygen compounds.
Purity Control: Atomic hydrogen is continuously generated to etch non-diamond bonded materials (graphite/amorphous carbon) from the surface.
Substrate: Single-crystal CVD (scCVD) diamond is used as a substrate to ensure high purity and single-crystal structure, though this limits crystal size (max 5 x 5 mm²).

2. Electrode and Sensor Architecture

Ohmic Contact: Electrodes are formed by metalization (e.g., Cr-Au or Ti-W alloy) followed by annealing to ensure an ohmic contact between the metal and the diamond.
Segmentation: Planar sensors are segmented into pads or strips using masks or lithographic processes during metalization.
Double Diamond (DD) Architecture: Two identical diamond crystals are glued onto opposite sides of a hybrid board, with corresponding pads connected to the same pre-amplification channel. This doubles the collected charge (improving SNR) while maintaining the same electronic noise level.

3. Front-End Electronics Design

Goal: Achieve a high Signal-to-Noise Ratio (SNR) and a fast signal rise time (low jitter).
Amplifier Placement: The first stage of amplification (based on discrete SiGe:C transistors) is placed within a few millimeters of the sensor to minimize parasitic capacitance (C_p).
Input Impedance: High input impedance (few kΩ) is used to integrate the charge generated by the diamond, optimizing the RC time constant to approximately 1 ns.
Bias Voltage: Detectors are operated with high bias voltage (up to 500 V) to ensure carrier drift velocity saturation (fastest signal).

4. Signal Digitization and Timing Extraction

Digitization Methods:
- Sampling: Use of fast digitizers (e.g., SAMPIC) capable of sampling up to 10 GSa/s for high-fidelity waveform capture.
- Discriminator/TDC: Use of ultra-fast discriminators (e.g., NINO chip) coupled with high-resolution TDCs (e.g., TRB3, HPTDC, resolution better than 10 ps) for high-rate applications.
Time Walk Correction: Algorithms are applied offline or implemented in hardware to remove the dependence of the measured time on the signal amplitude (due to Landau fluctuations in energy deposition).
- Constant Fraction Discriminator (CFD): Measures the time of zero crossing of a shaped, attenuated, and delayed signal.
- Time Over Threshold (TOT): Measures the duration the signal stays above a threshold, which is correlated to the total charge/amplitude, allowing for correction.

Commercial Applications

The unique combination of speed, radiation hardness, and thermal stability makes scCVD diamond detectors ideal for extreme environments and high-precision timing requirements:

High Energy Physics (HEP):
- 4D Tracking: Providing both spatial and temporal coordinates for particle trajectories (e.g., NA62 Gigatraker).
- Pile-up Mitigation: Discriminating between multiple interactions occurring in the same bunch crossing at high-luminosity colliders (e.g., CMS/TOTEM PPS at LHC).
- Time Of Flight (TOF): Precise particle identification based on arrival time (e.g., HADES START detector).
- Future Upgrades: Essential components for timing detectors in the ATLAS and CMS Phase-II upgrades (HL-LHC era).
High Radiation Environments:
- Monitoring and detection in nuclear reactors, fusion experiments, and high-intensity beam lines where silicon sensors would quickly degrade.
Medical Applications:
- High-rate particle counting and timing for beam monitoring in particle therapy (e.g., proton or heavy ion therapy).
- Potential use in fast timing medical imaging technologies.
General Scientific Instrumentation:
- Any application requiring ultra-fast, robust particle detection, such as neutron detection or high-speed photon counting.

View Original Abstract

Timing detectors are a well established part of High Energy Physics experimental instrumentation. The choice of sensors with fast (less than 10 ns) and precise (better than 100 ps) signals is an essential part of the design of a timing detector, together with radiation resistance considerations. Single crystal diamond sensors are one of the most promising technologies in this field. In this paper, the main characteristics that make single diamond crystal sensors ideal for timing applications will be described and an introduction to the design of fast front-end electronics will be given. Finally, two examples of diamond timing detectors used in High Energy Physics, the START detector of HADES and the TOTEM/CMS timing detector, will be discussed.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2020.00248

References

2019 - The proton timing system of the TOTEM experiment at LHC [Crossref]
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1990 - Nucleation of diamond crystals [Crossref]
2009 - Diamond-metal contacts: interface barriers and real-time characterization [Crossref]
2001 - The interface diffusion and reaction between Cr layer and diamond particle during metallization [Crossref]
2011 - Laser graphitization for polarization of diamond sensors [Crossref]
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