The CMS Precision Proton Spectrometer timing system - performance in Run 2, future upgrades and sensor radiation hardness studies

At a Glance

Metadata	Details
Publication Date	2020-05-21
Journal	Journal of Instrumentation
Authors	E. Bossini
Institutions	European Organization for Nuclear Research
Citations	9
Analysis	Full AI Review Included

Technical Documentation & Analysis: scCVD Diamond for High-Precision Timing Detectors

Reference Paper: E. Bossini, on behalf of CMS and TOTEM collaborations, The CMS Precision Proton Spectrometer timing system: performance in Run 2, future upgrades and sensor radiation hardness studies (arXiv:2004.11068v1)

Executive Summary

This research confirms the critical role of high-purity Single Crystal Chemical Vapor Deposition (scCVD) diamond in achieving ultra-fast timing resolution for High Energy Physics (HEP) applications, specifically the CMS Precision Proton Spectrometer (PPS) at the LHC.

Material Validation: Ultrapure scCVD diamond is the proven material choice for timing detectors operating in the highly non-uniform, high-radiation environment of the LHC Roman Pots (RPs).
Radiation Hardness: Sensors successfully sustained peak integrated fluences up to 5 * 10¹⁵ protons/cm², confirming diamond’s superior radiation tolerance compared to silicon alternatives.
Innovative Architecture: The novel Double Diamond (DD) architecture, utilizing two SCD crystals connected to a single channel, achieved a 1.7x improvement in time resolution over the Single Diamond (SD) design.
Achieved Resolution: A single DD plane demonstrated a time resolution of approximately 50 ps under nominal test beam conditions, validating the sensor and front-end design.
Future Goals: The Run 3 upgrade aims for an ultimate sector timing resolution better than 30 ps by increasing the number of DD layers per sector from 4 to 8.
Custom Requirements: Successful implementation relies on custom-sized (4.5x4.5 mm²), precisely segmented, 500 µm thick SCD wafers with specialized metalization for low-capacitance bonding.

Technical Specifications

The following hard data points were extracted from the analysis of the PPS timing system components and performance metrics.

Parameter	Value	Unit	Context
Sensor Material	scCVD Diamond	N/A	High purity, radiation-hard timing sensor
Sensor Thickness	500	µm	Required thickness for timing planes
Individual Crystal Area	4.5 x 4.5	mm²	Dimensions of SCD wafers used in SD/DD architectures
Strip Separation	100	µm	Segmentation pitch on the top electrode face
Edge Clearance	150	µm	Clearance area from crystal edges
Peak Integrated Fluence	5 * 10¹⁵	protons/cm²	Maximum radiation dose sustained in the near-beam region
Nominal Bias Voltage	~500	V	Required operating voltage in vacuum
Single DD Plane Resolution	~50	ps	Achieved resolution in test beam (nominal conditions)
Run 3 Target Resolution	< 30	ps	Ultimate goal for timing resolution per sector
Pre-Amplifier Input Capacitance	0.2 to 2	pF	Dominated by strip capacitance
Parasitic Capacitance (Bonding)	~0.2	pF	Achieved using 0.25 µm bonding wire diameter
HPTDC Binning Resolution	25	ps	Nominal resolution of the Time to Digital Converter

Key Methodologies

The successful deployment and performance of the PPS timing system rely on highly specialized material processing and electronic integration techniques.

Material Specification: Use of ultra-high purity Single Crystal CVD (scCVD) diamond to maximize charge collection distance (CCD) and intrinsic radiation hardness.
Custom Electrode Patterning: Segmentation of the top diamond face into strips (100 µm separation) via metalization (e.g., Ti/Pt/Au stack), with the bottom face serving as a single HV pad.
Double Diamond (DD) Architecture: Implementation of a novel design where two SCD crystals (500 µm thick) are glued back-to-back on a hybrid board, connecting corresponding strips to the same pre-amplification channel to maximize signal-to-noise ratio.
Low-Capacitance Bonding: Direct connection of sensor strips to the pre-amplifier input using fine bonding wires (0.25 µm diameter) to minimize parasitic capacitance (estimated ~0.2 pF).
Vacuum Operation & Discharge Mitigation: Application of a special coating to sensitive areas of the detector and electronics to reduce discharge probability while operating at high bias voltage (~500 V) in the LHC primary vacuum.
Time Walk Correction: Utilization of the NINO discriminator chip coupled with the HPTDC to measure the Time Over Threshold (TOT), which is then used offline to correct for signal amplitude fluctuations (time walk effect).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced scCVD diamond materials required to replicate, upgrade, and extend the performance demonstrated by the CMS PPS timing system. Our expertise in custom dimensions, high-purity growth, and specialized metalization directly addresses the stringent requirements of HEP timing detectors.

Applicable Materials

To achieve the required timing resolution and radiation hardness, the following 6CCVD material is recommended:

Optical Grade Single Crystal Diamond (SCD): This material offers the highest purity and lowest defect density, ensuring maximum charge collection efficiency (CCE) and minimal intrinsic noise, which is essential for achieving the target < 30 ps resolution.

Customization Potential

The PPS project requires highly specific material processing that aligns perfectly with 6CCVD’s core capabilities.

Requirement from Research Paper	6CCVD Solution & Capability	Technical Advantage
Precise Thickness (500 µm)	Custom Thickness Control (SCD up to 500 µm)	We guarantee tight thickness tolerances, critical for maintaining the precise alignment and electrical characteristics of the Double Diamond (DD) architecture.
Custom Dimensions (4.5x4.5 mm²)	High-Precision Laser Cutting	6CCVD provides custom laser cutting services to produce the exact small-area wafers required for integration into the Roman Pot (RP) mechanics.
Segmented Electrodes (100 µm pitch)	Advanced Photolithography & Patterning	We offer custom patterning services to create the required strip segmentation (100 µm separation) and single-pad back electrode geometry.
Multi-Layer Metalization	In-House Metalization (Au, Pt, Pd, Ti, W, Cu)	We can deposit the necessary multi-layer metal stacks (e.g., Ti/Pt/Au) optimized for low-capacitance wire bonding and reliable operation at high bias voltages (~500 V) in vacuum.
Surface Finish	Ultra-Low Roughness Polishing (Ra < 1 nm)	Superior surface quality is crucial for robust metal adhesion and minimizing leakage current and discharge risk in high-voltage, vacuum environments.
Volume & Scale	Wafers up to 125 mm (PCD) / Custom SCD Plates	While this project uses small SCD pieces, 6CCVD can scale production for future large-area or high-volume detector arrays.

Engineering Support

6CCVD’s in-house PhD team specializes in optimizing CVD diamond properties for extreme environments. We offer comprehensive engineering support for projects requiring:

Radiation Damage Mitigation: Consultation on material selection and processing to ensure long-term stability under fluences exceeding 5 * 10¹⁵ protons/cm².
Signal Optimization: Assistance with material purity and surface preparation to maximize charge collection efficiency (CCE) and minimize noise for ultra-fast timing applications.
Custom Integration: Support for defining optimal metalization schemes and surface finishes necessary for hybrid board integration and vacuum operation.

For custom specifications or material consultation regarding High Energy Physics Timing Detectors, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Central exclusive processes can be studied in CMS by combining the information of the central detector with the Precision Proton Spectrometer (PPS). PPS detectors, placed symmetrically at more than 200 m from the interaction point, can detect the scattered protons that survive the interaction. PPS has taken data at high luminosity while fully integrated in the CMS experiment. The total amount of collected data corresponds to more than 100 fb$^{-1}$ during the LHC Run 2. PPS consists of 3D silicon tracking stations as well as timing detectors that measure both the position and direction of protons and their time-of-flight with high precision. The detectors are hosted in special movable vacuum chambers, the Roman Pots, which are placed in the primary vacuum of the LHC beam pipe. The sensors reach a distance of few mm from the beam. Detectors have to operate in vacuum and must be able to sustain highly non-uniform irradiation: sensors used in Run 2 have accumulated an integrated dose with a local peak of $\sim 5 \cdot 10^{15}$ protons/cm$^2$. The timing system is made with high purity scCVD diamond sensors. A new architecture with two diamond crystals read out in parallel by the same electronic channel has been used to enhance the detector performance. In this paper, after a general overview of the PPS detector, we describe the timing system in detail. The sensor and the dedicated amplification chain are described, together with the signal digitization technique. Performance of the detector in Run 2 is reported. Recently the sensors used in Run 2 have been tested for efficiency and timing performance in a dedicated test beam at DESY. Preliminary results on radiation damage are reported. Important upgrades of the timing system are ongoing for the LHC Run 3, with the goal of reaching an ultimate timing resolution better than 30 ps; they are also discussed here.

Tech Support

Original Source

DOI: https://doi.org/10.1088/1748-0221/15/05/c05054