The CMS Precision Proton Spectrometer timing system - performance in Run 2, future upgrades and sensor radiation hardness studies
At a Glance
Section titled âAt a Glanceâ| 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 |
Executive Summary
Section titled âExecutive SummaryâThis analysis focuses on the development and performance of scCVD diamond timing detectors for the CMS Precision Proton Spectrometer (PPS) at the LHC, designed to measure proton Time-of-Flight (TOF) for Central Exclusive Processes (CEP).
- Core Innovation: The Double Diamond (DD) sensor architecture was developed, connecting two crystals in parallel to a single electronic channel. This design achieved a factor of 1.7 improvement in time resolution compared to the Single Diamond (SD) architecture while maintaining the same channel count.
- Test Beam Performance: A single DD plane demonstrated a time resolution of approximately 50 ps under nominal conditions, meeting the stringent requirements for high-precision timing.
- Run 2 Operational Results: The overall sector resolution during LHC Run 2 (2016-2018) was 90-120 ps. This performance was limited by external factors, including radio-frequency (RF) oscillations, reduced operating voltages (350-400 V instead of 500 V), and non-optimal coupling between the sensor and the NINO discriminator chip.
- Radiation Hardness: Sensors accumulated a peak integrated dose of ~5 * 1015 protons/cm2. Post-Run 2 tests confirmed that efficiency remained high (>95%) even in the most irradiated regions, although localized signal amplitude loss was observed near the beam edge.
- Readout Strategy: The system utilizes the NINO ultra-fast discriminator coupled with the HPTDC (25 ps binning), employing Time Over Threshold (TOT) measurements for crucial offline time-walk corrections.
- Run 3 Upgrade Goal: Significant upgrades are underway, including remote control of front-end low voltage (LV) and the installation of 8 DD layers per sector (up from 4 total layers in Run 2), targeting an ultimate sector resolution better than 30 ps.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Sensor Material | scCVD Diamond | N/A | High purity, single crystal. |
| Sensor Thickness | 500 | ”m | Standard thickness for timing layers. |
| Active Surface Area (Single Crystal) | 4.5 x 4.5 | mm2 | Surface area of individual diamond crystals. |
| Strip Separation | 100 | ”m | Segmentation on the top face. |
| Bias Voltage (Nominal) | ~500 | V | Required operating voltage in vacuum. |
| Bias Voltage (Run 2 Actual) | 350-400 | V | Reduced due to beam-induced discharges. |
| Integrated Dose (Peak) | ~5 * 1015 | protons/cm2 | Accumulated during LHC Run 2 (100 fb-1). |
| Single Plane Resolution (DD, Test Beam) | ~50 | ps | Measured under nominal conditions. |
| Resolution Improvement (DD vs. SD) | 1.7 | Factor | Improvement achieved by Double Diamond architecture. |
| Sector Resolution (Run 2 Operational) | 90-120 | ps | Achieved resolution per sector. |
| Target Sector Resolution (Run 3) | Better than 30 | ps | Ultimate goal for pile-up rejection. |
| NINO Input Charge Range | 0.01-2 | pC | Range for the ultra-fast discriminator. |
| HPTDC Binning | 25 | ps | Nominal resolution of the Time to Digital Converter. |
| Pre-Amplifier Input Capacitance | 0.2-2 | pF | Dominated by strip size and bonding wires. |
Key Methodologies
Section titled âKey MethodologiesâThe PPS timing system relies on specialized scCVD diamond fabrication, a novel sensor architecture, and a dedicated high-speed readout chain optimized for time-walk correction.
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Sensor Material and Fabrication:
- Ultrapure Single Crystal Chemical Vapor Deposition (scCVD) diamonds were selected for their high radiation hardness and fast charge collection time.
- Crystals were segmented via metallization (strips on the top face, single pad on the bottom face for HV) performed at GSI and PRISM.
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Double Diamond (DD) Architecture:
- Two identical scCVD crystals were mounted on opposite sides of a hybrid board.
- Corresponding strips from both crystals were connected in parallel to the same amplification channel.
- This technique effectively doubled the signal amplitude while keeping the noise floor constant (dominated by the pre-amplification input), leading to enhanced timing resolution.
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Front-End Amplification:
- A three-stage amplification chain was used, optimized for the characteristic ~1 ”A triangular current pulse from the diamond.
- The pre-amplification stage utilized a BFP840 SiGe BJT transconductance amplifier in a common emitter configuration, minimizing parasitic capacitance by direct bonding to the sensor strips.
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Signal Digitization and Readout (NINO/HPTDC):
- The NINO chip, an 8-channel ultra-fast fixed-threshold discriminator, was used for signal discrimination.
- The NINO output duration (Time Over Threshold, TOT) was measured by the HPTDC (High Performance Time to Digital Converter).
- TOT is correlated with the collected charge (Q) and used offline to correct for the time-walk effect, which is critical due to large energy release fluctuations in thin detectors.
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Radiation Damage Assessment (DESY Test Beam):
- Irradiated Run 2 sensors were tested at the DESY T24 beam line using a 4.8 GeV electron beam.
- A high-resolution EUDET tracker provided precise position reconstruction (~100 ”m precision).
- The SAMPIC fast sampler (6.4 GSa/s) was used to acquire analog waveforms, allowing detailed mapping of signal amplitude and rise time degradation as a function of position (irradiation dose).
Commercial Applications
Section titled âCommercial ApplicationsâThe technology developed for the CMS PPS, particularly the use of highly radiation-hard, ultra-fast scCVD diamond detectors and associated high-speed electronics, has direct relevance across several high-tech sectors.
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High Energy Physics (HEP) and Nuclear Science:
- Next-generation 4D tracking systems (position and time) for future high-luminosity colliders (e.g., HL-LHC, FCC).
- Beam monitoring and diagnostics in particle accelerators requiring detectors capable of surviving extreme, non-uniform particle fluxes.
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Radiation Hard Electronics and Sensing:
- Dosimetry and radiation monitoring in harsh environments, such as nuclear power facilities, space exploration, and fusion reactors, where silicon-based sensors fail rapidly.
- Development of robust, high-speed front-end electronics (like the NINO/HPTDC chain) for use in high-radiation fields.
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Medical Physics and Imaging:
- Ultra-fast timing detectors for improved spatial resolution in Positron Emission Tomography (PET) and Time-of-Flight PET (TOF-PET).
- Real-time beam monitoring and quality assurance in proton and hadron therapy facilities.
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High-Speed Signal Processing:
- Applications requiring precise time synchronization and digitization (e.g., the SAMPIC fast sampler technology) in fields outside of physics, such as high-speed communications or radar systems.
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.