VELO Upgrade Research Articles

The LHCb VELO Upgrade module construction

The LHCb detector has undergone a major upgrade for LHC Run 3. This Upgrade I detector facilitates operation at higher luminosity and utilises full-detector information at the LHC collision rate, critically including the use of vertex information. A new vertex locator system, the VELO Upgrade, has been constructed. The core element of the new VELO are the double-sided pixelated hybrid silicon detector modules which operate in vacuum close to the LHC beam in a high radiation environment. The construction and quality assurance tests of these modules are described in this paper. The modules incorporate 200 μm thick, n-on-p silicon sensors bump-bonded to 130 nm technology ASICs. These are attached with high precision to a silicon microchannel substrate that uses evaporative CO2 cooling. The ASICs are controlled and read out with flexible printed circuits that are glued to the substrate and wire-bonded to the chips. The mechanical support of the module is given by a carbon fibre plate, two carbon fibre rods and an aluminium plate. The sensor attachment was achieved with an average precision of 21 μm, more than 99.5% of all pixels are fully functional, and a thermal figure of merit of 3 Kcm2W-1 was achieved. The production of the modules was successfully completed in 2021, with the final assembly and installation completed in time for data taking in 2022.

The LHCb VELO Upgrade II: Design and development of the readout electronics

In 2032–2034, the LHCb will be upgraded to efficiently explore the new HL-LHC conditions. The increased number of interactions per event and radiation level brings several challenges including high non-uniform radiation doses (up to 6×1016MeVneq/cm2), operation under vacuum (power dissipation below 1.5 W/cm2), high data rates (up to order of 100 Gbps/ASIC) and ASIC timing resolution (30 ps or better). This proceedings describes the requirements for two different upgrade scenarios (different pixel pitch, radiation doses and material budget while keeping the same scattering terms), application specific integrated circuit status to readout the sensor pixels (PicoPix and IGNITE), electronic integration (Anisotropic Conductive Bonding) and data transmission (FALAPHEL). In summary, the open challenges mentioned before are being addressed in the current R&D phase and the most suitable candidate solutions will be consolidated in the technical design report in the coming years.

SystemC framework for architecture modelling of electronic systems in future particle detectors

The prototyping cost in advanced technology nodes and the complexity of future detectors require the adoption of a system design approach common in the industry: design space exploration through high-level architectural studies to achieve clear and optimized specifications.This contribution proposes a configurable SystemC framework to simulate the readout chain from the front-end chips to the detector back-end. The model is transaction accurate, includes an event generator, interfaces with real physics events, and provides metrics such as readout efficiency, latency, and average queue occupancy.This contribution details the structure of the framework and describes a case study based on the LHCb VeLo upgrade II.

The Upgrade of LHCb VELO

LHCb physics achievements to date include some of the world’s most precise flavour physics measurements, including the CKM phase γ and the discovery of CP violation in the charm sector. These achievements have been possible thanks to the enormous data samples collected and the performance of the subdetectors, with the efficiency and precision of the silicon vertex detector (VELO) being a major contributor to this success. The experiment has been upgraded to run at higher luminosity, requiring 40 MHz readout. The VELO Upgrade modules are composed of hybrid pixel detectors and electronics circuits glued onto a cooling substrate, made of thin silicon plates with embedded micro-channels, allowing for the circulation of evaporative CO 2 . The detectors are located in vacuum, separated from the beam by a thin aluminium box. The upgraded VELO is composed of 52 modules placed along the beam axis divided into two retractable halves. The design, production, installation and commissioning of the VELO Upgrade system is presented together with test results. • LHCb experiment relies on the VELO detector for accurate vertex reconstruction. • The VELO Upgrade is the brand new silicon pixel tracker of LHCb. • Detector comprises 52 double sided modules with 4 pixel sensors each. • Micro-channels circulating evaporative CO 2 allows for efficient heat dissipation. • Modules Electrical and thermal performance validated in operation conditions.

Spatial resolution and efficiency of prototype sensors for the LHCb VELO Upgrade

A comprehensive study of the spatial resolution and detection efficiency of sensor prototypes developed for the LHCb VELO upgrade is presented. Data samples were collected at the CERN SPS H8 beam line using a hadron mixture of protons and pions with momenta of approximately 180 GeV/c. The sensor performance was characterised using both irradiated and non-irradiated sensors. Irradiated samples were subjected to a maximum fluence of 8 × 1015 1 MeV neq cm-2, of both protons and neutrons. The spatial resolution is measured comparing the detected hits to theposition as predicted by tracks reconstructed by the Timepix3 telescope. The resolution is presented for different applied bias voltages and track angles, sensor thickness and implant size.

Microchannel cooling for the LHCb VELO Upgrade I

The LHCb VELO Upgrade I, currently being installed for the 2022 start of LHC Run 3, uses silicon microchannel coolers with internally circulating bi-phase CO2 for thermal control of hybrid pixel modules operating in vacuum. This is the largest scale application of this technology to date. Production of the microchannel coolers was completed in July 2019 and the assembly into cooling structures was completed in September 2021. This article describes the R&D path supporting the microchannel production and assembly and the motivation for the design choices, together with the achieved fluidic and thermal performance. The Thermal Figure of Merit of the microchannel coolers is measured on the final modules to be between 1.5 and 3.5 K cm2 W−1, depending on glue thickness. The microchannel coolers constitute 18% of the total radiation length of the VELO and less than 2% of the material seen before the second measured point on the tracks. Microchannel cooling is well suited to the VELO implementation due to the uniform mass distribution, close thermal expansion match with the module components and resistance to radiation.

LHCb VELO Timepix3 telescope

The LHCb VELO Timepix3 telescope is a silicon pixel tracking system constructed initially to evaluate the performance of LHCb VELO Upgrade prototypes. The telescope consists of eight hybrid pixel silicon sensor planes equipped with the Timepix3 ASIC . The planes provide excellent charge measurement, timestamping and spatial resolution and the system can function at high track rates. This paper describes the construction of the telescope and its data acquisition system and offline reconstruction software. A timing resolution of 350 ps was obtained for reconstructed tracks. A pointing resolution of better than 2\\mum was determined for the 180 GeV/c mixed hadron beam at the CERN SPS . The telescope has been shown to operate at a rate of 5 million particles s−1⋅ cm−2 without a loss in efficiency.

Physics benchmarks of the VELO upgrade

The LHCb Experiment at the LHC is successfully performing precision measurements primarily in the area of flavour physics. The collaboration is preparing an upgrade that will start taking data in 2021 with a trigger-less readout at five times the current luminosity. The vertex locator has been crucial in the success of the experiment and will continue to be so for the upgrade. It will be replaced by a hybrid pixel detector and this paper discusses the performance benchmarks of the upgraded detector. Despite the challenging experimental environment, the vertex locator will maintain or improve upon its benchmark figures compared to the current detector. Finally the long term plans for LHCb, beyond those of the upgrade currently in preparation, are discussed.

CO2 evaporative cooling: The future for tracking detector thermal management

In the last few years, CO2 evaporative cooling has been one of the favourite technologies chosen for the thermal management of tracking detectors at LHC. ATLAS Insertable B-Layer and CMS Pixel phase 1 upgrade have adopted it and their systems are now operational or under commissioning. The CERN PH-DT team is now merging the lessons learnt on these two systems in order to prepare the design and construction of the cooling systems for the new Upstream Tracker and the Velo upgrade in LHCb, due by 2018. Meanwhile, the preliminary design of the ATLAS and CMS full tracker upgrades is started, and both concepts heavily rely on CO2 evaporative cooling. This paper highlights the performances of the systems now in operation and the challenges to overcome in order to scale them up to the requirements of the future generations of trackers. In particular, it focuses on the conceptual design of a new cooling system suited for the large phase 2 upgrade programmes, which will be validated with the construction of a common prototype in the next years.

The VELO Upgrade

A significant upgrade of the LHCb detector is scheduled to be installed in 2018–2019. Afterwards all sub-detectors will be read out at the LHC bunch crossing frequency of 40 MHz and the trigger will be fully implemented in software. The silicon strip vertex detector will be replaced by a hybrid pixel detector. In these proceedings the following items are discussed: frontend ASIC, data rates, data transmission, cooling, radiation hard sensors, module design and simulated performance.

Digital column readout architectures for hybrid pixel detector readout chips

In this paper, two digital column architectures suitable for sparse readout of data from a pixel matrix in trigger-less applications are presented. Each architecture reads out a pixel matrix of 256 x 256 pixels with a pixel pitch of 55 μm. The first architecture has been implemented in the Timepix3 chip, and this is presented together with initial measurements. Simulation results and measured data are compared. The second architecture has been designed for Velopix, a readout chip planned for the LHCb VELO upgrade. Unlike Timepix3, this has to be tolerant to radiation-induced single-event effects. Results from post-layout simulations are shown with the circuit architectures.

The Timepix Telescope for high performance particle tracking

The Timepix particle tracking telescope has been developed as part of the LHCb VELO Upgrade project, supported by the Medipix Collaboration and the AIDA framework. It is a primary piece of infrastructure for the VELO Upgrade project and is being used for the development of new sensors and front end technologies for several upcoming LHC trackers and vertexing systems. The telescope is designed around the dual capability of the Timepix ASICs to provide information about either the deposited charge or the timing information from tracks traversing the 14×14mm matrix of 55×55μm pixels. The rate of reconstructed tracks available is optimised by taking advantage of the shutter driver readout architecture of the Timepix chip, operated with existing readout systems. Results of tests conducted in the SPS North Area beam facility at CERN show that the telescope typically provides reconstructed track rates during the beam spills of between 3.5 and 7.5kHz, depending on beam conditions. The tracks are time stamped with 1ns resolution with an efficiency of above 98% and provide a pointing resolution at the centre of the telescope of ∼1.6μm. By dropping the time stamping requirement the rate can be increased to ∼15kHz, at the expense of a small increase in background. The telescope infrastructure provides CO2 cooling and a flexible mechanical interface to the device under test, and has been used for a wide range of measurements during the 2011–2012 data taking campaigns.

Production, measurement and simulation of a low mass flex cable for multi gigabit/s readout for the LHCb VELO upgrade

The goal of this project is to examine the feasibility ofdata transmission up to ∼ 5 Gbit/s on a short ( ∼ 60 cm) low massflex cable, for the readout of the upgraded vertex detector (VELO) of theLHCb experiment. They will be in a vacuum and very high radiationenvironment and also partly in the particle acceptance. For the full system1600 readout links will be required. A set of single-ended and differential(edge-coupled) striplines, with a variety of line parameters have beenprototyped using a material specifically tailored for this type ofapplication (Dupont Pyralux AP-plus polyimide). To reduce mass, the totalthickness of the cable is kept to 0.7 mm. We will present measurements ofthe characteristic impedance, insertion and return loss, obtained both fromtime and frequency domain, as well as a comparison with simulations andexpectations. Also the effectiveness of grounded guard traces and the use ofground via holes to reduce crosstalk will be reported. From the measurementswe were also able to extract the material properties such as the dielectricconstant and loss factor up to several GHz. The measurements were done witha Vector Network Analyzer (VNA), TDR/TDT Digital Sampling Oscilloscope,serial PRBS generator and analyzer for eye diagram and CAD tools such asAgilent ADS and ANSYS HFSS simulators.

Performance of a double metal n-on-n and a Czochralski silicon strip detector read out at 40 MHz

The R&D undertaken by the VELO group in order to produce a sensor that satisfies the tight radiation hardness, efficiency, resolution and low material requirements of LHCb has resulted in the choice of an n-on-n double metal layer solution. First, measurements of the performance of the latest prototype and its related front end electronics, designed to function at the LHC speed of 40 MHz, are presented here. In addition, research has been carried out into new materials which could retain good performance in high-radiation environments at and beyond the LHC, and could provide an alternative for a possible VELO upgrade. For the first time, a full size Czochralski silicon detector sample with 50 μ m pitch strips has been irradiated with high energy protons and its performance has been measured in a test beam with 40 MHz electronics. The results of this test will be presented.