The ATLAS detector as installed in its experimental cavern at point 1 at CERN is described in this paper. A brief overview of the expected performance of the detector when the Large Hadron Collider begins operation is also presented.
The International School for Advanced Studies (SISSA) was founded in 1978 and was the first institution in Italy to promote post-graduate courses leading to a Doctor Philosophiae (or PhD) degree. A centre of excellence among Italian and international universities, the school has around 65 teachers, 100 post docs and 245 PhD students, and is located in Trieste, in a campus of more than 10 hectares with wonderful views over the Gulf of Trieste.
SISSA hosts a very high-ranking, large and multidisciplinary scientific research output. The scientific papers produced by its researchers are published in high impact factor, well-known international journals, and in many cases in the world's most prestigious scientific journals such as Nature and Science. Over 900 students have so far started their careers in the field of mathematics, physics and neuroscience research at SISSA.
ISSN: 1748-0221
Journal of Instrumentation (JINST) is a multidisciplinary, peer-reviewed and online-only journal designed to support the needs of this expanding community. JINST was created jointly by the International School of Advanced Studies (SISSA) and IOP Publishing.
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The CMS Collaboration et al 2008 JINST 3 S08004
The Compact Muon Solenoid (CMS) detector is described. The detector operates at the Large Hadron Collider (LHC) at CERN. It was conceived to study proton-proton (and lead-lead) collisions at a centre-of-mass energy of 14 TeV (5.5 TeV nucleon-nucleon) and at luminosities up to 1034 cm−2 s−1 (1027 cm−2 s−1). At the core of the CMS detector sits a high-magnetic-field and large-bore superconducting solenoid surrounding an all-silicon pixel and strip tracker, a lead-tungstate scintillating-crystals electromagnetic calorimeter, and a brass-scintillator sampling hadron calorimeter. The iron yoke of the flux-return is instrumented with four stations of muon detectors covering most of the 4π solid angle. Forward sampling calorimeters extend the pseudorapidity coverage to high values (|η| ⩽ 5) assuring very good hermeticity. The overall dimensions of the CMS detector are a length of 21.6 m, a diameter of 14.6 m and a total weight of 12500 t.
The LHCb Collaboration et al 2008 JINST 3 S08005
The LHCb experiment is dedicated to precision measurements of CP violation and rare decays of B hadrons at the Large Hadron Collider (LHC) at CERN (Geneva). The initial configuration and expected performance of the detector and associated systems, as established by test beam measurements and simulation studies, is described.
Lyndon Evans and Philip Bryant 2008 JINST 3 S08001
The Large Hadron Collider (LHC) at CERN near Geneva is the world's newest and most powerful tool for Particle Physics research. It is designed to collide proton beams with a centre-of-mass energy of 14 TeV and an unprecedented luminosity of 1034 cm−2 s−1. It can also collide heavy (Pb) ions with an energy of 2.8 TeV per nucleon and a peak luminosity of 1027 cm−2 s−1. In this paper, the machine design is described.
K. Batani 2023 JINST 18 C09012
Recent experiments with high-intensity lasers have shown record production of α-particles by irradiating boron-hydrogen targets. This opened the way to completely new studies on proton-boron fusion with multiple goals:
i) studies related to nuclear fusion. The proton-boron fusion reaction produces 3 α-particles and releases a large energy. It is considered an interesting alternative to deuterium-tritium fusion because it produces no neutrons, therefore no activation and radioactive wastes.
ii) generation of novel laser-driven α-particle sources. Laser-driven α-particle sources are promising for their potential high brightness while remaining compact. They could be used for multidisciplinary applications, including medical ones.
The COST Action CA21128 — PROBONO (PROton BOron Nuclear fusion: from energy production to medical applicatiOns) is the first international programme which aims at understanding the physics involved in laser-driven pB fusion, including the study of Equation of State of boron and boron compounds. Action's goals are to facilitate access to experimental infrastructures, maximize production of new knowledge, boost the career of young researchers by fostering opportunities for training, and finally interconnect researchers across countries building a well-organized community focused on pB research.
K.M. Black et al 2024 JINST 19 T02015
A multi-TeV muon collider offers a spectacular opportunity in the direct exploration of the energy frontier. Offering a combination of unprecedented energy collisions in a comparatively clean leptonic environment, a high energy muon collider has the unique potential to provide both precision measurements and the highest energy reach in one machine that cannot be paralleled by any currently available technology. The topic generated a lot of excitement in Snowmass meetings and continues to attract a large number of supporters, including many from the early career community. In light of this very strong interest within the US particle physics community, Snowmass Energy, Theory and Accelerator Frontiers created a cross-frontier Muon Collider Forum in November of 2020. The Forum has been meeting on a monthly basis and organized several topical workshops dedicated to physics, accelerator technology, and detector R&D. Findings of the Forum are summarized in this report.
X. Llopart et al 2022 JINST 17 C01044
Timepix4 is a 24.7 × 30.0 mm2 hybrid pixel detector readout ASIC which has been designed to permit detector tiling on 4 sides. It consists of 448 × 512 pixels which can be bump bonded to a sensor with square pixels at a pitch of 55 µm. Like its predecessor, Timepix3, it can operate in data driven mode sending out information (Time of Arrival, ToA and Time over Threshold, ToT) only when a pixel has a hit above a pre-defined and programmable threshold. In this mode hits can be tagged to a time bin of <200 ps and Timepix4 can record hits correctly at incoming rates of ∼3.6 MHz/mm2/s. In photon counting (or frame-based) mode it can count incoming hits at rates of up to 5 GHz/mm2/s. In both modes data is output via between 2 and 16 serializers each running at a programmable data bandwidth of between 40 Mbps and 10 Gbps. The specifications, architecture and circuit implementation are described along with first electrical measurements and measurements with radioactive sources. In photon counting mode X-ray images have been taken at a threshold of 650 e− (with <10 masked pixels). In data driven mode images were taken of ToA/ToT data using a 90Sr source at a threshold of 800 e− (with ∼120 masked pixels).
M.J. Christensen and T. Richter 2020 JINST 15 T09005
User Datagram Protocol (UDP) is a commonly used protocol for data transmission in small embedded systems. UDP as such is unreliable and packet losses can occur. The achievable data rates can suffer if optimal packet sizes are not used. The alternative, Transmission Control Protocol (TCP) guarantees the ordered delivery of data and automatically adjusts transmission to match the capability of the transmission link. Nevertheless UDP is often favored over TCP due to its simplicity, small memory and instruction footprints. Both UDP and TCP are implemented in all larger operating systems and commercial embedded frameworks. In addition UDP also supported on a variety of small hardware platforms such as Digital Signal Processors (DSP) Field Programmable Gate Arrays (FPGA). This is not so common for TCP. This paper describes how high speed UDP based data transmission with very low packet error ratios was achieved. The near-reliable communications link is used in a data acquisition (DAQ) system for the next generation of extremely intense neutron source, European Spallation Source. This paper presents measurements of UDP performance and reliability as achieved by employing several optimizations. The measurements were performed on Xeon E5 based CentOS (Linux) servers. The measured data rates are very close to the 10 Gb/s line rate, and zero packet loss was achieved. The performance was obtained utilizing a single processor core as transmitter and a single core as receiver. The results show that support for transmitting large data packets is a key parameter for good performance. Optimizations for throughput are: MTU, packet sizes, tuning Linux kernel parameters, thread affinity, core locality and efficient timers.
B. Acar et al 2024 JINST 19 P04015
This paper describes the experience with the calibration, reconstruction and evaluation of the timing capabilities of the CMS HGCAL prototype in the beam tests in 2018. The calibration procedure includes multiple steps and corrections ranging from tens of nanoseconds to a few hundred picoseconds. The timing performance is studied using signals from positron beam particles with energies between 20 GeV and 300 GeV. The performance is studied as a function of particle energy against an external timing reference as well as standalone by comparing the two different halves of the prototype. The timing resolution is found to be 60 ps for single-channel measurements and better than 20 ps for full showers at the highest energies, setting excellent perspectives for the HGCAL calorimeter performance at the HL-LHC.
The ALICE Collaboration et al 2008 JINST 3 S08002
ALICE (A Large Ion Collider Experiment) is a general-purpose, heavy-ion detector at the CERN LHC which focuses on QCD, the strong-interaction sector of the Standard Model. It is designed to address the physics of strongly interacting matter and the quark-gluon plasma at extreme values of energy density and temperature in nucleus-nucleus collisions. Besides running with Pb ions, the physics programme includes collisions with lighter ions, lower energy running and dedicated proton-nucleus runs. ALICE will also take data with proton beams at the top LHC energy to collect reference data for the heavy-ion programme and to address several QCD topics for which ALICE is complementary to the other LHC detectors. The ALICE detector has been built by a collaboration including currently over 1000 physicists and engineers from 105 Institutes in 30 countries. Its overall dimensions are 16 × 16 × 26 m3 with a total weight of approximately 10 000 t. The experiment consists of 18 different detector systems each with its own specific technology choice and design constraints, driven both by the physics requirements and the experimental conditions expected at LHC. The most stringent design constraint is to cope with the extreme particle multiplicity anticipated in central Pb-Pb collisions. The different subsystems were optimized to provide high-momentum resolution as well as excellent Particle Identification (PID) over a broad range in momentum, up to the highest multiplicities predicted for LHC. This will allow for comprehensive studies of hadrons, electrons, muons, and photons produced in the collision of heavy nuclei. Most detector systems are scheduled to be installed and ready for data taking by mid-2008 when the LHC is scheduled to start operation, with the exception of parts of the Photon Spectrometer (PHOS), Transition Radiation Detector (TRD) and Electro Magnetic Calorimeter (EMCal). These detectors will be completed for the high-luminosity ion run expected in 2010. This paper describes in detail the detector components as installed for the first data taking in the summer of 2008.
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P. Amedo et al 2024 JINST 19 C05001
Argon gas doped with 1% wavelength-shifter (CF4) has been employed in an optical time projection chamber (OTPC) to image cosmic radiation. We present results obtained during the system commissioning, performed with two stacked glass thick gaseous electron multipliers (THGEMs) and an electron-multiplying charge coupled device (EMCCD camera) at 1 bar. Preliminary estimates indicate that the combined optical gain was of the order of 106 (ph/e), producing sharp and high-contrast raw images without resorting to any filtering or post-processing. A first assessment of the impact of pressurization showed no change in the attainable gains when operating at 1.5 bar.
Yoshiharu Mori et al 2024 JINST 19 P05001
A new scheme of low energy negative muon source with muon catalyzed fusion (MuCF) is described. In the MuCF reaction process, muonic helium ions (μHe+) are created. By re-accelerating and stripping μHe+ ions, a low emittance negative muon beam is generated.
G.X. Chen et al 2024 JINST 19 P05002
A compact accelerator-driven neutron source is proposed at Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-Sen University, called Sun Yat-Sen University Proton Accelerator Facility (SYSU-PAFA). The proton accelerator is composed of a proton electron cyclotron resonance source, a four-vane radio frequency quadrupole (RFQ), and an alternative phase focusing drift tube linac (APF-DTL). It can accelerate 10 mA proton beam to 8 MeV. Due to the high current, beam matching is particularly important. In order to achieve beam matching between various components, beam transport sections are needed. The beam transport line is divided into three segments. The Low Energy Beam Transport (LEBT) ensures that the beam parameters are matched before entering the RFQ. The Medium Energy Beam Transport (MEBT) segment efficiently transfers the beam between the RFQ and DTL. The High Energy Beam Transport (HEBT) focuses on transporting the beam to the targets. The design goal of beam transport line is as short as possible while ensuring high efficiency of beam transportation. SYSU-PAFA has an overall transmission efficiency of 99%, with optimal transverse matching conditions between beam transport and RFQ or DTL accelerators. The efficient use of solenoids and magnets allows for a compact transmission section, resulting in a total length of 13.6 meters, shorter than most accelerators at the same beam energy. This paper will provide the detailed beam dynamics of the compact accelerator.
Fernando Torales Acosta et al 2024 JINST 19 P05003
Score based generative models are a new class of generative models that have been shown to accurately generate high dimensional calorimeter datasets. Recent advances in generative models have used images with 3D voxels to represent and model complex calorimeter showers. Point clouds, however, are likely a more natural representation of calorimeter showers, particularly in calorimeters with high granularity. Point clouds preserve all of the information of the original simulation, more naturally deal with sparse datasets, and can be implemented with more compact models and data files. In this work, two state-of-the-art score based models are trained on the same set of calorimeter simulation and directly compared.
Wenchang Liu et al 2024 JINST 19 T05001
The large area trigger detector is a key instrument in cosmic ray telescope system. One large area detector, sensitive size 1.3 * 2 m2, is proposed in this paper based on plastic scintillator tiles and wavelength shift optical fibers. Thanks to the wavelength shift fibers coupling to the scintillator tiles, only one photomultiplier tube is used to output the signals for whole large area detector. So this detector is simple and economy. The signal uniformity of this detector is better than 96% over the whole surface including the edges or corners. The detection efficiency of the muon is higher than 95%, and the time resolution is better than 10 ns over the entire detector. These performances are sufficient for the trigger detector in most cosmic ray telescope system.
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Łukasz Kamil Graczykowski et al 2022 JINST 17 C07016
Particle identification (PID) is one of the main strengths of the ALICE experiment at the LHC. It is a crucial ingredient for detailed studies of the strongly interacting matter formed in ultrarelativistic heavy-ion collisions. ALICE provides PID information via various experimental techniques, allowing for the identification of particles over a broad momentum range (from around 100 MeV/c to around 50 GeV/c). The main challenge is how to combine the information from various detectors effectively. Therefore, PID represents a model classification problem, which can be addressed using Machine Learning (ML) solutions. Moreover, the complexity of the detector and richness of the detection techniques make PID an interesting area of research also for the computer science community. In this work, we show the current status of the ML approach to PID in ALICE. We discuss the preliminary work with the Random Forest approach for the LHC Run 2 and a more advanced solution based on Domain Adaptation Neural Networks, including a proposal for its future implementation within the ALICE computing software for the upcoming LHC Run 3.
T. Akiyama et al 2022 JINST 17 C01052
A fast wave interferometer (FWI), which can measure ion mass density, has been developed on DIII-D for its use on future fusion reactors, as well as for the study of ion behavior in current plasma devices. The frequency of the fast waves used for the FWI is around 60 MHz, and require antennas and coaxial cables or waveguides, which, unlike traditional mirror-based optical interferometers, are less susceptible to neutron/gamma-ray radiation and are relatively immune to impurity deposition and erosion as well as alignment issues. The bulk ion density evaluated using FWI show good agreement with that derived from CO2 interferometry within about 15%. When the ion mass density measurement by FWI is combined with an electron density measurement from CO2 interferometry, Zeff measurements are also enabled and are in agreement with those from visible Bremsstrahlung measurements. Additionally, large-bandwidth FWI measurements clearly resolve 10–100 kHz coherent modes and demonstrate its potential as a core fluctuation diagnostic, sensitive to both magnetic and ion density perturbations.
C. Guidi et al 2021 JINST 16 C10004
KM3NeT (Cubic Kilometer Neutrino Telescope) is a research infrastructure that comprises two underwater neutrino detectors located at different sites in the Mediterranean Sea: KM3NeT-Fr (ORCA) (offshore the coast of Toulon, France, at a depth of around 2500 m) and KM3NeT-It (ARCA) (off Capo Passero, Sicily, Italy, at a depth of around 3500 m). The experiment consists of vertical structures, called strings, along which the optical modules are positioned. A hydrophone, located on the base of each string, is used for the reconstruction of the position of the KM3NeT elements with an accuracy of 10 cm. The presence of acoustic sensors in an underwater environment gives the opportunity to detect and study the sound emissions of marine mammals present in the area. The presented work describes the identification programs of the signals emitted by dolphins (clicks and whistles) and sperm whales (clicks) and the results of the analysis of real data collected between spring 2020 and spring 2021.
S. Sharakin and O.I. Ruiz Hernandez 2021 JINST 16 T07013
The Tracking Ultraviolet Set-up (TUS) is the world's first orbital imaging detector of Ultra High Energy Cosmic Rays (UHECR) and it operated in 2016–2017 as part of the scientific equipment of the Lomonosov satellite. The TUS was developed and manufactured as a prototype of the larger project K-EUSO with the main purpose of testing the efficiency of the method for measuring the ultraviolet signal of extensive air shower (EAS) in the Earth's night atmosphere. Despite the low spatial resolution (∼5 × 5 km2 at sea level), several events were recorded which are very similar to EAS as for the signal profile and kinematics. Reconstruction of the parameters of such events is complicated by a short track length, an asymmetry of the image, and an uncertainty in the sensitivity distribution of the TUS channels. An advanced method was developed for the determination of event kinematic parameters including its arrival direction. In the present article, this method is applied for the analysis of 6 EAS-like events recorded by the TUS detector. All events have an out of space arrival direction with zenith angles less than 40°. Remarkably they were found to be over the land rather close to United States airports, which indicates a possible anthropogenic nature of the phenomenon. Detailed analysis revealed a correlation of the reconstructed tracks with direction to airport runways and Very High Frequency (VHF) omnidirectional range stations. The method developed here for reliable reconstruction of kinematic parameters of the track-like events, registered in low spatial resolution, will be useful in future space missions, such as K-EUSO.
V Chepel and H Araújo 2013 JINST 8 R04001
We review the current status of liquid noble gas radiation detectors with energy threshold in the keV range, which are of interest for direct dark matter searches, measurement of coherent neutrino scattering and other low energy particle physics experiments. Emphasis is given to the operation principles and the most important instrumentation aspects of these detectors, principally of those operated in the double-phase mode. Recent technological advances and relevant developments in photon detection and charge readout are discussed in the context of their applicability to those experiments.
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Wenchang Liu et al 2024 JINST 19 T05001
The large area trigger detector is a key instrument in cosmic ray telescope system. One large area detector, sensitive size 1.3 * 2 m2, is proposed in this paper based on plastic scintillator tiles and wavelength shift optical fibers. Thanks to the wavelength shift fibers coupling to the scintillator tiles, only one photomultiplier tube is used to output the signals for whole large area detector. So this detector is simple and economy. The signal uniformity of this detector is better than 96% over the whole surface including the edges or corners. The detection efficiency of the muon is higher than 95%, and the time resolution is better than 10 ns over the entire detector. These performances are sufficient for the trigger detector in most cosmic ray telescope system.
R.R. Raylman et al 2024 JINST 19 P04034
Large arrays of pixelated scintillator potentially have application in economical construction of PET scanners. In this investigation, we constructed and evaluated a detector with an active area of 32.26 × 13.47 cm2. It is based on a 218 × 91 array of 1.4 × 1.4 × 15 mm3 LYSO elements (pitch 1.48 mm). Scintillation light is detected with a 5 × 12 array of silicon photomultipliers (SiPM) arrays. Each array consists of an 8 × 8 array of 3 × 3 mm2 (pitch 3.35 mm) SiPMs. Performance of these devices are enhanced and stabilized by cooling them. Testing revealed that the detector was able to detect 90% of the theoretically detectable 511 keV photons. The resolvability index (a measure of the ability to identify individual detector elements from background) is 0.24 ± 0.04. Additionally, the average energy resolution for the complete detector is 18.3%. These results compare well with those reported for much smaller detector modules.
L. Teruzzi et al 2024 JINST 19 P04035
Instruments based on light obscuration sensors are widely used for measuring the size distribution of insoluble sub-visible particles in liquid suspensions, being fast and suitable for in situ and real-time measurements. Such instruments are typically calibrated by means of reference polystyrene spherical particles with a specific refractive index, which unavoidably leads to systematic errors when determining the size of particles of different materials. In this paper, we propose a reliable and consistent method to overcome this limitation by setting the refractive index value according to the sample, thus achieving an improved particle size distribution (PSD) measurement. An ad hoc, ready-to-use, open source code with a graphical interface able to drive an in-line instrument and obtain a real-time correction to the PSD has been developed. The method has been extensively validated with several oil emulsions characterized by different refractive index values and the results have been compared with an independent optical method. As an example of application, we have adopted this approach for the analysis of dust suspended in meltwater of an ice core from a glacier in the Aosta Valley (Italy). We believe that our approach will strongly improve the accuracy in characterizing liquid suspensions and reduce discrepancies between data obtained with different methods. The code has been made publicly available at:https://instrumentaloptics.fisica.unimi.it/dedalo/ and on the GitHub page of the corresponding author (https://github.com/LucaTeruzzi/DEDALO).
S. Lai et al 2024 JINST 19 P04037
A neural network for software compensation was developed for the highly granular CALICE Analogue Hadronic Calorimeter (AHCAL). The neural network uses spatial and temporal event information from the AHCAL and energy information, which is expected to improve sensitivity to shower development and the neutron fraction of the hadron shower. The neural network method produced a depth-dependent energy weighting and a time-dependent threshold for enhancing energy deposits consistent with the timescale of evaporation neutrons. Additionally, it was observed to learn an energy-weighting indicative of longitudinal leakage correction. In addition, the method produced a linear detector response and outperformed a published control method regarding resolution for every particle energy studied.
A.G. McLean et al 2024 JINST 19 P04038
Low pressure gaseous Time Projection Chambers (TPCs) are a viable technology for directional Dark Matter (DM) searches and have the potential for exploring the parameter space below the neutrino fog [1,2]. Gases like CF4 are advantageous because they contain flourine which is predicted to have heightened elastic scattering rates with a possible Weakly Interacting Massive Particle (WIMP) DM candidate [3,4,5]. The low pressure of CF4 must be maintained, ideally lower than 100 Torr, in order to elongate potential Nuclear Recoil (NR) tracks which allows for improved directional sensitivity and NR/Electron Recoil (ER) discrimination [6]. Recent evidence suggests that He can be added to heavier gases, like CF4, without significantly affecting the length of 12C and 19F recoils due to its lower mass. Such addition of He has the advantage of improving sensitivity to lower mass WIMPs [1]. Simulations can not reliably predict operational stability in these low pressure gas mixtures and thus must be demonstrated experimentally. In this paper we investigate how the addition of He to low pressure CF4 affects the gas gain and energy resolution achieved with a single Thick Gaseous Electron Multiplier (ThGEM).
M. Kovacs et al 2024 JINST 19 C04052
The CMS Tracker Phase-2 Upgrade requires the production of new sensor modules to cope with the requirements of the HL-LHC. The two main building blocks of the Outer Tracker are the Strip-Strip and Pixel-Strip modules. All-together 47'520 hybrid circuits will be produced to construct 8'000 Strip-Strip and 5'880 Pixel-Strip modules. The circuit designs for the mass production were fine-tuned and kick-off batches were manufactured to verify the latest changes in the designs before the series production. This contribution focuses on lessons learned from the prototyping stage, design optimization details for the mass production as well as test results and production yield from the kick-off batches.
Grzegorz Węgrzyn and Robert Szczygieł 2024 JINST 19 C04053
We present the implementation of the indirect voltage measurement using a noise distribution algorithm [1] in the prototype application-specific integrated circuit (ASIC) SMAUG_ND_1 designed in CMOS 28 nm technology. The chip implements the matrix of 7×7 pixels with the size of 68×68 μm. Each pixel contains eight independent comparators implementing the described algorithm and optional correlated-double-sampling method. The paper describes the ASIC architecture and briefly presents preliminary test results and encountered problems.
N. Pino et al 2024 JINST 19 C04054
Liquid Argon (LAr) Time Projection Chambers (TPC) operating in double-phase can detect the nuclear recoils (NR) possibly caused by the elastic scattering of WIMP dark matter particles via light signals from both scintillation and ionization processes. In the scenario of a low-mass WIMP (< 2 GeV/c2), the energy range for the NRs would be below 20 keV, thus making it crucial to characterize the ionization response in LAr TPCs as the lone available detection channel at such low energy. The Recoil Directionality (ReD) project, within the Global Argon Dark Matter Collaboration, aims to measure the ionization yield of a LAr TPC in the recoil energy range of 2–5 keV. The measurement was performed in winter 2023 at the INFN Sezione of Catania and the analysis is ongoing.
Piotr Kaczmarczyk and Piotr Kmon 2024 JINST 19 C04056
In this paper we introduce a prototype Radiation Energy Measuring Integrated Circuit (REMIC) fabricated in a 28 nm CMOS process. The chip operates in a single-photon counting (SPC) mode and contains 100 pixels with a size of 50 μm × 50 μm. It is designed for precise energy measurements using asynchronous analogue-to-digital conversion. The proposed architecture allows both fast signal processing and precise energy measurement of incoming photons to be performed independently in each pixel, occupying a small pixel area. The integrated circuit (IC) has dimensions of 1.1 mm × 1.1 mm and is currently undergoing preliminary measurements. The paper focuses on the methodology used to mitigate process variations in each of the recording channels.
C. Zhang et al 2024 JINST 19 C04059
A series of monolithic High Voltage CMOS (HV-CMOS) pixel sensor prototypes have been developed by the CERN-RD50 CMOS working group for potential use in future high luminosity experiments. The aim is to further improve the performance of HV-CMOS sensors, especially in terms of pixel granularity, timing resolution and radiation tolerance. The evaluation of one of this series, RD50-MPW3, is presented in this contribution, including laboratory and test beam measurements. The design of the latest prototype, RD50-MPW4, which resolves issues found in RD50-MPW3 and implements further improvements, is described.