Low-Jitter Clock Receivers for Fast Timing Applications.

Precision timing is a key requirement for emerging 4D particle tracking, Positron Emission Tomography (PET), beam and fusion plasma diagnostics, and other systems. Time-to-Digital Converters (TDCs) are commonly used to provide digital estimates of the relative timing between events, but the jitter performance of a TDC can be no better than the performance of the circuits that acquire the pulses and deliver them to the TDC. Several clock receiver and distribution circuits were evaluated, and a differential amplifier with resistive loads driving a pseudo-differential clock distribution network, developed using design guidelines for radiation tolerance and cryogenic compatibility, was fabricated as part of three prototypes: an analog front-end testbed chip for high-precision timing pixel readout, a dedicated TDC evaluation chip, and a Low-Gain Avalanche Detector (LGAD) readout circuit. Based on TDC measurements of the prototypes, we infer that the jitter added by the clock receiver and distribution circuits is less than 2.25 ps-rms. This performance meets the requirements of many future precision timing systems. The clock receiver and on-chip pseudo-differential driver were fabricated in commercial 28-nm CMOS technology and occupy 2288 µm2

Dose rate dependence of TID damage to 65 nm CMOS transistors in X-ray irradiations of the ATLAS ITk Pixel ASIC (ITkPix)

The ATLAS Inner Tracker (ITk) upgrade for the High-Luminosity LHC (HL-LHC) requires a radiation-tolerant pixel readout chip, which must withstand a total ionising dose (TID) of up to 1 Grad. The readout ASIC for the ITk upgrade has been designed by the RD53 collaboration using 65 nm CMOS technology. In order to characterise the radiation tolerance of the chip digital logic, the RD53 ASICs include ring oscillators, which can be used to measure gate delay degradation. Extensive X-ray irradiation studies of the ring oscillators have been performed on the ITk Pixel pre-production readout ASIC, ITkPixV1. A dependence of radiation damage on dose rate has been observed in 65 nm CMOS technology. This paper aims to quantify the dose rate dependence of TID damage to the ITkPix ring oscillators and, therefore, the ITkPix ASIC digital logic. X-ray irradiations at different dose rates between 20 krad/h and 30 Mrad/h are compared. A dose rate dependence is observed, with 2-3 times more damage at the lowest dose rate of 20 krad/h, compared to 4 Mrad/h. The dose rate dependence was also observed to be dependent on transistor size and type

A-Se thin film detector on ITkpix CMOS readout: time-over-threshold tuning and pixel performance analysis

Amorphous Selenium (a-Se) has been extensively studied as a direct conversion detector material for x-ray imaging, and can be readily deposited as uniform layer by thermal evaporation. On the other hand, readout ASICs developed for high-energy physics experiments provide front-end electronics with low noise, high granularity and fast charge readout. Thin film technology to enable large-area, low-cost precision tracking is in turn also of interest to the HEP community. In our initial study presented at SPIE 2023, we verified the fabrication process of the a-Se layer for integration on a CMOS ASIC using the RD53B (ITkpix v.1.0) chip. The hybrid detector concept consists of the pixelated readout ASIC with 50x50µm pixel pitch, a polyimide hole-blocking layer, the active a-Se layer with 15-100 µm thickness, and a gold top electrode layer. In this work, we now present the evaluation of the a-Se detector with an advanced version of the chip, RD53C or ATLAS ITkpix v.2, which provides full time-over-threshold control and functionality and allows operation in self-triggered hit counting mode. We demonstrate the detection of low-energy beta electrons as well as x-rays of energies between 20 and 50 keV with this a-Se/ITkpixv2 assembly. We study the impact of the operation bias voltage of the a-Se layer on the hit rate and signal charge as represented through the time-over-threshold. The charge collection efficiency of the a-Se, as expected, strongly depends on the electric field applied across the layer. This manifests as both higher hit rates when more interactions pass the trigger threshold, as well as increased ToT for larger amounts of collected charge. As this ASIC was originally designed for hybrid silicon pixel detector readout, some operation parameters of the front-end preamplifier circuit were adjusted in data acquisition for more efficient readout of the small charges provided by the thin a-Se layer

Serial powering scheme and performance analysis for the innermost layer (L0) of ATLAS ITk modules

In this study, we powered in series 4 triplets based on the pre-production ATLAS FE chip for HL-LHC. We ensured that the chosen operational parameters were within our theoretical specs and resulted in the stable operation of the modules within the serial power chain. Triplets were also powered in Low Power mode (LP), used to operate the module at a lower current, and their performance was tested without cooling requirements. The performance of the under-shunt and over-voltage protection were also analyzed

Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton–proton collisions at the LHC

This paper presents the method and performance of primary vertex reconstruction in proton–proton collision data recorded by the ATLAS experiment during Run 1 of the LHC. The studies presented focus on data taken during 2012 at a centre-of-mass energy of √s=8 TeV. The performance has been measured as a function of the number of interactions per bunch crossing over a wide range, from one to seventy. The measurement of the position and size of the luminous region and its use as a constraint to improve the primary vertex resolution are discussed. A longitudinal vertex position resolution of about 30μm is achieved for events with high multiplicity of reconstructed tracks. The transverse position resolution is better than 20μm and is dominated by the precision on the size of the luminous region. An analytical model is proposed to describe the primary vertex reconstruction efficiency as a function of the number of interactions per bunch crossing and of the longitudinal size of the luminous region. Agreement between the data and the predictions of this model is better than 3% up to seventy interactions per bunch crossing

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

Qualification of irradiated 3D pixel sensors produced by FBK for the pre-production of the ATLAS ITk detector

To be ready for the challenging conditions of the High Luminosity phase of the LHC accelerator at CERN, the ATLAS Inner Detector will be completely replaced with a new all-silicon Inner Tracker, the ITk. Sensors in the innermost layer will be exposed to a fluence up to 1.9⋅10¹⁶~nₑq/cm² (considering a safety factor of 1.5) at the half of the HL-LHC program, after which it is scheduled to be replaced together will the full innermost system. Pixel sensors with 3D technology have been chosen to instrument it due to their radiation hardness. Sensors with 25x100~μm² pixel pitch will be used in the central region of the innermost layer (barrel) while sensors with a pitch of 50x50~μm² will instrument its two side regions (end-caps). The Fondazione Bruno Kessler (FBK) has been chosen as one of the two vendors for the production of these sensors. This paper will present the performance of 3D pre-production sensors with both pixel pitches produced by FBK measured in test beams with devices irradiated up to and beyond the sensor end-of-life fluence

The ABC130 barrel module prototyping programme for the ATLAS strip tracker

For the Phase-II Upgrade of the ATLAS Detector, its Inner Detector, consisting of silicon pixel, silicon strip and transition radiation sub-detectors, will be replaced with an all new 100 % silicon tracker, composed of a pixel tracker at inner radii and a strip tracker at outer radii. The future ATLAS strip tracker will include 11,000 silicon sensor modules in the central region (barrel) and 7,000 modules in the forward region (end-caps), which are foreseen to be constructed over a period of 3.5 years. The construction of each module consists of a series of assembly and quality control steps, which were engineered to be identical for all production sites. In order to develop the tooling and procedures for assembly and testing of these modules, two series of major prototyping programs were conducted: an early program using readout chips designed using a 250 nm fabrication process (ABCN-25) and a subsequent program using a follow-up chip set made using 130 nm processing (ABC130 and HCC130 chips). This second generation of readout chips was used for an extensive prototyping program that produced around 100 barrel-type modules and contributed significantly to the development of the final module layout. This paper gives an overview of the components used in ABC130 barrel modules, their assembly procedure and findings resulting from their tests.Comment: 82 pages, 66 figure