Valentina Sola

In this paper we report on the timing resolution obtained in a beam test with pions of 180 GeV/c momentum at CERN for the first production of 45 µm thick Ultra-Fast Silicon Detectors (UFSD). UFSD are based on the Low-Gain Avalanche Detector (LGAD) design, employing n-on-p silicon sensors with internal charge multiplication due to the presence of a thin, low-resistivity diffusion layer below the junction. The UFSD used in this test had a pad area of 1.7 mm2. The gain was measured to vary between 5 and 70 depending on the sensor bias voltage. The experimental setup included three UFSD and a fast trigger consisting of a quartz bar readout by a SiPM. The timing resolution was determined by doing Gaussian fits to the time-of-flight of the particles between one or more UFSD and the trigger counter. For a single UFSD the resolution was measured to be 34 ps for a bias voltage of 200 V, and 27 ps for a bias voltage of 230 V. For the combination of 3 UFSD the timing resolution was 20 ps for a bias voltage of 200 V, and 16 ps for a bias voltage of 230 V.

In this paper, we report on the radiation resistance of 50-micron thick LGAD detectors manufactured at the Fondazione Bruno Kessler employing several different doping combinations of the gain layer. LGAD detectors with gain layer doping of Boron, Boron low-diffusion, Gallium, Carbonated Boron and Carbonated Gallium have been designed and successfully produced. These sensors have been exposed to neutron fluences up to $\phi_n \sim 3 \cdot 10^{16}\; n/cm^2$ and to proton fluences up to $\phi_p \sim 9\cdot10^{15}\; p/cm^2$ to test their radiation resistance. The experimental results show that Gallium-doped LGADs are more heavily affected by initial acceptor removal than Boron-doped LGAD, while the presence of Carbon reduces initial acceptor removal both for Gallium and Boron doping. Boron low-diffusion shows a higher radiation resistance than that of standard Boron implant, indicating a dependence of the initial acceptor removal mechanism upon the implant width. This study also demonstrates that proton irradiation is at least twice more effective in producing initial acceptor removal, making proton irradiation far more damaging than neutron irradiation.

In this contribution we will review the progresses toward the construction of a tracking system able to measure the passage of charged particles with a combined precision of ∼10 ps and ∼10 μm, either using a single type of sensor, able to concurrently measure position and time, or a combination of position and time sensors.

This paper presents the principles of operation of Resistive AC-Coupled Silicon Detectors (RSDs) and measurements of the temporal and spatial resolutions using a combined analysis of laser and beam test data. RSDs are a new type of n-in-p silicon sensor based on the Low-Gain Avalanche Diode (LGAD) technology, where the n+ implant has been designed to be resistive, and the read-out is obtained via AC-coupling. The truly innovative feature of RSD is that the signal generated by an impinging particle is shared isotropically among multiple read-out pads without the need for floating electrodes or an external magnetic field. Careful tuning of the coupling oxide thickness and the n+ doping profile is at the basis of the successful functioning of this device. Several RSD matrices with different pad width-pitch geometries have been extensively tested with a laser setup in the Laboratory for Innovative Silicon Sensors in Torino, while a smaller set of devices have been tested at the Fermilab Test Beam Facility with a 120 GeV/c proton beam. The measured spatial resolution ranges between 2.5μm for 70–100 pad-pitch geometry and 17μm with 200–500 matrices, a factor of 10 better than what is achievable in binary read-out (binsize∕12). Beam test data show a temporal resolution of ∼40ps for 200 μm pitch devices, in line with the best performances of LGAD sensors at the same gain.

Is it possible to design a detector able to concurrently measure time and position with high precision? This question is at the root of the research and development of silicon sensors presented in this contribution. Silicon sensors are the most common type of particle detectors used for charged particle tracking, however their rather poor time resolution limits their use as precise timing detectors. A few years ago we have picked up the gantlet of enhancing the remarkable position resolution of silicon sensors with precise timing capability. I will be presenting our results in the following pages.

In this paper we report results from a neutron irradiation campaign of Ultra-Fast Silicon Detectors (UFSD) with fluences of 1e14, 3e14, 6e14, 1e15, 3e15 and 6e15 neq/cm2. The UFSD used in this study are circular 50 μ m thick Low-Gain Avalanche Detectors (LGAD), with a 1.0 mm diameter active area. Hamamatsu Photonics (HPK), Japan, produced the UFSD with pre-irradiation internal gain in the range 5–70 depending on the bias voltage. The sensors were tested pre-irradiation and post-irradiation with minimum ionizing particles (MIPs) from a 90Sr β-source. The leakage current, internal gain and the timing resolution were measured as a function of bias voltage at −20 °C and −30 °C. The timing resolution of each device under test was extracted from the time difference with a second calibrated UFSD in coincidence, using the constant fraction discriminator (CFD) method for both. The dependence of the gain upon the irradiation fluence is consistent with the acceptor removal mechanism; at −20 °C the highest gain decreases from 70 before radiation to 2 after a fluence of 6e15 n/cm2. Consequently, the timing resolution was found to deteriorate from 20 ps to 50 ps. The results indicate that the most accurate time resolution is obtained varying with fluence the CFD value used to determine the time of arrival, from 0.1 for pre-irradiated sensors to 0.6 at the highest fluence. Key changes to the pulse shape induced by irradiation, i.e. (i) the contribution of charge multiplication not limited to the gain layer zone, (ii) the shortening of the rise time and (iii) the reduced pulse height, were compared with the WF2 simulation program and were found to be in agreement.

Fondazione Bruno Kessler (FBK, Trento, Italy) has recently delivered its first 50 $\mu$m thick production of Ultra-Fast Silicon Detectors (UFSD), based on the Low-Gain Avalanche Diode design. These sensors use high resistivity Si-on-Si substrates, and have a variety of gain layer doping profiles and designs based on Boron, Gallium, Carbonated Boron and Carbonated Gallium to obtain a controlled multiplication mechanism. Such variety of gain layers will allow identifying the most radiation hard technology to be employed in the production of UFSD, to extend their radiation resistance beyond the current limit of $\phi \sim$ 10$^{15}$ n$_{eq}$/cm$^2$. In this paper, we present the characterisation, the timing performances, and the results on radiation damage tolerance of this new FBK production.

Abstract This paper presents the measurements on first very thin Ultra-Fast Silicon Detectors (UFSDs) produced by Fondazione Bruno Kessler; the data have been collected in a beam test setup at the CERN PS, using beam with a momentum of 12 GeV/c. UFSDs with a nominal thickness of 25 and 35 $$\mu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> m and an area of 1 $$\times$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>×</mml:mo> </mml:math> 1 $$\text {mm}^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>mm</mml:mtext> <mml:mn>2</mml:mn> </mml:msup> </mml:math> have been considered, together with an additional HPK 50- $$\mu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> m thick sensor, taken as reference. Their timing performances have been studied as a function of the applied voltage and gain. A time resolution of about 25 ps and of 22 ps at a voltage of 120 and 240 V has been obtained for the 25 and 35 $$\mu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> m thick UFSDs, respectively.

The DC-coupled Resistive Silicon Detectors (DC-RSD) are the evolution of the AC-coupled RSD (RSD) design, both based on the Low-Gain Avalanche Diode (LGAD) technology. The DC-RSD design concept intends to address a few known issues present in RSDs (e.g., baseline fluctuation, long tail-bipolar signals) while maintaining their advantages (e.g., signal spreading, 100% fill factor). The simulation of DC-RSD presents several unique challenges linked to the complex nature of its design and the large pixel size. The defining feature of DC-RSD, charge sharing over distances that can be as large as a millimetre, represents a formidable challenge for Technology-CAD (TCAD), the standard simulation tool. To circumvent this problem, we have developed a mixed-mode approach to the DC-RSD simulation, which exploits a combination of two simulation tools: TCAD and Spice. Thanks to this hybrid approach, it has been possible to demonstrate that, according to the simulation, the key features of the RSD, excellent timing and spatial resolutions (few tens of picoseconds and few microns), are maintained in the DC-RSD design. In this work, we present the developed models and methodology, mainly showing the results of device-level numerical simulation, which have been obtained with the state-of-the-art Synopsys Sentaurus TCAD suite of tools. Such results will provide all the necessary information for the first batch of DC-RSD produced by Fondazione Bruno Kessler (FBK) foundry in Trento, Italy.

In this paper we present the numerical simulation of silicon detectors with internal gain as the main tool for 4-dimensional (4D) particle trackers design and optimization. The Low-Gain Avalanche Diode (LGAD) technology and its present limitations are reviewed with the aim of introducing the Resistive AC-Coupled Silicon Detectors (RSD) paradigm as a case study of our investigation. Authors here present Spice-like and 2D/3D Technological Computer-Aided Design (TCAD) simulations to characterize sensors in terms of both their electrostatic behavior, capacitive (dynamic) coupling and radiation-hardness performances, showing the methodological approach used in order to extract the set of layout rules allowing the release of RSD1, the incoming production run at Fondazione Bruno Kessler (FBK) of next-generation silicon detectors for 4D tracking with intrinsic 100% fill-factor.

Several future high-energy physics facilities are currently being planned. The proposed projects include high energy e+e− circular and linear colliders, hadron colliders, and muon colliders, while the Electron–Ion Collider (EIC) is expected to construct at the Brookhaven National Laboratory in the future. Each proposal has its advantages and disadvantages in terms of readiness, cost, schedule, and physics reach, and each proposal requires the design and production of specific new detectors. This paper first presents the performances necessary for future silicon tracking systems at the various new facilities. Then it illustrates a few possibilities for the realization of such silicon trackers. The challenges posed by the future facilities require a new family of silicon detectors, where features such as impact ionization, radiation damage saturation, charge sharing, and analog read-out are exploited to meet these new demands.

Recent advances in segmented low-gain avalanche detectors (LGADs) make them promising for the position-sensitive detection of low-energy X-ray photons thanks to their internal gain. LGAD microstrip sensors fabricated by Fondazione Bruno Kessler have been investigated using X-rays with both charge-integrating and single-photon-counting readout chips developed at the Paul Scherrer Institut. In this work it is shown that the charge multiplication occurring in the sensor allows the detection of X-rays with improved signal-to-noise ratio in comparison with standard silicon sensors. The application in the tender X-ray energy range is demonstrated by the detection of the sulfur Kα and Kβ lines (2.3 and 2.46 keV) in an energy-dispersive fluorescence spectrometer at the Swiss Light Source. Although further improvements in the segmentation and in the quantum efficiency at low energy are still necessary, this work paves the way for the development of single-photon-counting detectors in the soft X-ray energy range.

Low Gain Avalanche Diodes (LGADs) are now considered a viable solution for 4D-tracking thanks to their excellent time resolution and good resistance to high radiation fluence. However, the currently available LGAD technology is well suited only for applications that require coarse space precision, pixels with pitch in the range 500 µm–1 mm, due to the presence of a no-gain region between adjacent pixels of about 50μm, in which the gain is completely suppressed. In this paper, we will discuss the segmentation issues in the LGAD technology and we will present two new segmentation strategies aimed at producing LGADs with high spatial resolution and high fill factor. The first presented design is the so-called Trench-Isolated LGAD (TI-LGAD). Here, the pixel isolation is provided by trenches, physically etched in the silicon and then filled with silicon oxide. The second design is the Resistive AC-coupled Silicon Detector (RSD), an evolution of LGADs, where the segmentation is obtained by means of AC-coupled electrodes. Prototypes of both designs have been produced at FBK and characterized at the Laboratories for Innovative Silicon Sensors (INFN and University of Turin) by means of a laser setup to estimate the space resolution and the fill factor. The functional characterization shows that both the technologies yield fully working small pixel LGADs (down to 50 µm), providing the first examples of sensors able to concurrently measure space and time with excellent precision.

The book describes the development of innovative silicon sensors known as ultra-fast silicon detectors for use in the space-time tracking of charge particles. The first comprehensive collection of information on the topic, otherwise currently scattered in existing literature, this book presents a comprehensive introduction to the development of ultra-fast silicon detectors with the latest technology and applications from the field. It will be an ideal reference for graduate and postgraduates studying high energy and particle physics and engineering, in addition to researchers in the area. Key features Authored by a team of subject area specialists, whose research group first invented ultra-fast silicon detectors The first book on the topic to explain the details of the design of silicon sensors for 4-dimensional tracking Presents state-of-the-art results, and prospects for further performance evolutions &nbsp; The Open Access version of this book, available at&nbsp;www.taylorfrancis.com/books/oa-mono/10.1201/9781003131946/, has been made available under a Creative Commons Attribution-Non Commercial-No Derivatives 4.0 license. Cover image credit goes to Marta Tornago &nbsp; &nbsp;

Abstract RSDs (Resistive AC-Coupled Silicon Detectors) are n-in-p silicon sensors based on the LGAD (Low-Gain Avalanche Diode) technology, featuring a continuous gain layer over the whole sensor area. The truly innovative feature of these sensors is that the signal induced by an ionising particle is seen on several pixels, allowing the use of reconstruction techniques that combine the information from many read-out channels. In this contribution, the first application of a machine learning technique to RSD devices is presented. The spatial resolution of this technique is compared to that obtained with the standard RSD reconstruction methods that use analytical descriptions of the signal sharing mechanism. A Multi-Output regressor algorithm, trained with a combination of simulated and real data, leads to a spatial resolution of less than 2 μm for a sensor with a 100 μm pixel. The prospects of future improvements are also discussed.

The past ten years have seen the advent of silicon-based precise timing detectors for charged particle tracking. The underlying reason for this evolution is a design innovation: the Low-Gain Avalanche Diode (LGAD). In its simplicity, the LGAD design is an obvious step with momentous consequences: low gain leads to large signals maintaining sensors stability and low noise, allowing sensor segmentation. Albeit introduced for a different reason, to compensate for charge trapping in irradiated silicon sensors, LGAD found fertile ground in the design of silicon-based timing detectors. Spurred by this design innovation, solid-state-based timing detectors for charged particles are going through an intense phase of R&D, and hybrid and monolithic sensors, with or without internal gain, are being explored. This contribution offers a review of this booming field.

Segmented silicon sensors with internal gain, the so called Ultra-FAST Silicon Detectors (UFSD), have been produced at FBK for the first time. UFSD are based on the concept of Low-Gain Avalanche Detectors (LGAD), which are silicon detectors with an internal, low multiplication mechanism (gain ∼ 10). This production houses two main type of devices: one type where the gain layer is on the same side of the read-out electrodes, the other type where the gain layer is on the side opposite to the pixellated electrodes (reverse-LGAD). Several technological splits have been included in the first production run, with the aim to tune the implantation dose of the multiplication layer, which controls the gain value of the detector. An extended testing on the wafers has been performed and the results are in line with simulations: the fabricated detectors show good performances, with breakdown voltages above 1000 Volts, and gain values in the range of 5–60 depending on the technological split. The detectors timing resolution has been measured by means of a laboratory setup based on an IR picosecond laser. The sample with higher gain shows time resolution of 55 ps at high reverse bias voltage, indicating very promising performance for future particle tracking applications.

In the past few years, there has been growing interest in the development of silicon sensors able to simultaneously measure accurately the time of passage and the position of impinging charged particles. In this contribution, a review of the progresses in the design of UFSD (Ultra-Fast Silicon Detectors) sensors, manufactured at the FBK (Fondazione Bruno Kessler) Foundry, aiming at tracking charged particles in 4 dimensions, is presented. The state-of-the-art UFSD sensors, with excellent timing capability, are planned to be used in both ATLAS and CMS experiments detector upgrade, in order to reduce the background due to the presence of overlapping events in the same bunch crossing. The latest results on sensors characterization including time resolution, radiation resistance and uniformity of the response are here summarized, pointing out the interplay between the design of the gain layer and the UFSD performances. The research is now focusing on the maximization of the sensor fill factor, to be able to reduce the pixel size, exploring the implementation of shallow trenches for the pixel isolation and the development of resistive AC-coupled UFSD sensors. In conclusion, a brief review on research paths tailored for detection of low energy X-rays or for low material budget applications is given.

In the past few years, the need of measuring accurately the spatial and temporal coordinates of the particles generated in high-energy physics experiments has spurred a strong R&D in the field of silicon sensors. Within these research activities, the so-called Ultra-Fast Silicon Detectors (UFSDs), silicon sensors optimized for timing based on the Low-Gain Avalanche Diode (LGAD) design, have been proposed and adopted by the CMS and ATLAS collaborations for their respective timing layers. The defining feature of the Ultra-Fast Silicon Detectors (UFSDs) is the internal multiplication mechanism, determined by the gain layer design. In this paper, the performances of several types of gain layers, measured with a telescope instrumented with a 90Sr β-source, are reported and compared. The measured sensors are produced by Fondazione Bruno Kessler (FBK) and Hamamatsu Photonics (HPK). The sensor yielding the best performance, both when new and irradiated, is an FBK 45 μm-thick sensor with a carbonated deep gain implant, where the carbon and the boron implants are annealed concurrently with a low thermal load. This sensor is able to achieve a time resolution of 40 ps up to a radiation fluence of 2.5⋅1015 neq/cm2, delivering at least 5 fC of charge.

In this work, we introduce a new design concept: the DC-coupled Resistive Silicon Detectors, based on the LGAD technology.This new design intends to address a few known drawbacks of the first generation of AC-coupled Resistive Silicon Detectors (RSD).The sensor behaviour is simulated using a fast hybrid approach based on a combination of two packages, Weightfield2 and LTSpice.The simulation demonstrates that the key features of the RSD design are maintained, yielding excellent space and time resolutions: a few tens of ps and a few microns.In this report, we will outline the optimization methodology and the results of the simulation.We will also present detailed studies on the effects induced by the choice of key design parameters on the space and time resolutions provided by this sensor.

We review the progress toward the development of a novel type of silicon detectors suited for tracking with a picosecond timing resolution, the so called Ultra-Fast Silicon Detectors. The goal is to create a new family of particle detectors merging excellent position and timing resolution with GHz counting capabilities, very low material budget, radiation resistance, fine granularity, low power, insensitivity to magnetic field, and affordability. We aim to achieve concurrent precisions of ∼ 10 ps and ∼ 10 μm with a 50 μm thick sensor. Ultra-Fast Silicon Detectors are based on the concept of Low-Gain Avalanche Detectors, which are silicon detectors with an internal multiplication mechanism so that they generate a signal which is factor ∼ 10 larger than standard silicon detectors.

To fully exploit the physics potentials of particle therapy in delivering dose with high accuracy and selectivity, charged particle therapy needs further improvement. To this scope, a multidisciplinary project (MoVeIT) of the Italian National Institute for Nuclear Physics (INFN) aims at translating research in charged particle therapy into clinical outcome. New models in the treatment planning system are being developed and validated, using dedicated devices for beam characterization and monitoring in radiobiological and clinical irradiations. Innovative silicon detectors with internal gain layer (LGAD) represent a promising option, overcoming the limits of currently used ionization chambers. Two devices are being developed: one to directly count individual protons at high rates, exploiting the large signal-to-noise ratio and fast collection time in small thicknesses (1 ns in 50 μm) of LGADs, the second to measure the beam energy with time-of-flight techniques, using LGADs optimized for excellent time resolutions (Ultra Fast Silicon Detectors, UFSDs). The preliminary results of first beam tests with therapeutic beam will be presented and discussed.

The basic principle of operation of silicon sensors with resistive read-out is built-in charge sharing. Resistive Silicon Detectors (RSD, also known as AC-LGAD), exploiting the signals seen on the electrodes surrounding the impact point, achieve excellent space and time resolutions even with very large pixels. In this paper, a TCT system using a 1064 nm picosecond laser is used to characterize RSD sensors produced by Fondazione Bruno Kessler. The paper first introduces the parametrization of the errors in the determination of the position and time coordinates in RSD, then outlines the reconstruction method, and finally presents the results. Three different pixel pitches are used in the analysis: 200 × 340, 450 × 450, and 1300 × 1300 μm2. At gain = 30, the 450 × 450 μm2 pixel achieves a time jitter of 20 ps and a spatial resolution of 15 μm concurrently, while the 1300 × 1300 μm2 pixel achieves 30 ps and 30 μm, respectively. The implementation of cross-shaped electrodes improves considerably the response uniformity over the pixel surface.

Abstract The recently developed Low-Gain Avalanche Diode (LGAD) technology has gained growing interest within the high-energy physics (HEP) community, thanks to its capability of internal signal amplification that improves the particle detection. Since the next generation of HEP experiments will require tracking detectors able to efficiently operate in environments where expected fluences will exceed 1 × 10 17 1 MeV n eq /cm 2 , the design of radiation-resistant particle detectors becomes of utmost importance. To this purpose, Technology Computer-Aided Design (TCAD) simulations are a relevant part of the current detector R&amp;D, not only to support the sensor design and optimization, but also for a better understanding and modelling of radiation damage. In this contribution, the recent advances in the TCAD modelling of non-irradiated and irradiated LGAD sensors are presented, whose validation relies on the agreement between the simulated and experimental data — in terms of current-voltage (I-V), capacitance-voltage (C-V), and gain-voltage (G-V) characteristics, coming from devices manufactured by Hamamatsu Photonics (HPK), and accounting for different irradiation levels and temperatures.

Abstract RSDs are LGAD silicon sensors with 100% fill factor, based on the principle of AC-coupled resistive read-out. Signal sharing and internal charge multiplication are the RSD key features to achieve picosecond-level time resolution and micron-level spatial resolution, thus making these sensors promising candidates as 4D-trackers for future experiments. This paper describes the use of a neural network to reconstruct the hit position of ionizing particles, an approach that can boost the performance of the RSD with respect to analytical models. The neural network has been trained in the laboratory and then validated on test beam data. The device-under-test in this work is a 450 μm-pitch matrix from the FBK RSD2 production, which achieved a resolution of about 65 μm at the DESY Test Beam Facility, a 50% improvement compared to a simple analytical reconstruction method, and a factor two better than the resolution of a standard pixel sensor of equal pitch size with binary read-out. The test beam result is compatible with the laboratory ones obtained during the neural network training, confirming the ability of the machine learning model to provide accurate predictions even in environments very different from the training one. Prospects for future improvements are also discussed.

This paper reports on a measurement campaign to characterize the inter-pad region of Ultra-Fast Silicon Detectors (UFSDs) manufactured by Fondazione Bruno Kessler. The devices under test are either pixel or strip arrays, featuring a large number of different inter-pad layouts; both pre-irradiation and irradiated sensors have been measured. The aim of the study is to link the design parameters of the inter-pad region to the operation of the sensors, providing insights into the design of UFSD arrays with narrow inter-pad gaps. We concluded that, in the UFSD design, the doping level and the area of the p-stop should be kept low, in order to avoid the early breakdown of the device and the micro-discharges effect; UFSDs with such characteristics proved also rather insensitive to floating pads and irradiation. Thanks to these findings, it was possible to design a UFSD array that yields the expected performance with an inter-pad width as small as 25 μm, significantly improving its fill factor with respect to standard designs. Two innovative experimental techniques are presented in this work: the first one is based on a TCT setup, the second makes use of an ultra-low light CCD camera.

Monitoring Ultra-High Dose Rate (UHDR) beams is one of the multiple challenges posed by the emergent FLASH radiotherapy. Technologies (i.e., gas-filled ionization chambers) nowadays used in conventional radiotherapy are no longer effective when applied to UHDR regimes, due to the recombination effect they are affected by, and the time required to collect charges. Exploiting the expertise in the field of silicon sensors’ applications into clinics, the medical physics group of the University and INFN Torino is investigating thin silicon sensors as possible candidates for UHDR beam monitoring, exploiting their excellent spatial resolution and well-developed technology. Silicon sensors of 30 and 45 µm active thicknesses and 0.25, 1 and 2 mm 2 active areas were tested at the SIT ElectronFlash machine (CPFR, Pisa) on 9 MeV electron beams, featuring a pulse duration of 4 µs, a frequency of 1 Hz, and a dose-per-pulse ranging from 1.62 to 10.22 Gy/pulse. The silicon sensors were positioned at the exit of the ElectronFlash applicator, after a solid water build-up slab, and were readout both with an oscilloscope and with a multi-channel front-end readout chip (TERA08). A response linearity extending beyond 10 Gy/pulse was demonstrated by comparison with a reference dosimeter (FlashDiamond), thus fulfilling the first requirement of a potential application in UHDR beam monitoring.

We present an analysis of the fluence profile at the JSI TRIGA neutron reactor facility in Ljubljana. For the study, multi-pad Low-Gain Avalanche Diodes (LGADs) are used. The deactivation of acceptor doping in the gain layer implant due to the irradiation, typical of LGAD devices, is exploited to map the fluence profile inside the irradiation channels. The amount of active doping of the LGAD gain layer is extracted via capacitance-voltage measurements for each pad before and after irradiation to a fluence of 1.5×1015n˙eq/cm2, where neq stands for 1 MeV equivalent neutron count, providing a precise and prompt measurement of the fluence distribution over the LGAD sensor. Experimental results are compared to neutron fluence expectations calculated with Monte Carlo techniques.

Single ion counting in particle therapy may lead to new beam monitoring systems, enabling innovative delivery strategies that are faster and more sensitive than those currently used in clinics. Previous studies carried out by the University and the National Institute of Nuclear Physics (INFN) of Turin have demonstrated the feasibility of using thin silicon detectors to count single protons in clinical beams (Monaco et al., 2023) [1]. The aim of this work is to report the performance of a strip-segmented 60-μm thick silicon PIN sensor used for single carbon ion discrimination. All measurements were performed using the CNAO synchrotron at different beam energies covering the clinical energy range (115–399 MeV/u). Signals from the sensor strips were read using a custom amplifier board and sampled with a 5 GS/s digitizer. The carbon ion signals were analyzed in terms of amplitude, duration, and deposited charge at different sensor bias voltages.

Abstract EXFLU1 is a new batch of radiation-resistant silicon sensors manufactured at Fondazione Bruno Kessler (FBK, Italy). The EXFLU1 sensors utilize thin substrates that remain operable even after extensive irradiation. They incorporate Low-Gain Avalanche Diode (LGAD) technology, enabling internal multiplication of charge carriers to boost the small signal produced by a particle crossing their thin active thicknesses, ranging from 15 to 45 μ m. To address current challenges related to acceptor removal, the EXFLU1 production incorporates improved defect engineering techniques. This includes the so called carbonated LGADs, where carbon doping is implanted alongside boron in the gain layer. This contribution focuses on evaluating the performances of thin sensors with carbonated gain layer from the EXFLU1 production, before and after irradiation up to 2.5· 10 15 n 1 Mev eq. /cm 2 . The conducted tests involve static and transient characterizations, including I-V and C-V measurements, as well as laser and β -source tests. This work aims to present the state of the art in LGAD sensor technology with a carbonated gain layer and shows the characterization of the most radiation-resistant LGAD sensors produced to date.

This paper summarizes the beam test results obtained with a Resistive Silicon Detector (RSD) (also called AC-Low Gain Avalanche Diode, AC-LGAD) pixel array tested at the DESY beam test facility with a 5 GeV/c electron beam. Furthermore, it describes in detail the simulation results of DC-RSD, an evolution of the RSD design. The simulations campaign described in this paper has been instrumental in the definition of the structures implemented in the Fondazione Bruno Kessler FBK first DC-RSD production. The RSD matrix used in this study is part of the second FBK RSD production, RSD2. The best position resolution reached in this test is σx=15 μm, about 3.4% of the pitch. DC-RSD LGAD, are an evolution of the AC-coupled design, eliminating the dielectric and using a DC-coupling to the electronics. The concept of DC-RSD has been finalized using full 3D Technology-CAD simulations of the sensor behavior. TCAD simulations are an excellent tool for designing this innovative class of detectors, enabling the evaluation of different technology options (e.g., the resistivity of the n+ layer, contact materials) and geometrical layouts (shape and distance of the read-out pads).

In this paper we report measurements of the uniformity of time resolution, signal amplitude, and charged particle detection efficiency across the sensor surface of low-gain avalanche detectors (LGAD). Comparisons of the performance of sensors with different doping concentrations and different active thicknesses are presented, as well as their temperature dependence and radiation tolerance up to 6×1014 n/cm2. Results were obtained at the Fermilab test beam facility using 120 GeV proton beams, and a high precision pixel tracking detector. LGAD sensors manufactured by the Centro Nacional de Microelectrónica (CNM) and Hamamatsu Photonics (HPK) were studied. The uniformity of the sensor response in pulse height before irradiation was found to have a 2% spread. The signal detection efficiency and timing resolution in the sensitive areas before irradiation were found to be 100% and 30–40 ps, respectively. A “no-response” area between pads was measured to be about 130 μm for CNM and 170μm for HPK sensors. After a neutron fluence of 6×1014 n/cm2 the CNM sensor exhibits a large gain variation of up to a factor of 2.5 when comparing metalized and non-metalized sensor areas. An irradiated CNM sensor achieved a time resolution of 30 ps for the metalized area and 40 ps for the non-metalized area, while a HPK sensor irradiated to the same fluence achieved a 30 ps time resolution.

The properties of 50 um thick Low Gain Avalanche Diode (LGAD) detectors manufactured by Hamamatsu photonics (HPK) and Fondazione Bruno Kessler (FBK) were tested before and after irradiation with 1 MeV neutrons. Their performance were measured in charge collection studies using b-particles from a 90Sr source and in capacitance-voltage scans (C-V) to determine the bias to deplete the gain layer. Carbon infusion to the gain layer of the sensors was tested by FBK in the UFSD3 production. HPK instead produced LGADs with a very thin, highly doped and deep multiplication layer. The sensors were exposed to a neutron fluence from 4e14 neq/cm2 to 4e15 neq/cm2. The collected charge and the timing resolution were measured as a function of bias voltage at -30C, furthermore the profile of the capacitance over voltage of the sensors was measured.

The performance of the Ultra-Fast Silicon Detectors (UFSD) after irradiation with neutrons and protons is compromised by the removal of acceptors in the thin layer below the junction responsible for the gain. This effect is tested both with capacitance–voltage, C–V, measurements of the doping concentration and with measurements of charge collection, CC, using charged particles. We find a perfect linear correlation between the bias voltage to deplete the gain layer determined with C–V and the bias voltage to collect a defined charge, measured with charge collection. An example for the usefulness of this correlation is presented.

Fast procedures for the beam quality assessment and for the monitoring of beam energy modulations during the irradiation are among the most urgent improvements in particle therapy. Indeed, the online measurement of the particle beam energy could allow assessing the range of penetration during treatments, encouraging the development of new dose delivery techniques for moving targets. Towards this end, the proof of concept of a new device, able to measure in a few seconds the energy of clinical proton beams (from 60 to 230 MeV) from the Time of Flight (ToF) of protons, is presented. The prototype consists of two Ultra Fast Silicon Detector (UFSD) pads, featuring an active thickness of 80 um and a sensitive area of 3 x 3 mm2, aligned along the beam direction in a telescope configuration, connected to a broadband amplifier and readout by a digitizer. Measurements were performed at the Centro Nazionale di Adroterapia Oncologica (CNAO, Pavia, Italy), at five different clinical beam energies and four distances between the sensors (from 7 to 97 cm) for each energy. In order to derive the beam energy from the measured average ToF, several systematic effects were considered, Monte Carlo simulations were developed to validate the method and a global fit approach was adopted to calibrate the system. The results were benchmarked against the energy values obtained from the water equivalent depths provided by CNAO. Deviations of few hundreds of keV have been achieved for all considered proton beam energies for both 67 and 97 cm distances between the sensors and few seconds of irradiation were necessary to collect the required statistics. These preliminary results indicate that a telescope of UFSDs could achieve in a few seconds the accuracy required for the clinical application and therefore encourage further investigations towards the improvement and the optimization of the present prototype.

The properties of 60-μm thick Ultra-Fast Silicon Detectors (UFSD) detectors manufactured by Fondazione Bruno Kessler (FBK), Trento (Italy) were tested before and after irradiation with minimum ionizing particles (MIPs) from a 90Sr β-source. This FBK production, called UFSD2, has UFSDs with gain layer made of Boron, Boron low-diffusion, Gallium, carbonated Boron and carbonated Gallium. The irradiation with neutrons took place at the TRIGA reactor in Ljubljana, while the proton irradiation took place at CERN SPS. The sensors were exposed to a neutron fluence of 4⋅1014, 8⋅1014, 1.5⋅1015, 3⋅1015, 6⋅ 1015 neq/cm2 and to a proton fluence of 9.6⋅ 1014 p/cm2, equivalent to a fluence of 6⋅ 1014 neq/cm2. The internal gain and the timing resolution were measured as a function of bias voltage at -20oC. The timing resolution was extracted from the time difference with a second calibrated UFSD in coincidence, using the constant fraction method for both.

The upgrades of the CMS and ATLAS experiments for the high luminosity phase of the Large Hadron Collider will employ precision timing detectors based on Low Gain Avalanche Detectors (LGADs). We present a suite of results combining measurements from the Fermilab Test Beam Facility, a beta source telescope, and a probe station, allowing full characterization of the HPK type 3.1 production of LGAD prototypes developed for these detectors. We demonstrate that the LGAD response to high energy test beam particles is accurately reproduced with a beta source. We further establish that probe station measurements of the gain implant accurately predict the particle response and operating parameters of each sensor, and conclude that the uniformity of the gain implant in this production is sufficient to produce full-sized sensors for the ATLAS and CMS timing detectors.

Abstract In this work, the results of Technology-CAD (TCAD) device-level simulations of non-irradiated and irradiated Low-Gain Avalanche Diode (LGAD) detectors and their validation against experimental data will be presented. Thanks to the intrinsic multiplication of the charge within these silicon sensors, it is possible to improve the signal to noise ratio thus limiting its drastic reduction with fluence, as it happens instead for standard silicon detectors. Therefore, special attention has been devoted to the choice of the avalanche model, which allows the simulation findings to better fit with experimental data. Moreover, a radiation damage model (called “New University of Perugia TCAD model”) has been fully implemented within the simulation environment, to have a predictive insight into the electrical behavior and the charge collection properties of the LGAD detectors, up to the highest particle fluences expected in the future High Energy Physics (HEP) experiments. This numerical model allows to consider the comprehensive bulk and surface damage effects induced by radiation on silicon sensors. By coupling the “New University of Perugia TCAD model” with an analytical model that describes the mechanism of acceptor removal in the multiplication layer, it has been possible to reproduce experimental data with high accuracy, demonstrating the reliability of the simulation framework.

This paper describes the performance of a prototype timing detector, based on 50 μm thick Ultra Fast Silicon Detector, as measured in a beam test using a 180 GeV/c momentum pion beam. The dependence of the time precision on the pixel capacitance and bias voltage is investigated in this paper. A timing precision from 30 ps to 100 ps (RMS), depending on the pixel capacitance, has been measured at a bias voltage of 180 V.

The combination of precision space and time information in particle tracking, the so called 4D tracking, is being considered in the upgrade of the ATLAS, CMS and LHCb experiments at the High-Luminosity LHC, set to start data taking in 2024–2025. Regardless of the type of solution chosen, space–time tracking brings benefits to the performance of the detectors by reducing the background and sharpening the resolution; it improves tracking performances and simplifies tracks combinatorics. Space–time tracking also allows investigating new physics channels, for example it opens up the possibilities of new searches in long-living particles by measuring accurately the time of flight between the production and the decay vertexes. The foreseen applications of 4D tracking in experiments with very high acquisition rates, for example at HL-LHC, add one more dimension to the problem, increasing dramatically the complexity of the read-out system and that of the whole detector design: we call 5D tracking the application of 4D tracking in high rate environments.

In this contribution we describe the second run of RSD (Resistive AC-Coupled Silicon Detectors) designed at INFN Torino and produced by Fondazione Bruno Kessler (FBK), Trento. RSD are n-in-p detectors intended for 4D particle tracking based on the LGAD technology that get rid of any segmentation implant in order to achieve the 100% fill-factor. They are characterized by three key-elements, (i) a continuous gain implant, (ii) a resistive n-cathode and (iii) a dielectric coupling layer deposited on top, guaranteeing a good spatial reconstruction of the hit position while benefiting from the good timing properties of LGADs. We will start from the very promising results of our RSD1 batch in terms of tracking performances and then we will move to the description of the design of the RSD2 run. In particular, the principles driving the sensor design and the specific AC-electrode layout adopted to optimize the signal confinement will be addressed.

Abstract The next generation of high-energy physics experiments at future hadronic colliders will require tracking detectors able to efficiently operate in extreme radiation environments, where expected fluences will exceed 1 × 10 17 n eq /cm 2 . This new operating scenario imposes many efforts on the design of effective and radiation-resistant particle detectors. Low-Gain Avalanche Diode (LGAD) represents a remarkable advance because the radiation damage effects can be mitigated by exploiting its charge multiplication mechanism after heavy irradiation. To obtain the desired gain (about 10–20) on the sensor output signal, a careful implementation of the “multiplication” region is needed (i.e. the high-field junction implant). Moreover, a proper design of the peripheral region (namely, the guard-ring structure) is crucial to prevent premature breakdown and large leakage currents at very high fluences, when the bias voltage applied creates an electric field higher than 15 V/μm. In this contribution, the design of LGAD sensors for extreme fluence applications is discussed, addressing the critical technological aspects such as the choice of the active substrate thickness, the gain layer design and the optimization of the sensor periphery. The impact of several design strategies is evaluated with the aid of Technology-CAD (TCAD) simulations based on a recently proposed model for the numerical simulation of radiation damage effects on LGAD devices.

In this contribution we present our most recent numerical investigations towards the development of silicon particle detectors able to provide accurate measurements in both space and time (4D tracking). In particular, we discuss the performances of different Low-Gain Avalanche Diode (LGAD) detectors, by presenting comparisons between measurements and TCAD (Technology Computer-Aided Design) simulations, performed on several detectors fabricated by Fondazione Bruno Kessler (FBK, Italy), Centro Nacional de Microelectrónica (CNM, Spain) and Hamamatsu Photonics K.K. (HPK, Japan). To have a satisfactory timing resolution, carriers multiplication in LGAD has to be properly controlled through the implantation of a specific highly-dopedp-type layer underneath the n-cathode. This internal multiplication process is so crucial in view of having large output signals for accurate time measurements, that numerical simulation turns out to be one of the main tools in designing LGADs. For this reason, in this paper we present a simulation framework, where the most robust avalanche models - Massey, van Overstraeten-de Man and Okuto-Crowell - have been tested. Thus, at the end, we propose a reliable designing tool which is highly predictive in the field of research and development of LGADs.

Abstract Beam monitoring in particle therapy is a critical task that, because of the high flux and the time structure of the beam, can be challenging for the instrumentation. Recent developments in thin silicon detectors with moderate internal gain, optimized for timing applications (Ultra Fast Silicon Detectors, UFSD), offer a favourable technological option to conventional ionization chambers. Thanks to their fast collection time and good signal-to-noise ratio, properly segmented sensors allow discriminating and counting single protons up to the high fluxes of a therapeutic beam, while the excellent time resolution can be exploited for measuring the proton beam energy using time-of-flight techniques. We report here the results of the first tests performed with UFSD detector pads on a therapeutic beam. It is found that the signal of protons can be easily discriminated from the noise, and that the very good time resolution is confirmed. However, a careful design is necessary to limit large pile-up inefficiencies and early performance degradation due to radiation damage.

The Ultra Fast Silicon Detectors (UFSD) are a novel concept of silicon detectors based on the Low Gain Avalanche Diode (LGAD) technology, which are able to obtain time resolution of the order of few tens of picoseconds. First prototypes with different geometries (pads/pixels/strips), thickness (300 and 50 μm) and gain (between 5 and 20) have been recently designed and manufactured by CNM (Centro Nacional de Microelectrónica, Barcelona) and FBK (Fondazione Bruno Kessler, Trento). Several measurements on these devices have been performed in laboratory and in beam test and a dependence of the gain on the temperature has been observed. Some of the first measurements will be shown (leakage current, breakdown voltage, gain and time resolution on the 300 μm from FBK and gain on the 50 μm-thick sensor from CNM) and a comparison with the theoretically predicted trend will be discussed.

A new pixel detector for the CMS experiment was built in order to cope with the instantaneous luminosities anticipated for the Phase~I Upgrade of the LHC. The new CMS pixel detector provides four-hit tracking with a reduced material budget as well as new cooling and powering schemes. A new front-end readout chip mitigates buffering and bandwidth limitations, and allows operation at low comparator thresholds. In this paper, comprehensive test beam studies are presented, which have been conducted to verify the design and to quantify the performance of the new detector assemblies in terms of tracking efficiency and spatial resolution. Under optimal conditions, the tracking efficiency is $99.95\pm0.05\,\%$, while the intrinsic spatial resolutions are $4.80\pm0.25\,\mu \mathrm{m}$ and $7.99\pm0.21\,\mu \mathrm{m}$ along the $100\,\mu \mathrm{m}$ and $150\,\mu \mathrm{m}$ pixel pitch, respectively. The findings are compared to a detailed Monte Carlo simulation of the pixel detector and good agreement is found.

In this contribution we review the progress towards the development of a novel type of silicon detectors suited for tracking with a picosecond timing resolution, the so called Ultra-Fast Silicon Detectors. The goal is to create a new family of particle detectors merging excellent position and timing resolution with GHz counting capabilities, very low material budget, radiation resistance, fine granularity, low power, insensitivity to magnetic field, and affordability. We aim to achieve concurrent precisions of ∼ 10 ps and ∼ 10 μm with a 50 μm thick sensor. Ultra-Fast Silicon Detectors are based on the concept of Low-Gain Avalanche Detectors, which are silicon detectors with an internal multiplication mechanism so that they generate a signal which is factor ∼10 larger than standard silicon detectors. The basic design of UFSD consists of a thin silicon sensor with moderate internal gain and pixelated electrodes coupled to full custom VLSI chip. An overview of test beam data on time resolution and the impact on this measurement of radiation doses at the level of those expected at HL-LHC is presented. First I-V and C-V measurements on a new FBK sensor production of UFSD, 50 μm thick, with B and Ga, activated at two diffusion temperatures, with and without C co-implantation (in Low and High concentrations), and with different effective doping concentrations in the Gain layer, are shown. Perspectives on current use of UFSD in HEP experiments (UFSD detectors have been installed in the CMS-TOTEM Precision Protons Spectrometer for the forward physics tracking, and are currently taking data) and proposed applications for a MIP timing layer in the HL-LHC upgrade are briefly discussed.

The design of future high-energy and high-intensity hadronic machines, such as FCC-hh, relies on the ability of detectors to sustain harsh radiation environments while keeping excellent performances on tracking and tagging all the interaction products.In order to face the challenge, a vast R&D effort is required.In this paper, we propose a novel concept of tracking system, that combines the possibility to track particles up to fluences of the order of 10 17 n eq /cm 2 together with a precise time information, σ t ∼ 10 ps.For this purpose, Low-Gain Avalanche Diodes (LGAD) are the suited technology.For the innermost, most irradiated portion of the detector, very thin sensors (20 -40 µm) with moderate gain (∼ 5 -10) can provide the required tolerance to the radiation.For such detectors, the internal gain mechanism of LGAD allows to provide the same amount of charge released by a particle passing 100 -200 µm of standard PiN diodes up to Φ ∼ 0.5•10 16 n eq /cm 2 .Above those fluences, the thin doped layer responsible for the signal multiplication gets deactivated, but if operated at the proper bias voltage (∼ 500 V) the signal multiplication happens inside the whole irradiated bulk volume.Moreover, in the region of the tracker detector where the level of overall fluence keeps ≤ 0.5 -1•10 16 n eq /cm 2 , LGAD with a geometry optimised for timing measurement, the so-called Ultra-Fast Silicon Detectors (UFSD), can be used to provide precise position and timing information at the same time.Considering the current timing performances of UFSD under irradiation and assuming a σ t ∼ 40 ps from sensor + ASIC, the usage of track-timing layers alternated to tracking only layers can provide an ultimate σ t ∼ 10 ps per single track.

In this contribution, we present an innovative design of the Low-Gain Avalanche Diode (LGAD) gain layer, the p+ implant responsible for the local and controlled signal multiplication. In the standard LGAD design, the gain layer is obtained by implanting ∼5E16/cm3 atoms of an acceptor material, typically Boron or Gallium, in the region below the n++ electrode. In our design, we aim at designing a gain layer resulting from the overlap of a p+ and an n+ implants: the difference between acceptor and donor doping will result in an effective concentration of about 5E16/cm3, similar to standard LGADs. At present, the gain mechanism of LGAD sensors under irradiation is maintained up to a fluence of ∼1–2E15/cm2, and then it is lost due to the acceptor removal mechanism. The new design will be more resilient to radiation, as both acceptor and donor atoms will undergo removal with irradiation, but their difference will maintain constant. The compensated design will empower the 4D tracking ability typical of the LGAD sensors well above 1E16/cm2.

In this paper we report results from a neutron irradiation campaign of Ultra-Fast Silicon Detectors (UFSD) with fluences of 1e14, 3e14, 6e14, 1e15, 3e15, 6e15 n/cm2. The UFSD used in this study are circular 50 micro-meter thick Low-Gain Avalanche Detectors (LGAD), with a 1.0 mm diameter active area. They have been produced by Hamamatsu Photonics (HPK), Japan, with pre-radiation internal gain in the range 10-100 depending on the bias voltage. The sensors were tested pre-irradiation and post-irradiation with minimum ionizing particle (MIPs) from a 90Sr based \b{eta}-source. The leakage current, internal gain and the timing resolution were measured as a function of bias voltage at -20C and -30C. The timing resolution was extracted from the time difference with a second calibrated UFSD in coincidence, using the constant fraction method for both. The dependence of the gain upon the irradiation fluence is consistent with the concept of acceptor removal and the gain decreases from about 80 pre-irradiation to 7 after a fluence of 6e15 n/cm2. Consequently, the timing resolution was found to deteriorate from 20 ps to 50 ps. The results indicate that the most accurate time resolution is obtained at a value of the constant fraction discriminator (CFD) threshold used to determine the time of arrival varying with fluence, from 10% pre-radiation to 60% at the highest fluence. Key changes to the pulse shape induced by irradiation, i.e. (i) a reduce sensitivity of the pulse shape on the initial non-uniform charge deposition, (ii) the shortening of the rise time and (iii) the reduced pulse height, were compared with the WF2 simulation program and found to be in agreement.

In this contribution we present new developments in the production of Ultra Fast Silicon Detectors (UFSD) at Fondazione Bruno Kessler (FBK) in Trento, Italy. FBK after having in 2016 a successfully first production of UFSD sensors 300-micrometer thick, has produced in 2017 its first 50-micrometer thick UFSD sensors. These sensors use high resistivity Silicon on Silicon substrate and have different doping profile configurations of the gain layer based on Boron, Gallium, Carbonated Boron and Carbonated Gallium to obtain a controlled multiplication mechanism. The motivation of variety of gain layers it is to identify the most radiation hard technology to be employed in the production of UFSD for applications in high-radiation environments.

The MIUR PRIN 4DInSiDe collaboration aims at developing the next generation of 4D (i.e., position and time) silicon detectors based on Low-Gain Avalanche Diodes (LGAD) that guarantee to operate efficiently in the future high-energy physics experiments. To this purpose, different areas of research have been identified, involving the development, design, fabrication and test of radiation-hard devices. This research has been enabled thanks to ad-hoc advanced TCAD modelling of LGAD devices, accounting for both technological issues as well as physical aspects, e.g. different avalanche generation models and combined surface and bulk radiation damage effects modelling. In this contribution, it is reviewed the progress and the relevant detector developments obtained during the research activities in the framework of the 4DInSiDe project. • TCAD modelling for the design of radiation-hard LGAD sensors for 4D tracking. • Gain layer compensation, (p ＋ - and n ＋ -doping) to preserve the gain at high fluences. • New design approach to resistive read-out sensors: DC-coupled RSD. • DC-RSD employs a direct coupling of the resistive layer to the read-out pads. • DC-coupled low resistivity strips between read-out pads to improve the resolution.

Resistive AC-coupled Silicon Detectors (RSDs) are based on the Low Gain Avalanche Diode (LGAD) technology, characterized by a continuous gain layer, and by the innovative introduction of resistive read-out. Thanks to a novel electrode design aimed at maximizing signal sharing, RSD2, the second RSD production by Fondazione Bruno Kessler (FBK), achieves a position resolution on the whole pixel surface of about 8 $\mu m$ for 200-$\mu m$ pitch. RSD2 arrays have been tested using a Transient Current Technique setup equipped with a 16-channel digitizer, and results on spatial resolution have been obtained with machine learning algorithms.

For the Phase-2 upgrade, the CMS experiment foresees the installation of a MIP Timing Detector (MTD) to assign a precise timestamp to every charged particle up to pseudorapidity |η|=3, empowering the CMS detector with unique and new capabilities. The target timing resolution of MTD of less than 40 ps per track will mitigate the effects of the severe pile-up expected at the High-Luminosity LHC, maintaining CMS performances similar to current LHC ones. To match the requirements on radiation tolerance and occupancy, the forward region of the MTD, 1.6 <|η|< 3, will be equipped with silicon low-gain avalanche diodes (LGADs) coupled to the Endcap Timing Read Out Chip (ETROC), currently under development. We will present the current status of LGAD sensor testing, their qualification from beam tests, bench measurements, and the performance of the final ETROC design. Finally, we will discuss the challenges and the road map necessary to achieve timely installation of the Endcap Timing Layer (ETL).

This paper describes the new concept of the double-LGAD. The goal is to increase the charge at the input of the electronics, keeping a time resolution equal or better than a standard (single) LGAD; this has been realized by adding the charges of two coupled LGADs while still using a single front-end electronics. The study here reported has been done starting from single LGAD with a thickness of 25 \textmu{m}, 35 \textmu{m} and 50 \textmu{m}.

Abstract This paper describes the new concept of the double LGAD (low-gain avalanche diodes). The goal was to increase the charge at the input of the electronics, keeping a time resolution equal to or better than a standard (single) LGAD; this has been realized by adding the charges of two coupled LGADs while still using a single front-end electronics. The study here reported has been done starting from single LGAD with a thickness of 25 µm, 35 µm and 50 µm.

Recent developments in sensor design are opening the way to high resolution 4D-tracking detectors, able to measure concurrently position and time of passage of charged particles using the same sensitive device. The ongoing R&D path, based on the Low Gain Avalanche Diode (LGAD) technology, brings a new paradigm by introducing controlled signal sharing in the principles of operation of silicon sensors with the internal gain. The device under development is a thin LGAD with a resistive DC-coupled read-out (DC-coupled Resistive Silicon Detector - DC-RSD). The goal is to achieve a spatial resolution of a few micrometres using large pixels (150-200 micrometres), providing an excellent time resolution (~20-30 ps).The concept of DC-RSD has been finalized using an innovative mixed-mode approach to simulation: SPICE-based fast modeling to derive the sensor design parameters, followed by full 3D TCAD simulations of the sensor behaviour.This contribution reports the latest outcome of the simulations, which have been instrumental for the definition of the design technical implementation. This contribution also describes the characteristics of the first DC-RSD production at FBK, to be submitted in early summer, aimed at exploring multiple technological options and electrode layouts.Interesting information on the expected DC-RSD performance, extracted from recent experimental results obtained on AC-coupled resistive read-out sensors and on DC-RSD test structures, will be presented.

The Ultra Fast Silicon Detectors (UFSDs) are a new kind of silicon detectors based on Low Gain Avalanche Diodes technology. The UFSDs are optimised for time measurements with the goal of both excellent space and time resolution, which makes them a very good candidate for 4D tracking. In this paper, we will briefly explain their innovative design and show the status of the latest development. Recent measurements at the H8 beam line (CERN) will be reported, based on the UFSDs from two manufacturers: FBK and HPK. In particular, UFSDs of different thicknesses, with different doping concentrations and with different dopants of the gain layer have been studied. A time resolution of 35 ps has been achieved for a 50μm thick design and the results have been found to be in very good agreement with the expectations.

The recent development in the design of Ultra Fast Silicon Detector (UFSD), aimed at combining radiation resistance up to fluences of 1015 neq/cm2 and fine read-out segmentation, makes these sensors suitable for high energy physics applications. UFSD is an evolution of standard silicon sensor, optimized to achieve excellent timing resolution (∼30 ps), thanks to an internal low gain (∼20). UFSD sensors are n in p Low Gain Avalanche Diode (LGAD) with an active thickness of ∼5 μm. The internal gain in LGAD is obtained by implanting an appropriate density of acceptors (of the order of ∼ 1016/cm3) close to the p-n junction, that, when depleted, locally generates an electric field high enough to activate the avalanche multiplication; this layer of acceptors is called gain layer. The two challenges in the development of UFSD for high energy physics detectors are the radiation hardness and the fine segmentation of large area sensors. Irradiation fluences of the order of 1015 neq/cm2 have a dramatic effect on the UFSD: neutrons and charged hadrons reduce the active acceptor density forming the gain layer; this mechanism, called initial acceptor removal, causes the complete disappearance of the internal gain above fluence of 1015 neq/ cm2. For the segmentation of UFSDs, the crucial point is the electrical insulation of pads and the extension of the inactive area between pads. In this paper we present the latest results on radiation resistance of LGADs with different gain layer designs, irradiated up to 3⋅1015 neq/ cm2. Three different segmentation technologies, developed by Fondazione Bruno Kessler in Trento, will also be discussed in detail in the second part of the paper.

Abstract The University of Torino (UniTO) and the National Institute for Nuclear Physics (INFN-TO) are investigating the use of Ultra Fast Silicon Detectors (UFSD) for beam monitoring in radiobiological experiments with therapeutic proton beams. The single particle identification approach of solid state detectors aims at increasing the sensitivity and reducing the response time of the conventional monitoring devices, based on gas detectors. Two prototype systems are being developed to count the number of beam particles and to measure the beam energy with time-of-flight (ToF) techniques. The clinically driven precision (&lt; 1%) in the number of particles delivered and the uncertainty &lt; 1 mm in the depth of penetration (range) in radiobiological experiments (up to 10 8 protons/s fluxes) are the goals to be pursued. The future translation into clinics would allow the implementation of faster and more accurate treatment modalities, nowadays prevented by the limits of state-of-the-art beam monitors. The experimental results performed with clinical proton beams at CNAO (Centro Nazionale di Adroterapia Oncologica, Pavia) and CPT (Centro di Protonterapia, Trento) showed a counting inefficiency &lt;2% up to 100 MHz/cm 2 , and a deviation of few hundreds of keV of measured beam energies with respect to nominal ones. The progresses of the project are reported.

Abstract Future particle trackers will have to measure concurrently position and time with unprecedented accuracy, aiming at ∼5 μm and a few 10s ps resolution respectively. A promising good candidate for such a task are the resistive AC-LGADs, solid state silicon sensors of novel design, characterized by an internal moderate gain and an AC-coupled resistive read-out to achieve signal sharing among pads. The sensor design leads to a drastic reduction in the number of read-out channels, has an intrinsic 100% fill factor, and adapts easily to any read-out geometry. This report describes the design challenges, the signal formation and recent test results obtained with the first prototypes. A part is also dedicated to the reconstruction techniques that exploit the distributed nature of the signal, including machine learning. An outlook to a future development for optimized read-out electrodes and electronics is also presented.

Resistive Silicon Detectors (RSD, also known as AC-LGAD) are innovative silicon sensors, based on the LGAD technology, characterized by a continuous gain layer that spreads across the whole sensor active area. RSDs are very promising tracking detectors, thanks to the combination of the built-in signal sharing with the internal charge multiplication, which allows large signals to be seen over multiple read-out channels. This work presents the first experimental results obtained from a 3$\times$4 array with 200~\mum~pitch, coming from the RSD2 production manufactured by FBK, read out with a 16-ch digitizer. A machine learning model has been trained, with experimental data taken with a precise TCT laser setup, and then used to predict the laser shot positions, finding a spatial resolution of $\sim$~5.5~\mum.

The “Perugia Surface and Bulk” radiation damage model is a Synopsys Sentaurus Technology CAD (TCAD) numerical model which accounts for surface and bulk damage effects induced by radiation on silicon particle detectors. In this work, the significance of the input parameters of the model, such as electron/hole cross sections and acceptor/donor introduction rates is investigated, with respect to the changes in leakage current, full depletion voltage, charge collection efficiency and the current-related damage factor α (an irradiated device’s figure of merit) of a PIN diode. Different types (IV, 1/C2-V) of comparisons are made between simulation outputs and experimental data taken from irradiated PIN diodes. Finally, the possibility of the analytical model’s validation with the examination of the Low-Gain Avalanche Detector (LGAD) case, and its general application for future silicon sensors is discussed.

Next generation Low Gain Avalanche Diodes (LGAD) produced by Hamamatsu photonics (HPK) and Fondazione Bruno Kessler (FBK) were tested before and after irradiation with ~1MeV neutrons at the JSI facility in Ljubljana. Sensors were irradiated to a maximum 1-MeV equivalent fluence of 2.5E15 N eq /cm 2 . The sensors analysed in this paper are an improvement after the lessons learned from previous FBK and HPK productions that were already reported in precedent papers. The gain layer of HPK sensors was fine-tuned to optimize the performance before and after irradiation. FBK sensors instead combined the benefit of Carbon infusion and deep gain layer to further the radiation hardness of the sensors and reduced the bulk thickness to enhance the timing resolution. The sensor performance was measured in charge collection studies using β-particles from a 90Sr source and in capacitance-voltage scans (C-V) to determine the bias to deplete the gain layer. The collected charge and the timing resolution were measured as a function of bias voltage at -30C. Finally a correlation is shown between the bias voltage to deplete the gain layer and the bias voltage needed to reach a certain amount of gain in the sensor. HPK sensors showed a better performance before irradiation while maintaining the radiation hardness of the previous production. FBK sensors showed exceptional radiation hardness allowing a collected charge up to 10 fC and a time resolution of 40 ps at the maximum fluence.

This paper presents a new version of the FAST family of ASICs and its comparison with the previous version. The new design, FAST3, aims at a timing jitter below 15 picoseconds when coupled to Ultra-Fast Silicon detectors (UFSD). The FAST3 integrated circuit is designed in standard 110 nm CMOS technology; it comes in two different versions: the amplifier-comparator version comprises 20 readout channels, while the amplifier-only version 16 channels. The ASIC power rail is at +1.2 V, and the power consumption for the front-end stage is 2.4 mW/ch and about 5 mW/ch for the output driver. In our tests, the FAST2 ASIC, coupled to a UFDS with a capacitance of 3.4 pF, achieves timing jitters of about 25 ps at an input charge of about 15 fC, while the simulation indicates that the FAST3 jitter will be about 15 ps at the same charge. Furthermore, FAST3 enhances its dynamic range up to 55 fC compared to the 15 fC of FAST2.

In this contribution we review the growing interest in implementing large-area fast-timing detectors with a time resolution of 30 -35 ps, based on Low-Gain Avalanche Detectors.Precise time information added to tracking brings benefits to the performance of the detectors by reducing the background and sharpening the resolution; it improves tracking performances and simplifies tracking combinatorics.Large-scale high-precision timing detectors have to face formidable challenges in almost every aspect: sensors performance, segmentation and radiation tolerance, very low-power and low-noise electronics, cooling, low material budget, and large data volumes.We will report on the current status and new development of such detectors for high energy physics, in view of their possible use in the experiment upgrades at the High Luminosity LHC and beyond.

Low Gain Avalanche Detectors (LGADs) are thin silicon detectors with moderate internal signal amplification and time resolution as good as 17 ps for minimum ionizing particles. However the current major limiting factor in granularity is due to protection structures preventing breakdown caused by high electric fields at the edge of the segmented implants. This structure, called Junction Termination Extension (JTE), causes a region of 50-100 μm of inactive space. Therefore, the granularity of LGAD sensors is currently limited to the mm scale. This challenge could be overcome by employing AC coupled LGADs (AC-LGADs) which can provide spatial resolution on the 10‘s of um scale. This is achieved by un-segmented gain layer and N-layer, then a di-electric layer separates the N+ and the metal readout pads. This design allows for 100% fill factor with no dead regions inside the sensor. The high spatial precision is achieved by using the information from multiple pads, exploiting the intrinsic charge sharing capabilities of the AC-LGAD provided by the common N-layer. Using a focused IR-Laser scans directed either at the read-out side on the front and the bias side on the back of the sensor, several detector parameters have been investigated with the goal of optimizing the sensor design: sheet resistance, thickness of the isolation di-electric, doping profile of the gain layer, and pitch and size of the readout pads. Furthermore we will show an interpolation technique for hit reconstruction and time-of-arrival measurement based on charge sharing. The data are used to recommend a base-line sensor for near-future large-scale application like the Electron-Ion Collider or the PIONEER experiment where simultaneous precision time and position resolution is required in the tracking detectors.

This paper presents the FAST2 family of ASICs. FAST2 aims to keep the jitter below 20 picoseconds when coupled to Ultra-Fast Silicon detectors (UFSD). FAST2 is designed in standard 110 nm CMOS technology, and it comes in 2 versions: the amplifier-comparator version comprises 20 readout channels while the amplifier-only version of 16 channels. The ASIC power rail is at +1.2 V, and its power consumption is 2.4 mW/ch. In our tests, the FAST2 ASIC, coupled to a UFDS with a capacitance of 3.4 pF, achieves timing jitters lower than 15 ps, at an input charge of about 15 fC. In tests with an Sr90 beta source, the FAST2 reached a time resolution below 45 ps.The worst case of Near-end and Far-end crosstalk due to the mutual capacitance between neighboring channels achieves an amplitude of -23 dB at 50 W of load.

In this paper we report on the timing resolution of the first production of 50 micro-meter thick Ultra-Fast Silicon Detectors (UFSD) as obtained in a beam test with pions of 180 GeV/c momentum. UFSD are based on the Low-Gain Avalanche Detectors (LGAD) design, employing n-on-p silicon sensors with internal charge multiplication due to the presence of a thin, low-resistivity diffusion layer below the junction. The UFSD used in this test belongs to the first production of thin (50 {\mu}m) sensors, with an pad area of 1.4 mm2. The gain was measured to vary between 5 and 70 depending on the bias voltage. The experimental setup included three UFSD and a fast trigger consisting of a quartz bar readout by a SiPM. The timing resolution, determined comparing the time of arrival of the particle in one or more UFSD and the trigger counter, for single UFSD was measured to be 35 ps for a bias voltage of 200 V, and 26 ps for a bias voltage of 240 V, and for the combination of 3 UFSD to be 20 ps for a bias voltage of 200 V, and 15 ps for a bias voltage of 240 V.

This paper describes the performance of a prototype timing detector, based on 50 micrometer thick Ultra Fast Silicon Detector, as measured in a beam test using a 180 GeV/c momentum pion beam. The dependence of the time precision on the pixel capacitance and the bias voltage is investigated here. A timing precision from 30 ps to 100 ps, depending on the pixel capacitance, has been measured at a bias voltage of 180 V. Timing precision has also been measured as a function of the bias voltage.

The medical physics group of the Turin section of the National Institute of Nuclear Physics, on the behalf of the MoVeIT collaboration, is working for the development of a new prototype of silicon strip detector for particle therapy applications. This device, based on 50 μm thin silicon sensors with internal gain, aims to detect the single beam particle and count their number up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /s fluxes, with a pileup probability <; 1%. A similar approach would lead to a drastic step forward, compared to the classical and widely used monitoring system based on ionization chambers. The better sensitivity, the higher dynamic range and the fact that the particle counting is independent of the beam energy, pressure and temperature, make this silicon detector suitable for the on-line dose monitoring in particle therapy applications. The prototype detector will cover a 3×3 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area and at the moment, two sets of strip sensors with different geometry and custom design, have been produced and are currently under investigation. The classic orthogonal strip positioning is used for beam profile measures. For what concerns the front-end electronics, the design of two different solutions is ongoing: one based on a transimpedance preamplifier, with a resistive feedback and the second one based on a charge sensitive amplifier. The challenging task for the design is the expected 3 fC - 130 fC wide input charge range (due to the Landau fluctuation spreading and different beam energies), dealing with a hundreds of MHz instantaneous rate (from 200 MHz up to 500 MHz ideally). To effectively design these components, it is crucial to perform preliminary investigation of the sensor response to the expected stimuli. For this reason an extensive work has been done and is still on going, using 1.2 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area and 50μm silicon pads with gain, performing test with the clinical beam of the Italian National Center of Oncological Hadrontherapy (CNAO) in Pavia, Italy.

In this contribution we review the progress towards the development of a novel type of silicon detectors suited for tracking with a picosecond timing resolution, the so called Ultra-Fast Silicon Detectors.The goal is to create a new family of particle detectors merging excellent position and timing resolution with GHz counting capabilities, very low material budget, radiation resistance, fine granularity, low power, insensitivity to magnetic field, and affordability.We aim to achieve concurrent precisions of ∼ 10 ps and ∼ 10µm with a 50 µm thick sensor.The first part of this contribution explains the basic concepts of low-gain silicon sensors, while in the following the main results are presented, together with the efforts to make the design radiation resistance.

Low Gain Avalanche Detectors (LGADs) are a type of thin silicon detector with a highly doped gain layer. LGADs manufactured by Fondazione Bruno Kessler (FBK) were tested before and after irradiation with neutrons. In this study, the Inter-pad distances (IPDs), defined as the width of the distances between pads, were measured with a TCT laser system. The response of the laser was tuned using $\beta$-particles from a 90Sr source. These insensitive "dead zones" are created by a protection structure to avoid breakdown, the Junction Termination Extension (JTE), which separates the pads. The effect of neutron radiation damage at \fluence{1.5}{15}, and \fluence{2.5}{15} on IPDs was studied. These distances are compared to the nominal distances given from the vendor, it was found that the higher fluence corresponds to a better matching of the nominal IPD.

Purpose For beam monitoring in particle therapy, silicon detectors could overcome the limitations of ionization chambers. In particular, silicon sensors with internal gain (Ultra Fast Silicon Detectors, UFSDs) provide high signal-to-noise ratio and fast collection times ( ∼1 ns in 50 μm thickness). A segmented sensor could allow discriminating and counting single protons up to high fluxes of therapeutic beams. Moreover, the excellent time resolution suggests using time-of-flight techniques for measuring the proton beam energy. Materials and Methods Several 50 μm thick UFSD prototypes, both single pads or segmented in strips, doped with Boron or Gallium, and with different doping concentrations were fully characterized at the Turin university laboratory and tested on the clinical proton beam of the CNAO particle therapy facility, up to fluxes of 109  p/s. Signal-to-noise ratio, pileup probability, internal gain and gain degradation with beam fluence were determined from the offline analysis of the collected waveforms and compared, whenever possible, to the laboratory measurements. Conclusions UFSDs are found to be a viable option for improving the qualification and the monitoring of a therapeutic proton beams. However, a careful design is necessary to avoid large pileup inefficiencies and early performance degradation.

The research of a few tens of pico-seconds accuracy in timing measurements is currently a hot topic not only in the field of high energy physics but also in several applied physics branches. The Ultra Fast Silicon Detector group of the Turin section of the INFN is involved in this challenge, developing extremely fast silicon sensors. This group spent the recent years simulating and designing UFSD devices, which are essentially a particular type of Low Gain Avalanche Diodes optimized for timing application. These innovative devices for particle tracking are furthermore suitable for a very accurate time measure. The novelty improving the time tagging capability is enabled by the inclusion of a controlled low gain in the detector response, therefore increasing the detector output signal amplitude while keeping controlled the noise. A fast detecting system requires a high-performance front-end electronics to be coupled with the sensors. In cutting-edge experiments like the High-Luminosity LHC, both high spatial and time resolutions are strict constraints. Therefore, highly segmented sensors are employed, implying high density of channels and integrated VLSI electronics. Thanks to the experience gained with the development and the characterization of other timing ASICs, the group is currently exploring various design possibilities for low power front-end electronics to be coupled with UFSDs. The design approach is based on the study of different amplifier architectures, with a dedicated study on the delays introduced by parasitic components. Moreover, taking advantage of different scaled and ultra-scaled CMOS technology nodes, it will be possible to compare among various technology features changing with thThe research of a few tens of pico-seconds accuracy in timing measurements is currently a hot topic not only in the field of high energy physics but also in several applied physics branches. The Ultra Fast Silicon Detector group of the Turin section of the INFN is involved in this challenge, developing extremely fast silicon sensors. This group spent the recent years simulating and designing Ultra-Fast Silicon Detectors (UFSDs), pe of Low Gain Avalanche Diodes optimized for timing application. These innovative devices for particle tracking are furthermore suitable for a very accurate time measure. The novelty improving the time tagging capability is enabled by the inclusion of a controlled low gain in the detector response, therefore increasing the detector output signal amplitude while keeping controlled the noise. A fast detecting system requires a high-performance front-end electronics to be coupled with the sensors. In cutting-edge experiments like the High-Luminosity LHC, both high spatial and time resolutions are strict constraints. Therefore, highly segmented sensors are employed, implying high density of channels and integrated VLSI electronics. Thanks to the experience gained with the development and the characterization of other timing ASICs, the group is currently exploring various design possibilities for low power front-end electronics to be coupled with UFSDs. The design approach is based on the study of different amplifier architectures, with a dedicated study on the delays introduced by parasitic components. Moreover, taking advantage of different scaled and ultra-scaled CMOS technology nodes, it will be possible to compare among various technology features changing with the technology scaling. After an extensive phase of study and design simulation, our group is planning to develop a multichannel ASIC in a commercial 110 μm technology. The planned prototype is a 2 mm × 5 mm chip consisting of 24 channels.

The Move-IT research project of the National Institute for Nuclear Physics aims at the study of models for biologically optimized treatment planning systems in particle therapy and the development of dedicated devices for plan verification. On behalf of this collaboration, the Turin medical physics group is working for the development of a new prototype of silicon strips detector. This device, based on 50 μm thin silicon sensors with internal gain, aims to detect the single beam particle and count their number up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /s fluxes, with a precision ≥ 99%. The prototype detector will cover a 3×3 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area, segmented in strips. The classic orthogonal strip positioning is used for beam profile measures. At the moment, two types of sensor geometry with different silicon design features have been characterized with laser, radioactive sources and with a clinical proton beam. For what concerns the front-end electronics, the challenging tasks are represented by the charge and dynamic range which are respectively the 3-150 fC and the hundreds of MHz instantaneous rate (at least 100 MHz, 250 MHz ideally). On this purpose, our group is exploring different solutions with the design of two prototypes of custom front-end electronics: one based on a resistive feedback differential transimpedance amplifier and a second one based on a charge sensitive amplifier with gain boost and a discrimination-activated reset of the feedback capacitance. Preliminary results on the ASIC characterization are presented in the following sections.

The upgrades of the CMS and ATLAS experiments for the high luminosity phase of the Large Hadron Collider will employ precision timing detectors based on Low Gain Avalanche Detectors (LGADs). We present a suite of results combining measurements from the Fermilab Test Beam Facility, a beta source telescope, and a probe station, allowing full characterization of the HPK type 3.1 production of LGAD prototypes developed for these detectors. We demonstrate that the LGAD response to high energy test beam particles is accurately reproduced with a beta source. We further establish that probe station measurements of the gain implant accurately predict the particle response and operating parameters of each sensor, and conclude that the uniformity of the gain implant in this production is sufficient to produce full-sized sensors for the ATLAS and CMS timing detectors.

Low Gain Avalanche Detectors (LGADs) are a type of thin silicon detector with a highly doped gain layer. LGADs manufactured by Fondazione Bruno Kessler (FBK) were tested before and after irradiation with neutrons. In this study, the Inter-pad distances (IPDs), defined as the width of the distances between pads, were measured with a TCT laser system. The response of the laser was tuned using $\beta$-particles from a 90Sr source. These insensitive dead zones are created by a protection structure to avoid breakdown, the Junction Termination Extension (JTE), which separates the pads. The effect of neutron radiation damage at \fluence{1.5}{15}, and \fluence{2.5}{15} on IPDs was studied. These distances are compared to the nominal distances given from the vendor, it was found that the higher fluence corresponds to a better matching of the nominal IPD.

In the past few years, the need of measuring accurately the spatial and temporal coordinates of the particles generated in high-energy physics experiments has spurred a strong R\&D in the field of silicon sensors. Within these research activities, the so-called Ultra-Fast Silicon Detectors (UFSDs), silicon sensors optimized for timing based on the Low-Gain Avalanche Diode (LGAD) design, have been proposed and adopted by the CMS and ATLAS collaborations for their respective timing layers. The defining feature of the Ultra-Fast Silicon Detectors (UFSDs) is the internal multiplication mechanism, determined by the gain layer design. In this paper, the performances of several types of gain layers, measured with a telescope instrumented with a $^{90}$Sr $\beta$-source, are reported and compared. The measured sensors are produced by Fondazione Bruno Kessler (FBK) and Hamamatsu Photonics (HPK). The sensor yielding the best performance, both when new and irradiated, is an FBK 45\mum-thick sensor with a carbonated deep gain implant, where the carbon and the boron implants are annealed concurrently with a low thermal load. This sensor is able to achieve a time resolution of 40~ps up to a radiation fluence of~\fluence{2.5}{15}, delivering at least 5~fC of charge.