Pile-up Effects Research Articles

Time-domain single photon-excited autofluorescence lifetime for label-free detection of T cell activation.

Fluorescence lifetime imaging microscopy (FLIM) is a powerful technique, capable of label-free assessment of the metabolic state and function within single cells. The FLIM measurements of autofluorescence were recently shown to be sensitive to the functional state and subtype of T cells. Therefore, autofluorescence FLIM could improve cell manufacturing technologies for adoptive immunotherapy, which currently require a time-intensive process of cell labeling with fluorescent antibodies. However, current autofluorescence FLIM implementations are typically too slow, bulky, and prohibitively expensive for use in cell manufacturing pipelines. Here we report a single photon-excited confocal whole-cell autofluorescence system that uses fast field-programmable gate array-based time tagging electronics to achieve time-correlated single photon counting (TCSPC) of single-cell autofluorescence. The system includes simultaneous near-infrared bright-field imaging and is sensitive to variations in the fluorescence decay profile of the metabolic coenzyme NAD(P)H in human T cells due to the activation state. The classification of activated and quiescent T cells achieved high accuracy and precision (area under the receiver operating characteristic curve, AUC = 0.92). The lower-cost, higher acquisition speed, and resistance to pile-up effects at high photon flux compared to traditional multiphoton-excited FLIM and TCSPC implementations with similar SNR make this system attractive for integration into flow cytometry, sorting, and quality control in cell manufacturing.

Anomalous edge response of cadmium telluride-based photon counting detectors jointly caused by high-flux radiation and inter-pixel communication

This work reports an edge enhancing effect experimentally observed in cadmium telluride (CdTe)-based photon counting detector (PCD) systems operated under the charge summing (CS) mode and irradiated by high-flux x-rays. Experimental measurements of the edge spread functions (ESFs) of a PCD system (100 μm pixel size, 88 ns deadtime) were performed at different input flux levels from 4.5 × 105 count per second (cps) mm−2 to 1.5 × 109 cps mm−2 for the single pixel mode (SP) and the CS mode. A theoretical model that incorporates the impacts of inter-pixel communications and the arbitration process involved in the CS mode was developed to help explain the physical origin of the observed edge enhancing effect. Compared with the monotonically increasing ESF of the SP mode, the ESF of the CS mode measured at high-flux levels shows a peak at an intermediate location (50 μm from the edge). The peak became more pronounced with increasing flux levels. The theoretically calculated ESFs agreed well with experimental results with relative errors less than 5% at all flux levels and tested. These results indicate that the anomalous edge enhancing effect is jointly caused by the pileup effect and the CS circuit that introduces negative correlations between adjacent pixels. When the input flux is high enough to deliver photons to multiple adjacent pixels within the same deadtime period, the CS mode may treat the coincident x-rays as shared charges and thus introduce count losses in addition to the well-known pileup count loss. When a high contrast object partially blocks certain pixels from x-rays, the adjacent unblocked pixels have an increased probability of registering counts as a result of the negative correlation. This leads to a peak on the ESF at a pixel-to-edge distance half of the pixel pitch.

Methodological comment on the nanoindentation of ion-irradiation hardened materials

This paper is dedicated to the discussion of some issues connected with the commonly applied methods to analyze radiation hardening using nanoindentation on ion-irradiated samples. These issues mainly comprise the sample surface preparation, the indentation measurement mode, pile-up and softer substrate effect, then the indentation size effect, especially applicability of the Nix-Gao model in the investigated depth range.

Episodes of chip formation in micro-to-nanoscale cutting of Inconel 625

Nickel-chromium superalloys (Inconel 625) are used for various conventional/macro-sized products as well as for micro/nano applications. However, machining of these ‘difficult-to-cut’ materials is challenging due to their extreme toughness, high temperature resistance and continuous work hardening. Thus, the tool-based cutting processes require improvements for micro/nano applications as the chip formation mechanisms in micro-to-nanoscale are not yet understood clearly. Hence, in this study, material removal mechanisms have been comprehensively analysed in ultra-precision machining of Inconel 625 at three distinct regimes of micrometer, sub-micrometer and nanometer level. The physical responses of material were analogized to the fundamental mechanisms of cutting, ploughing, sliding and burnishing by the change of removal scale. In microscale cutting, continuous chips with lamella-like structure were noticed akin to conventional cutting. Sub-microscale machining delivers significant variations of the material removal episodes as chip perforation, bifurcation, fragmentation and structural heterogeneity as the tool advances from grain to grain, thus, influencing the change of chip thickness, striations and machining forces. The fluctuation of force profiles noticed due to the material pile-up effect influencing the overall coefficient of friction (COF). Nanoscale removal persisted from the grain cutting phenomena with the transformation of material cutting to material deformation in sliding-like behaviour. Prevalence of damage on the machined surface, induced by tensile stress, diminishes from micro to sub-micrometer removal lengths. Nanoscale machined surface, affected by the high value of negative effective angle of rake and compressive stress, correlated with the slide burnishing (SB) phenomena for improved surface finishing. This study, thus, provides a fundamental basis on various aspects of tool-based processes of Ni-Cr superalloy in terms of material behaviour, tool-work interaction, chip formation and surface generation in micro-to-nanoscale operations.

Utilization of adaptive flattop time with large area silicon drift detectors

Energy-dispersive detection of X-rays using silicon drift detectors has a wide range of applications, especially for non-destructive material analysis. As X-ray spectra are acquired by signal processing of detector output signals, a key element is a shaping filter used for determination of X-ray energy values within digital signal processing. A flattop region in this filter prevents degradation of spectra by ballistic deficit effects. Detectors with large active areas demand high flattop times due to slow maximum signal rise times, causing pile-up effects and loss of count rate capability. In this work an approach using an energy filter with variable flattop time is presented, adapting the flattop time on a pulse-to-pulse basis to signal rise time. Implementation in hardware and experimental investigation is done using a 150mm2 active area silicon drift detector. Spectroscopic performance is investigated using a 55Fe source operating at 1 × 106counts/s. Peaking times of 50ns and 100ns are considered. In two series of measurements, influence of ballistic deficit is investigated for static flattop times between 150ns and 800ns, and for adaptive flattop time with 150ns to 450ns additional flattop time. Full width at half maximum, position, symmetry, and area of Mn-Kα peak are evaluated and compared. Results at similar influence of ballistic deficit show increased signal throughput for adaptive flattop time by 17% using the 50ns peaking time, and by 15% using the 100ns peaking time. Comparison at similar signal throughput, on the other hand, shows superior spectra quality achieved by adaptive flattop time, indicating a significantly lower impact of ballistic deficit.

Application of the MTSVD method in the experimental x-ray spectrum unfolding in the diagnostic energy range using CR film and Plexiglas wedge

Obtaining the quality of the beam is considered one of the most important quality control issues for X-ray tubes. X-ray spectrum is a very important aspect of quality control of medical imaging devices. However it may not be measured directly due to the very high photon flux, increasing the problems of the dead time and the Pile-up effect. The purpose of this study is to use a simple, and indirect method to unfold the primary X-ray spectrum. In this investigation, a system composing of a PMMA wedge, and a CR film was designed and constructed. The absorbed dose by the film is related to the primary spectrum by response matrix which was obtained using the MCNP Monte Carlo code exactly. The response matrix was used to unfold the spectrum, from the reading of the CR film. However, as the response matrix is ill-conditioned, it is not possible to solve the equation system in the normal way. Therefore the MTSVD regularization method in MATLAB software was used for unfolding the primary spectrum. The shape of the spectrum and the beam quality was compared with the IPEM 78 report catalog. The results indicate that the X-ray spectrum may be unfolded using this indirect method with sufficient accuracy.

Indentation size effect, geometrically necessary dislocations and pile-up effects in hardness testing of irradiated nickel

The indentation size effect response, based on dislocation density, of commercially pure Ni irradiated at 120°C with 6MeV protons to 0.1dpa, was compared with non-irradiated Ni. Irradiation-induced defects were characterised by TEM. Nano-scale and micro-scale indentation tests were carried out. The Nix-Gao (NG) model was applied to determine the increase in the bulk yield strength arising from irradiation induced defects, which agreed well with the predicted increase in yield strength from the Bacon-Kocks-Scattergood (BKS) obstacle hardening model, using a superposition of SFT, glissile perfect loops and sessile Frank loops. A bi-linear trend was observed in the NG model in both the non-irradiated and irradiated material, indicating a transition in deformation mechanisms for indents in the ‘nano-scale’ regime. The extent of the bi-linear trend was minimized if indentation pile-up was measured and accounted for. It is shown that, in this material, care must be taken when interpreting measurements of the increase in hardness due to irradiation induced defects when measurements are made from depths shallower than ̴500nm. It is experimentally shown using both strain gradient plasticity modelling and independent SEM/EBSD analysis that there is a higher density of geometrically necessary dislocations (GNDs) present in the deformation volume under an indent in the irradiated material when compared to the analogous indent in the non-irradiated material; this results from a more confined plastic deformation volume under the indent in the irradiated material. It is proposed that sessile Frank loops inhibit plastic deformation by acting as obstacles to dislocation motion which results in deformation under the indenter tip being more localised. The result is a larger difference in hardness between the irradiated and non-irradiated material when measured using nanoindentation compared to bulk hardness testing. The increase in GND density in nanoindentation of irradiated material can be accounted for by correcting for indentation pile-up.

Finite Element Analysis of Conical Indentation: Proposing a Contact Area Independent Procedure Based on Strain Energy Analysis to Estimate Materials Mechanical Properties

A systematic finite element analysis (FEA) of conical indentation in different isotropic elastic-plastic bulk solids was realized. In particular, the relation between the indentation Hardness (H)-to-Reduced Elastic Modulus (Er) ratio and the Elastic (Ue)-to-Total (Ut) strain energy ratio was studied. Results showed a linear dependency between both ratios as predicted by analytical studies, being the proportionality constant mainly dependent on cone semi-angle(𝜃) and material Poisson´s ratio (𝑣). A well-agreement with experimental data reported in the literature was observed for a wide variety of materials. By a simple analysis of the Oliver & Pharr method and our results, we proposed two analytical equations to calculate the H and E parameters overcoming the indenter contact area (AC) dependence in indentation data analysis. AC independent analytical procedures can become interesting to simplify the data analysis and overcome considerable uncertainties due to possible pile-up effects associated to different plastic deformation phenomena.

Fully digital PET is unaffected by any deterioration in TOF resolution and TOF image quality in the wide range of routine PET count rates

PurposeDigital PET involving silicon photomultipliers (SiPM) provides an enhanced time-of-flight (TOF) resolution as compared with photomultiplier (PMT)-based PET, but also a better prevention of the count-related rises in dead time and pile-up effects mainly due to smaller trigger domains (i.e., the detection surfaces associated with each trigger circuit). This study aimed to determine whether this latter property could help prevent against deteriorations in TOF resolution and TOF image quality in the wide range of PET count rates documented in clinical routine.MethodsVariations, according to count rates, in timing resolution and in TOF-related enhancement of the quality of phantom images were compared between the first fully digital PET (Vereos) and a PMT-based PET (Ingenuity). Single-count rate values were additionally extracted from the list-mode data of routine analog- and digital-PET exams at each 500-ms interval, in order to determine the ranges of routine PET count rates.ResultsRoutine PET count rates were lower for the Vereos than for the Ingenuity. For Ingenuity, the upper limits were estimated at approximately 21.7 and 33.2 Mcps after injection of respectively 3 and 5 MBq.kg-1 of current 18F-labeled tracers. At 5.8 Mcps, corresponding to the lower limit of the routine count rates documented with the Ingenuity, timing resolutions provided by the scatter phantom were 326 and 621 ps for Vereos and Ingenuity, respectively. At higher count rates, timing resolution was remarkably stable for Vereos but exhibited a progressive deterioration for Ingenuity, respectively reaching 732 and 847 ps at the upper limits of 21.7 and 33.2 Mcps. The averaged TOF-related gain in signal/noise ratio was stable at approximately 2 for Vereos but decreased from 1.36 at 5.8 Mcps to 1.14 and 1.00 at respectively 21.7 and 33.2 Mcps for Ingenuity.ConclusionContrary to the Ingenuity PMT-based PET, the Vereos fully digital PET is unaffected by any deterioration in TOF resolution and consequently, in the quality of TOF images, in the wide range of routine PET count rates. This advantage is even more striking with higher count-rates for which the preferential use of digital PET should be further recommended (i.e., dynamic PET recording, higher injected activities).

An Artificial Neural Network System for Photon-Based Active Interrogation Applications

Active interrogation (AI) is a promising technique to detect shielded special nuclear materials (SNMs). At the University of Michigan, we are developing a photon-based AI system that uses bremsstrahlung radiation from an electron linear accelerator (linac) as an ionizing source and trans-stilbene organic scintillating detectors for neutron detection. Stilbene scintillators are sensitive to fast neutrons and photons and have excellent pulse shape discrimination (PSD) capabilities. The traditional charge integration (CI) method commonly used for PSD analysis eliminates piled-up pulses and relies on a particle discrimination line to separate neutrons and photons. The presence of the intense photon flux during AI creates a significant number of piled-up events in the stilbene scintillator, thereby posing a great challenge to the traditional CI method. Identifying true single neutron pulses becomes challenging due to the presence of a pile-up cloud and overlapping neutron, photon and pile-up clouds in the PSD analysis. To mitigate the effect of pulse pile-up and identify true single neutron pulses from stilbene scintillators, an artificial neural network (ANN) system is developed. The developed ANN system identifies single neutron pulses and neutron-photon combinations from piled-up events. The results obtained from a 252Cf measurement in the presence of the intense photon flux show that the developed ANN system outperforms the traditional CI method. Since many piled-up events lie above the particle discrimination line, they get misclassified as neutrons by the traditional CI method resulting in 27% overestimation of the net neutron count rate during the linac pulse. The overall net neutron count rate (single and restored neutrons) during the linac pulse, estimated by the ANN system is 62.32% of the ground truth. Energy spectroscopy of the ANN attributed single neutron pulses further provides evidence on the detection of prompt fission neutrons from the 252Cf fission source.

A novel reconstruction framework for an imaging calorimeter for HL-LHC

To sustain the harsher conditions of the high-luminosity LHC, the CMS collaboration is designing a novel endcap calorimeter system. The new calorimeter will predominantly use silicon sensors to achieve sufficient radiation tolerance and will maintain highly-granular information in the readout to help mitigate the effects of pileup. In regions characterised by lower radiation levels, small scintillator tiles with individual on-tile SiPM readout are employed. A unique reconstruction framework (TICL: The Iterative CLustering) is being developed to fully exploit the granularity and other significant detector features, such as particle identification and precision timing, with a view to mitigate pileup in the very dense environment of HL-LHC. The inputs to the framework are clusters of energy deposited in individual calorimeter layers. Clusters are formed by a density-based algorithm. Recent developments and tunes of the clustering algorithm will be presented. To help reduce the expected pressure on the computing resources in the HL-LHC era, the algorithms and their data structures are designed to be executed on GPUs. Preliminary results will be presented on decreases in clustering time when using GPUs versus CPUs. Ideas for machine-learning techniques to further improve the speed and accuracy of reconstruction algorithms will be presented.

A High-Granularity Timing Detector for the Phase-II upgrade of the ATLAS Calorimeter system: detector concept, description and R&D and beam test results

The particle flux increase (pile-up) at the HL-LHC with luminosities of L = 7.5 × 1034 cm−2 s−1 will have a significant impact on the reconstruction of the ATLAS detector and on the performance of the trigger. The forward region and the end-cap where the internal tracker has poorer longitudinal track impact parameter resolution, and where the liquid argon calorimeter has coarser granularity, will be significantly affected. A High Granularity Time Detector (HGTD) is proposed to be installed in front of the LAr end-cap calorimeter for the mitigation of the pileup effect, as well as measurement of luminosity. It will have coverage of 2.4 to 4.0 from the pseudo-rapidity range. Two dual-sided silicon sensor layers will provide accurate timing information for minimum-ionizing particles with a resolution better than 30 ps per track (before irradiation), for assigning each particle to the correct vertex. The readout cells are about 1.3 mm × 1.3 mm in size, which leads to a high granular detector with 3 million channels. The technology of low-gain avalanche detectors (LGAD) with sufficient gain was chosen to achieve the required high signal-to-noise ratio. A dedicated ASIC is under development with some prototypes already submitted and evaluated. The requirements and general specifications of the HGTD will be maintained and discussed. R&D campaigns on the LGAD are carried out to study the sensors, the related ASICs and the radiation hardness. Both laboratory and test beam results will be presented.

Challenges to Accurate Evaluation of Bulk Hardness from Nanoindentation Testing at Low Indent Depths

Estimations of bulk hardness from nanoindentation data are frequently subject to considerable uncertainties, due to indentation size effects, potential pileup effects, and potential influence of surface quality or test methods. In this study, we examined materials science principles of nanoindentation test methods to enable accurate prediction of bulk hardness for a series of high purity Fe and Fe alloys containing 3-25 wt. %Cr. These materials were tested in as-annealed and thermally aged (100-900 hours at 475 oC for 14-25 wt. %Cr) conditions to produce Cr-rich a’ precipitates of varying size and density (bulk Vickers hardness values of ~50 to 350 VHN). Nanoindentation performed with a Berkovich indenter using constant strain rate (0.05 to 0.5 /s) and constant loading rate test methods provided comparable bulk equivalent hardness (H0) extracted by Nix-Gao model, indicating a weak strain rate sensitivity of the investigated materials at room temperature. Results from electropolished and fine mechanically polished samples were found to give comparable measured hardness. Conversely, material pileup adjacent to the indented area produced a 7-20% correction to the indent contact area. The Nix-Gao fitted nanoindentation H0 after pileup corrections agreed quantitatively better with the bulk Vickers hardness than the uncorrected H0 values. The model-predicted (dispersed barrier hardening superposition) and measured strength values agreed for aged Fe18Cr, indicating that Nix-Gao model combined with pileup corrections significantly improved the accuracy of hardness evaluation by nanoindentation. Analytic and experimental analyses demonstrate that inappropriate methods such as hardness ratios or changes at a reference depth (applied in many prior nanoindentation studies) can cause quantitative errors in bulk hardness estimates as large as 60% due to indentation size effects.

Performance studies of Time-of-Flight detectors at LHC

We present results of a toy model study of performance of the Time-of-Flight detectors integrated into forward proton detectors. The goal of the ToF device is to suppress effects of additional soft processes (so called pile-up) accompanying the hard-scale central diffractive event, characterized by two tagged leading protons, one on each side from the interaction point. The method of mitigation of the pile-up effects exemplified in this study is based on measuring a difference between arrival times of these leading protons at the forward proton detectors and hence estimate the z-coordinate of the production vertex. We evaluate effects of the pile-up background by studying in detail its components, and estimate the performance of the ToF method as a function of the time and spatial resolution of the ToF device and of the number of pile-up interactions per bunch crossing. We also propose a new observable with a potential to efficiently separate central diffractive signal from the harsh pile-up environment.

Anisotropy in the hardness of single crystal tungsten before and after neutron irradiation

In this work, the hardness of single crystal tungsten (W) after low, medium and high temperature neutron irradiation, reaching up to 1200°C, is studied. Micro-hardness tests with Vickers pyramid indenter are performed on reference and neutron irradiated samples by varying the orientation of the indenter on the single crystal tungsten contact surface. Due to the availability of multiple slip systems, the anisotropy of the hardness is characterized to allow the construction of uncertainty bands as well as dependence of the absolute value of the hardness on the orientation of the indenter wedges and principal crystallographic axes of the crystal. The applied neutron fluence and flux is representative for the ITER reactor, where tungsten is selected as armour material for plasma facing components. The neutron irradiation is performed in the BR2 reactor with mixed neutron spectrum up to 1 dpa (end of life fluence for ITER) but with appropriate measures to reduce the thermal neutron flux, such that the spurious transmutation into rhenium is reduced. The hardness study is accompanied with a detailed microscale examination to deduce the true indenter surface area. As a result, the true values of the hardness accounting for the pile-up effects are obtained as a function rotation angle of the indenter. The variation of the hardness as a function of indenter orientation before and after irradiation is discussed based on the indenter-slip system geometry. A simple but efficient methodology is proposed to deduce the true hardness and its angle-dependent variation for single crystal tungsten.

A high precision narrow-band neutrino beam: The ENUBET project

The knowledge of the initial flux, energy and flavor of current neutrino beams is the main limitation for a precise measurement of neutrino cross-sections. The ENUBET ERC project is studying a facility based on a narrow-band neutrino beam capable of constraining the neutrino fluxes normalization through the monitoring of the associated charged leptons in an instrumented decay tunnel. In ENUBET, the identification of large-angle positrons from [Formula: see text] decays at single particle level can potentially reduce the [Formula: see text] flux uncertainty at the level of 1%. This setup would allow for an unprecedented measurement of the [Formula: see text] cross-section at the GeV scale. This input would be highly beneficial to reduce the budget of systematic uncertainties in the next long baseline oscillation projects. Furthermore, in narrow-band beams, the transverse position of the neutrino interaction at the detector can be exploited to determine a priori with significant precision the neutrino energy spectrum without relying on the final state reconstruction. This contribution will present the advances in the design and simulation of the hadronic beam line. Special emphasis will be given to a static focusing system of secondary mesons that can be coupled to a slow extraction proton scheme. The consequent reduction of particle rates and pile-up effects makes the determination of the [Formula: see text] flux through a direct monitoring of muons after the hadron dump viable, and paves the way to a time-tagged neutrino beam. Time-coincidences among the lepton at the source and the neutrino at the detector would enable an unprecedented purity and the possibility to reconstruct the neutrino kinematics at source on an event-by-event basis. We will also present the performance of positron tagger prototypes tested at CERN beamlines, a full simulation of the positron reconstruction chain and the expected physics reach of ENUBET.

Estimation of the number of muons with muon counters

The origin and nature of the cosmic rays is still uncertain. However, a big progress has been achieved in recent years due to the good quality data provided by current and recent cosmic-rays observatories. The cosmic ray flux decreases very fast with energy in such a way that for energies ≳1015 eV, the study of these very energetic particles is performed by using ground based detectors. These detectors are able to detect the atmospheric air showers generated by the cosmic rays as a consequence of their interactions with the molecules of the Earth’s atmosphere. One of the most important observables that can help to understand the origin of the cosmic rays is the composition profile as a function of primary energy. Since the primary particle cannot be observed directly, its chemical composition has to be inferred from parameters of the showers that are very sensitive to the primary mass. The two parameters more sensitive to the composition of the primary are the atmospheric depth of the shower maximum and the muon content of the showers. Past and current cosmic-rays observatories have been using muon counters with the main purpose of measuring the muon content of the showers. Motivated by this fact, in this work we study in detail the estimation of the number of muons that hit a muon counter, which is limited by the number of segments of the counters and by the pile-up effect. We consider as study cases muon counters with segmentation corresponding to the underground muon detectors of the Pierre Auger Observatory that are currently taking data, and the one corresponding to the muon counters of the AGASA Observatory, which stopped taking data in 2004.

Development of a passive gold-foil Nested Neutron Spectrometer to validate the active current-mode He-3 measurements in a high neutron fluence rate radiotherapy environment

Nested Neutron SpectrometersTM (NNS) can be used to measure neutron fluence rate spectra under diverse circumstances with a working principle similar to Bonner sphere systems. Conventionally, the NNS consists of an active-readout He-3 detector core and concentric moderator shells. In environments where the neutron fluence rate exceeds ∽104 neutrons/s, these spectrometers may be operated in a current-mode to avoid the effects of pulse pile-up and deadtime. A current-to-pulse conversion factor is used to convert current-mode measurements to pulse-mode. However, the conversion factor can only be directly calibrated under low-flux conditions due to the pulse pile-up in high-flux situations. In order to have confidence in the use of the conversion factor in high neutron fluence rate environments such as in high-energy radiotherapy, its use must be experimentally validated. To perform this validation, we developed a passive-readout NNS with gold activation foils. Our work included the generation of system response functions using the Monte Carlo toolkit, GEANT4, and an experimental workflow. The passive NNS and the active NNS were then used to measure the secondary neutron fluence rate spectra produced by a Varian TrueBeamTM STx linac under identical experimental conditions. We found that the spectrum obtained using the active NNS agreed well with that obtained using the passive NNS within uncertainties. This serves as validation of the use of the current-mode of the active NNS in the high neutron fluence rate conditions encountered in radiotherapy.

DOSIMETRY WITH THE ABILITY TO DISTINGUISH PULSED AND NON-PULSED DOSE CONTRIBUTIONS

The concept of an active dosimetry system for pulsed radiation dose rate measurements is presented. Real-time distinction of pulsed and non-pulsed radiation contributions is based on the time structure of a single interaction. A fast tissue equivalent plastic scintillator is exploited to minimize the pile-up effect influence on absorbed energy measurements. Being connected to a fully digital signal processing board, the detector creates an active dosimetry system with adjustable parameters. With this system, absorbed dose rate measurements were carried out in a photon field with a time structure mimicking a radiotherapeutic beam, but also in the presence of a constant radiation field. Measurements show a linear dependence of a pulsed radiation contribution on the accelerator current in the investigated range of the total dose rate up to 8 μGy h-1. While increasing the accelerator current by 1 μA, the pulsed radiation dose rate grows by (26.2±0.9) nGy h-1 when considering pile-up events.

Design and characterization of the FAST chip: a front-end for 4D tracking systems based on Ultra-Fast Silicon Detectors aiming at 30 ps time resolution

Detectors able to measure the time of flight with very high accuracy (∼10 ps RMS) are becoming fundamental in the design of new High Energy Physics experiments, where accurate time measurements will be used to mitigate pileup effects. The development of such detectors has spurred intense R&D in both silicon sensors and the associated readout electronics, aiming at obtaining silicon-based detectors with a time resolution in the few tens-of-picosecond range. This work presents FAST, a family of three different 20 channel amplifier-comparator chips, tailored to the readout of Ultra Fast Silicon Detectors. These ASICs have been designed optimizing the sensor-readout interplay with the aim of reaching the smallest possible jitter term. The three chips of the FAST family differ in the architecture of the front-end while sharing the channel back-end, consisting of a leading-edge discriminator and a LVDS driver. The goal of these front-ends is to achieve a time resolution of about 30 ps RMS while coupled to a sensor with a few pico-Farad capacitance, keeping the power budget of the single channel below 1.3 mW. This paper reports the description of the FAST design architecture and summarizes the results on the initial characterization of one chip of the FAST family, in a stand-alone test structure and when coupled to a UFSD.