1,721,038 research outputs found
High Timing Resolution Front-end Circuit for Silicon Photomultiplier Detectors
Full text linkNegli ultimi decenni, i miglioramenti tecnologici nei processi planari per la produzione di circuiti integrati CMOS hanno contribuito allo sviluppo di nuovi dispositivi allo stato solido ad elevate prestazioni, tra i quali vale la pena menzionare i fotomoltiplicatori al silicio. La peculiarità di questi sensori, meglio conosciuti con l’acronimo SiPM, riguarda l’amplificazione intrinseca che essi presentano quando vengono fatti funzionare in modalità Geiger; infatti, quando la tensione di polarizzazione è superiore alla tensione di breakdown del dispositivo, i SiPM sono in grado di generare un segnale in corrente con un fronte di salita molto ripido, anche in corrispondenza dell'incidenza di un singolo fotone sulla regione attiva del dispositivo. Inoltre, dati i loro costi contenuti, la loro solidità e l’insensibilità ai campi magnetici, i SiPM rappresentano una valida alternativa ai più affermati tubi fotomoltiplicatori (PMT). Per questo motivo, l’utilizzo di questi dispositivi sta prendendo piede in numerosi campi di applicazione, specialmente in quelli in cui è importante rilevare bassi livelli di luce con alte risoluzioni temporali. Questo è il caso della Tomografia ad Emissione di Positroni (PET), una tecnica adoperata in medicina nucleare per la diagnosi di alcune patologie, dove i fotomoltiplicatori sono impiegati per rilevare i fotoni gamma emessi da sostanze radiomarcate iniettate nel corpo del paziente. Durante questo corso di dottorato, è stato sviluppato un nuovo circuito di front-end per fotomoltiplicatori al silicio in tecnologia CMOS 130nm. Questo progetto è stato condotto in collaborazione con il gruppo di circuiti integrati di SLAC National Accelerator Laboratory, con sede a Menlo Park, California, con lo scopo di realizzare un canale analogico per sistemi PET risoluzione temporale allo stato dell’arte. Difatti, questo circuito di lettura è in grado di misurare non solo l’energia dell’evento, ma anche l’istante di tempo in cui il fotone viene assorbito dal sensore con una risoluzione temporale di poche decine di picosecondi, conforme con le specifiche di progetto. Conseguentemente è stato sviluppato un circuito integrato multicanale allo scopo di testare il canale di lettura analogico, implementando tutti i blocchi necessari per la conversione, l’analisi e la trasmissione dei dati.Over the last few decades, the technological advancement in planar processes for the production of CMOS integrated circuits has also enabled the development of new high performance solid-state detectors, among which it is worth mentioning silicon photomultipliers. The distinctive feature of such sensors, also known with the acronym SiPMs, is the intrinsic amplification that they exhibit when operating in Geiger mode; in this condition these devices are able to generate a fast current signal with an adequate amplitude even in response to the detection of a single impinging photon. Moreover, given their low cost, robustness and insensitivity to magnetic fields, SiPMs represent a valid alternative to the most established Photomultiplier Tubes (PMTs). Therefore, the application of these detectors is taking hold in a number of fields, especially where low light levels and fine time resolutions are concerned. This is the case of the Positron Emission Tomography (PET), a medical imaging technique aimed at diagnosing specific diseases, where photomultipliers are employed to detect the gamma-ray photons emitted by the radiotracer injected into the patient’s body. During this doctoral program, a new front-end circuit for silicon photomultipliers has been designed in a standard 130 nm CMOS technology. This project has been carried out in collaboration with the IC group of SLAC National Accelerator Laboratory based in Menlo Park, California, with the aim of developing an analog channel for PET systems with groundbreaking performances in terms of temporal resolution. Indeed, this electronic circuit is able to provide not only the energy of the detected event, but also the occurrence time of the photon absorption with a time resolution of just few tens of picoseconds, compliant with the design specifications. Subsequently, a multichannel Application Specific Integrated Circuit (ASIC) has been developed with the purpose of testing the analog front-end, by implementing all the circuit blocks useful for the conversion, parsing and transmission of the digital data
Correction induced by irrelevant operators in the correlators of the 2d Ising model in a magnetic field
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Stochastic timing in gene expression for simple regulatory strategies
Full text linkTiming is essential for many cellular processes, from cellular responses to external stimuli to the cell cycle and circadian clocks. Many of these processes are based on gene expression. For example, an activated gene may be required to reach in a precise time a threshold level of expression that triggers a specific downstream process. However, gene expression is subject to stochastic fluctuations, naturally inducing an uncertainty in this threshold-crossing time with potential consequences on biological functions and phenotypes. Here, we consider such ‘timing fluctuations’ and we ask how they can be controlled. Our analytical estimates and simulations show that, for an induced gene, timing variability is minimal if the threshold level of expression is approximately half of the steady-state level. Timing fluctuations can be reduced by increasing the transcription rate, while they are insensitive to the translation rate. In presence of self-regulatory strategies, we show that self-repression reduces timing noise for threshold levels that have to be reached quickly, while self-activation is optimal at long times. These results lay a framework for understanding stochasticity of endogenous systems such as the cell cycle, as well as for the design of synthetic trigger circuits
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
VRG: A database of vascular dysfunctions related genes
Full text linkAbstractHeart and vascular defects occur in a large number of hereditary and sporadic human diseases as a result of a complex interplay of genetic factors. Since genome sequencing of many organisms disclosed similarities among genomes, animal models are crucial for the discovery of genes involved in those pathological processes. Therefore we propose a VRG database, in which human data have been manually managed and integrated with mouse information in order to create a catalogue of genes involved in vascular diseases
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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