1,720,966 research outputs found
Superintense Laser-driven Ion Beam Analysis
Full text linkIon beam analysis techniques are among the most powerful tools for advanced materials characterization. Despite their growing relevance in a widening number of fields, most ion beam analysis facilities still rely on the oldest accelerator technologies, with severe limitations in terms of portability and flexibility. In this work we thoroughly address the potential of superintense laser-driven proton sources for this application. We develop a complete analytical and numerical framework suitable to describe laser-driven ion beam analysis, exemplifying the approach for Proton Induced X-ray/Gamma-ray emission, a technique of widespread interest. This allows us to propose a realistic design for a compact, versatile ion beam analysis facility based on this novel concept. These results can pave the way for ground-breaking developments in the field of hadron-based advanced materials characterization
Laser-Driven Neutron Generation with Near-Critical Targets and Application to Materials Characterization
Full text linkLaser-driven neutrons arouse outstanding interest because of their promising uses in several fields, from basic science to materials inspection. Many experiments achieved neutron yields [108 - 1010 n/(sr s)] suitable for applications. These results were obtained by exploiting high-energy (approximately 10-100 J) lasers working at low repetition rates. Instead, adopting advanced target configurations like near-critical double-layer targets (DLTs) and compact, commercial lasers was slightly considered. Here, a theoretical study is performed to address neutron generation with commercial (40-400 TW, 1-15 J) systems and DLTs. We investigate proton acceleration and interaction with various materials to induce (p, n) reactions. DLTs allow achieving 1-2 orders of magnitude larger neutron yields and maximum energies 3 times higher than with single-layer targets. Then, the feasibility of two materials characterization techniques, namely fast neutron activation analysis and pulsed fast neutron resonance radiography, is assessed. The results indicate that they can be performed with commercial lasers and DLTs
Superintense laser-driven photon activation analysis
Full text linkLaser-driven radiation sources are attracting increasing attention for several materials science applications. While laser-driven ions, electrons and neutrons have already been considered to carry out the elemental characterization of materials, the possibility to exploit high-energy photons remains unexplored. Indeed, the electrons generated by the interaction of an ultra-intense laser pulse with a near-critical material can be turned into high-energy photons via bremsstrahlung emission when shot into a high-Z converter. These photons could be effectively exploited to perform Photon Activation Analysis (PAA). In the present work, laser-driven PAA is proposed and investigated. We develop a theoretical approach to identify the optimal experimental conditions for laser-driven PAA in a wide range of laser intensities. Lastly, exploiting the Monte Carlo and Particle-In-Cell tools, we successfully simulate PAA experiments performed with both conventional accelerators and laser-driven sources. Under high repetition rate operation (i.e. 1−10 Hz) conditions, the ultra-intense lasers can allow performing PAA with performances comparable with those achieved with conventional accelerators. Moreover, laser-driven PAA could be exploited jointly with complementary laser-driven materials characterization techniques under investigation in existing laser facilities
Role of energetic ions in the growth of fcc and ω crystalline phases in Ti films deposited by HiPIMS
Full text linkIn this work, we show how to control the nucleation of fcc and hexagonal (ω) crystalline phases in Ti films by adding a proper Ti ion flux to the film-forming species. To this aim, films with different thicknesses are grown by High Power Impulse Magnetron Sputtering (HiPIMS) and conventional Direct Current Magnetron Sputtering (DCMS) as a reference. HiPIMS depositions with different substrate bias voltage US (0 V, −300 V and −500 V) are performed to investigate different ion energy ranges. Microstructure, morphology and residual stress of the deposited films, as well as the DCMS and HiPIMS plasma composition, are analysed with different characterization techniques. The DCMS samples, where the species are mostly atoms, exhibit the Ti α-phase only. As far as HiPIMS samples are concerned, films deposited in low energy ion conditions (US = 0 V) show the presence of the Ti fcc phase up to a maximum thickness of about 370 nm. Differently, films deposited under high energy conditions (US = −300 V and −500 V) show the nucleation of the Ti ω-phase for thicknesses greater than 260 and 330 nm, respectively. The formation of these unusual Ti phases is discussed considering the different deposition conditions
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
Role of magnetic field and bias configuration on HiPIMS deposition of W films
Full text linkIn this work, the deposition of tungsten (W) films by High Power Impulse Magnetron Sputtering (HiPIMS) has been investigated. By adopting a combined modeling and experimental approach, the role of magnetic field strength and bias configuration on growth of W films has been studied since they are relevant parameters for the energetic and ionized HiPIMS environment. Modeling results showed that increasing the magnetic strength from 40 to 60 mT led to larger W ion fraction in the plasma and, contemporary, to higher ion back-attraction to the target. This, in turn, resulted in a similar W ion fraction in the flux towards the substrate for the two magnetic field strengths considered during the on-time of the voltage pulse. On the contrary, the W ion fraction became significantly different in the afterglow and the same happened to the ion flux composition. Exploiting the studied discharges, W films have been grown applying at substrate negative pulsed bias voltages, both synchronized and delayed to the voltage pulse onset. Films morphology, microstructure, residual stress, composition and density have been examined. In light of plasma differences retrieved from the numerical investigation, film growth and properties 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
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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