1,721,136 research outputs found
Positronium cooling at cryogenic temperature for advanced experiments
No full textNew Ps spectroscopy measurements, formation of antihydrogen for antimatter-matter comparison experiments, production of Ps beams require the efficient production of cooled positronium in vacuum. At present the most efficient positron-positronium converters are silica based ordered or disordered porous materials, in which formed Ps decreases its kinetic energy by collisional cooling. Recently new positron-positronium converters based on oxidized nanochannels in silicon were found to be very promising because of the tunability of the nanochannel size, which allows to overcome the limits imposed to the Ps cooling by the quantum confinement. With these converters, Ps with temperatures as low as 150 K was detected in vacuum by a TOF apparatus. The Ps formation, quantum confinement, collisional cooling and emission into vacuum from nanochanneled silicon will be discussed in light of recent results. © Published under licence by IOP Publishing Ltd
Collisional cooled Ps emitted into vacuum from silica-based porous materials: Experiment to measure the Ps cooling time
No full textIn recent experiments on positronium time of flight (Ps-TOF) we have studied emission of cooled and thermalized Ps into vacuum from oxidized nanochannels synthetized in silicon. Ps cools down through collisions with the walls of the channels before exiting into vacuum. An important unknown parameter in the Ps-TOF measurements is the permanence time in the medium, i.e. the Ps cooling time before emission into vacuum. In this paper we describe an experiment that allows us to estimate the cooling time of Ps by analyzing the Ps-TOF spectra of cool Ps at three different distances from the sample
Radiation damage characterization in implanted silica
No full textThe radiation damage produced in silica glass implanted with Ar+ and Au+ ions at different implantation energies and fluences was depth profiled by positron annihilation spectroscopy. The modification of the sub-nanovoids structure of the glass was investigated by measuring the change in the Doppler broadening of the 511keV positron annihilation line induced by the quenching of positronium, and the increasing of positron annihilation with oxygen defects. Two distinct defected regions were pointed out. The first region was found to be confined below the ion projected range Rp, and the second extended deep into the sample, up to two to three times the Rp plus the range straggling Rp. The type, origin and concentration of the defects are discussed
Time of Flight system to investigate positronium cooling
No full textA positronium Time of Flight (TOF) apparatus, conceived to work with continuous positron beams of intensity up to ∼10 6 positrons/s, was developed. The geometry of the TOF chamber and the acquisition chain are described in detail. The performances of the set up were preliminary tested with a laboratory positron beam of ∼5 × 10 3 positrons/s by measuring the Time of Flight of Ps emitted from oxidized nanochannels produced in a Si single crystal. A TOF spectrum of ∼10 4 events was collected in 9 days with a time resolution of 8 ns. The analysis of the TOF spectrum is discussed. This apparatus is going to be assembled at the intense positron source NEPOMUC at FRM-II reactor, where the measurement time of each spectrum will be reduced to less than 2 h, making this TOF system appropriate to investigate positronium emission after cooling in porous materials held at cryogenic temperature. © The Author(s) 2012
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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