1,721,006 research outputs found
Nuclear astrophysics deep underground : the Luna experiment
No full textLUNA (Laboratory for Underground Nuclear Astrophysics) is a nuclear astrophysics experiment running at the INFN Laboratori Nazionali del Gran Sasso (LNGS). Aim of the experiment is to measure the crosssection of fusion reactions that take place inside the stars and that, in the past, dominated the Big Bang nucleosynthesis (BBN). The low value of these cross-sections (varying from pb to fb and even smaller), in the astrophysical range of energies, prevent any kind of measurements at the Earth's surface. On the other hand, the low background of the LNGS underground laboratory allows LUNA to investigate these reactions at energies of astrophysical interest without the necessity of an extrapolation from the highest energies. Recently, the LUNA Collaboration has been engaged in the study of the 2H(α,γ) 6Li and 17O(p,γ) 18F reactions. In this paper the experimental set-up will be described and the preliminary data will be discussed. The future possibilities of LUNA will also be outlined
STUDY OF POSITRONIUM CONVERTERS IN THE AEGIS ANTIMATTER EXPERIMENT
Get PDFThis PhD thesis is focused on the AEGIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) antimatter experiment. The main work consisted in the design, realization and characterization of an electron beam. This machine, now fully operational, will be used for ageing measurements of positron to positronium converters for the AEGIS experiment. In addition, I have been working on the design of the final AEGIS positron source and participating in the studies and modelling needed for the choice of the appropriate converter for the experiment. Finally, I made measurements of positronium formation in liquid scintillators, of interest for solar neutrino detectors
Positronium Formation and Decay in Organic Scintillators for Neutrino Detection
No full textThe detection of electron anti-neutrinos is generally carried out by searching for the coincidence of the reaction products, neutron and positron, in liquid scintillators. However, in these last a positron may form positronium (Ps) with an electron of the medium; efficiency of the process may be high. Furthermore, the triplet ground state sublevel (o-Ps) has lifetimes of a few ns. These features introduce distortions in the time distribution of the emitted photons, which is essential for position reconstruction and pulse shape discrimination algorithms in anti-neutrino experiments. This drawback can be favorably exploited by using o-Ps as a probe to detect anti-neutrinos in the scintillator. We report results of positron annihilation lifetime measurements in some organic liquid scintillators used for neutrino experiments. The o-Ps induced distortion of the scintillation photon emission time distribution may enhance the anti-neutrino detection. © (2013) Trans Tech Publications, Switzerland
Swelling of Positronium Confined in a Small Cavity
Get PDFThe electron density at the positron (contact density) in the ground state positronium (Ps) formed in condensed matter is generally found to be lower than in vacuum. This is usually attributed to microscopic electric fields which polarize Ps, by acting on the two particles of the atom. In this paper we quantitatively investigate an opposite effect. It is due to the confinement of Ps in small cavities existing in the host solid (e.g. free volume in polymers), which increases the contact density. Although this phenomenon is greater, the smaller is the size of the cavity, Ps polarization seems to play anyway a predominant role
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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