1,721,013 research outputs found
Studying stars from the deep underground: the luna experiment and the case of
Full text linkUnderstanding the stellar evolution and the origin of chemical elements are the main goals of Nuclear Astrophysics. In the last century, many collaborations worked to develop experiments and accelerators to study in Earth laboratories the main nuclear processes taking place in stars at their relevant temperature. As an example, we present the measurement of the 13C(α,n)16O reaction performed by the LUNA collaboration
Introduction of the new LUNA experimental setup for high precision measurement of the 13C(α,n)16O reaction for astrophysical purposes
No full textThe 13C(α,n)16O reaction is the prevalent neutron source for the main s-process. The direct measurement of this reaction at stellar temperature (kT=8 keV) has so far not been possible due to the very low cross section at the corresponding energy. The extrapolation of the astrophysical S-factor of this reaction into the Gamow window (Eα,c.m.=140-230 keV) is complicated by the large uncertainties of the low-energy experimental data and the existence of a state of 17O near the α-threshold that can have a large effect on low energy cross section. The aim of this paper is to introduce the new LUNA experimental setup, dedicated to the investigation of 13C(α,n)16O reaction below Eα,lab=400 keV
Direct Measurement of the 13 C(a,n)16O Reaction at LUNA
No full textThe13 C(a,n)16 reaction is the main neutron source for the s-process in low mass AGB stars. Although several direct measurements have been performed, no dataset reaches the Gamow window (140–230�keV) due to the the nearly exponential drop of (E) with decreasing energy. The available dataset didnâ€TMt extend to lower energies because of the strong cosmic background and some difficulties to evaluate the target degradation. To study the13 C(a,n)16 cross section at low energies, ancillary measurements to characterize13 C enriched evaporated targets, under an high intensity proton beam (100–200 A), are carried out at Laboratori Nazionali del Gran Sasso (LNGS) in the framework of the LUNA experiment. The preliminary results are reported in this contribution
Direct 13C(x,n)16O Cross Section Measurement at Low Energies
No full textThe reaction C,n O is the main neutron source in the “s process”, which is responsible for the production of about half of the heavy elements in the universe. It operates in thermally pulsing low mass AGB stars at temperatures of about 90 MK. This translates to a Gamow window between 140 and 230 keV, far below the Coulomb barrier. Various measurements of the low energy cross section of C,n O have been performed in the past, and while remarkable results have been achieved, ultimately the environmental background on the surface of the earth has been a limiting factor. The LUNA collaboration is currently performing a measurement of C,n O in the low-background environment of the LNGS, where the environmental neutron flux is reduced by over three magnitudes with respect to the surface. This unique location, together with a high-efficiency low background detector and state of the art electronics that allow suppression of the intrinsic background, has already enabled us to push the low-energy cross section limit beyond what has been reached before. Here we present the current status of the experiment, the plans for an upcoming next measurement campaign and preliminary results
Direct Measurement of the (formula presented) Reaction at LUNA
No full textThe (formula presented) reaction is the main neutron source for the s-process in low mass AGB stars. Although several direct measurements have been performed, no dataset reaches the Gamow window (140–230 keV) due to the exponential drop of the cross section (formula presented)(E) with decreasing energy. The reaction rate becomes so low that the strong cosmic background would become predominant. In order to measure the (formula presented) cross section at low energies, ancillary measurements to understand the behaviour of 99% enriched (formula presented) evaporated targets, under a high intensity alpha beam (100–200 (formula presented)A). These measurements were carried out in deep underground laboratories of Laboratori Nazionali del Gran Sasso (LNGS) in the framework of the LUNA experiment. The preliminary results are reported in this contribution
Precise resonance energies measured for energy calibration of particle accelerator using thin silicon–nitride foils
No full textIn this work resonance parameters of the 28Si(p,p′γ)28Si (Eγ = 1779 keV) and 14N(p,p′γ)14N (Eγ = 2313 keV) reactions were measured between 3 and 4 MeV proton energies for absolute proton beam energy calibration of particle accelerators. For the measurements commercially available self-supporting thin silicon–nitride films and a HPGe gamma-detector were used which are readily available in most accelerator laboratories. Based on our measurement and on the assessment of literature data, the following resonance energies and widths are recommended: 3100.6 ± 0.9 keV (Γ = 11.9 ± 0.4 keV) and 3338.1 ± 1.0 keV (Γ = 11.0 ± 0.5 keV) in 28Si(p,p′γ)28Si and 3903.1 ± 1.7 keV (Γ = 98 ± 1 keV) and 3991.6 ± 1.1 keV (Γ = 10.8 ± 0.5 keV) in 14N(p,p′γ)14N. In addition to the resonance measurements, absolute thick target yields and gamma-ray angular distributions were measured for the 28Si(p,p′γ)28Si (Eγ = 1779 keV) reaction in the vicinity of Ep = 3.10 and 3.34 MeV for ion beam analysis purposes using a thick silicon wafer target
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
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