1,722,309 research outputs found
Long lived states in antiprotonic helium at low densities
No full textIt is now well established that a fraction of about 3% of the p's stopped in a helium target survives for a surprisingly long time (tens of microseconds). As suggested by Condo in 1964 the long lifetime of p's in helium is explained by the existence of metastable states of the exotic atom pHe/sup +/. Theoretical calculations predict an increase of such a fraction up to about 20% when lowering the pressure of the helium target to around 1 mbar. An analysis of data from p's stopped in helium targets at pressures from 150 mbar down to 0.2 mbar, collected by the Obelix apparatus at the LEAR decelerator at CERN, is presented. Evidence of a noticeable fraction of long lived states at helium target pressures lower than some millibar is reported. Moreover a dependence of this fraction on the p momentum distribution is observed. (16 refs)
Measurements of cascade times of antiprotons in hydrogen and helium
No full textThe Obelix experiment at CERN collected samples of antiproton annihilations at rest in different gaseous targets, such as hydrogen, deuterium and helium. We analyze a set of the Obelix data using a new technique for measuring, for the first time, the cascade times independent of the capture energy and of the antiproton stopping power. We report on measurements of the cascade times for hydrogen at 3.4, 5.8, 9.8 and 150 mbar and for helium at 8.2, 50 and 150 mbar pressure. An estimate of the antiproton capture energy in hydrogen is also given
Physics with low energy antiprotons (antihydrogen)
No full textThe first observations of antihydrogen (H) atoms [1, 2] in 1995 opened a new way of testing the fundamental symmetries of nature. These first experiments were not suited to precision comparison measurements having produced antihydrogen in small quantities and at high energy. A second generation of experiments was then built with the aim of creating antihydrogen in a controlled process inside an electromagnetic trap. ATHENA and ATRAP indeed succeeded in producing for the first time cold antihydrogen by mixing antiprotons (p’s) and positrons (e+’s) at low temperature (few K) in a nested Penning trap [3, 4]. This result was made possible thanks to the CERN Antiproton Decelerator (AD), the only facility in the world to deliver low energy antiprotons for antihydrogen physics. Since then, upgrades to the experiments were made to keep the antiatoms inside the traps, in order to perform atomic spectroscopy and thus test CPT. With such objective three experiments are at the moment on the floor: ALPHA, ASACUSA and ATRAP. Another experiment, AEgIS, was recently approved and will soon start construction in the AD hall. It aims at directly measuring the gravitational acceleration g on a beam of cold antihydrogen. An overview of past, present and future antihydrogen experimental activities will be given
Progress in Muon Tomography
No full textIn the last decades the interest and the research activities for applications of cosmic ray muons have grown considerably. Various groups around the world are considering these particles, that hit continuously our planet, to develop new techniques and systems. In the following the principles of muon radiography and tomography will be presented along with an overview of the present activities in this field
Muons: civil applications
No full textCosmic radiation has been known since the first decades of the 20th century: it has been considered for many years, the best source of projectiles to investigate the core of matter, from nuclei to elementary particles. Nowadays, cosmic ray muons are very important in particle and nuclear physics, because they are used for detector testing and calibration, and for detector alignment in complex measurement apparatuses, thanks to their high penetration capability. However, cosmic ray radiation has been already applied in fields beyond pure physics. An overview of the applications of the cosmic ray muons for civil usages, with special focus on the last development of the various techniques will be here presented
From demographic strategies to mathematical models: trends in population dynamics studies of aquatic Oligochaeta
No full textThe study of the population dynamics of aquatic oligochaetes is still largely a descriptive subject. However, going beyond description is essential in any field of science. This paper analyses the methodological approaches that can be used in this process, together with some examples from each of these approaches as
they have been applied to aquatic oligochaetes. Observation and experiments in the field, laboratory
experiments, and mathematical models are reviewed. Increased use of tools that are regarded as standard in
the field of population dynamics at large and interaction with applied disciplines are advocated as ways to
improve the quality of research in this field
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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