323,862 research outputs found
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
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
CO2 Laser irradiation of GeO2 planar waveguide fabricated by rf-sputtering
No full textGeO2 transparent glass ceramic planar waveguides were fabricated by a RF-sputtering technique and then irradiated by a pulsed CO2 laser. The effects of CO2 laser processing on the optical and structural properties of the waveguides were evaluated by different techniques including m-line, micro-Raman spectroscopy, atomic force microscopy, and positron annihilation spectroscopy. After laser annealing, an increase of the refractive index of approximately 0.04 at 1.5 μm and a decrease of the attenuation coefficient from 0.9 to 0.5 db/cm at 1.5 μm was observed. Raman spectroscopy and microscopy results put in evidence that the system embeds GeO2 nanocrystals and their phase varies with the irradiation time. Moreover, positron annihilation spectroscopy was used to study the depth profiling of the as prepared and laser annealed samples. The obtained results yielded information on the structural changes produced after the irradiation process inside the waveguiding films of approximately 1 μm thickness. In addition, a density value of the amorphous GeO2 samples was evaluated.Fil: Chiasera, A.. Consiglio Nazionale delle Ricerche. Istituto di Fotonica e Nanotecnologie; ItaliaFil: Macchi, Carlos Eugenio. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Instituto de Física de Materiales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Tandil. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires; ArgentinaFil: Mariazzi, C.. Università di Trento. Dipartimento di Fisica; ItaliaFil: Valligatla, S.. Consiglio Nazionale delle Ricerche. Istituto di Fotonica e Nanotecnologie; Italia. University of Hyderabad. School of Physics; India. Università di Trento. Dipartimento di Fisica; ItaliaFil: Lunelli, L.. Bruno Kessler Foundation; Italia. National Research Council Institute of Biophysics; ItaliaFil: Pederzolli, C.. Bruno Kessler Foundation; ItaliaFil: Rao, D. N.. University of Hyderabad. School of Physics; IndiaFil: Somoza, Alberto Horacio. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Instituto de Física de Materiales; Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Tandil. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires; ArgentinaFil: Brusa, R. S.. Università di Trento. Dipartimento di Fisica; ItaliaFil: Ferrari, M.. Consiglio Nazionale delle Ricerche. Istituto di Fotonica e Nanotecnologie; Itali
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
Diffusive author(s), cohesive author: Analysis of S/N (1994)
No full textThis study indicates the ways in which various aspects of the author(s) are brought forth in Dumb type’s performance art, the S/N production. Previous research has suggested a non-hierarchical organization of Dumb type and the absence of a “privileged author” in Dumb type’s collaborative work, S/N. However, the results that I have investigated from member’s interviews on the creative process of S/N along with my analysis of the recorded images of S/N, indicate a different aspect of the author(s). First, S/N was created through, so to speak, the collective ideas of the members of Dumb type. Further, S/N has at least nine quotations from previous performances, installations, and printed writings, besides the work-in-progress technique. Explicating one of the “author functions” as given by Michel Foucault, each text has plural subjects of the author. However, it has been revealed from members’ interviews that Teiji Furuhashi had a decision-making role in selecting the members’ ideas within the performance. Since then, S/N has had plural subjects of creation; however, Furuhashi is one of the subjects of creation along with the “privileged author.” S/N has plural authors (diffusive authors) yet at the same time, it has a “privileged author,” Teiji Furuhashi (cohesive author)
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Single-crystal silicon coimplanted by helium and hydrogen: Evolution of decorated vacancy like defects with thermal treatments
No full textSi p-type (100) samples were coimplanted at room temperature with He+ ions at 30 keV with a dose of 1×1016 ions/cm2 and successively with H+ ions at 24 keV with a dose of 1×1016 ions/cm2. A series of samples was thermally treated for 2 h from 100 to 900 °C at 100 °C steps to study the evolution of pointlike and extended defects by two complementary techniques: positron Doppler broadening spectroscopy and transmission electron microscopy. Depth profiling the samples with a positron beam led to the identification of five different traps and the evolution of their profile distributions with thermal treatments. The positron traps were identified as decorated vacancy clusters of different sizes. Their decoration by implanted ions and in some case by oxygen was probed by coincidence Doppler broadening spectroscopy. Up to 300 °C annealing temperature positrons probe three distributions of different decorated defects covering regions of the sample down to 400–450 nm. Starting from 300 °C annealing temperature no defects were revealed by positrons in the region next to the peak of the implanted ions distributions positioned around 280 nm, where extended defects are expected; this indicates complete filling of the defects by H and He. From 300 to 600 °C decorated vacancy clusters of different sizes appear progressively in the region below 280 nm, with a distribution moving deeper into the sample. Comparison with previous measurements on He-implanted samples points out the chemical action of H. Hydrogen atoms interact with the previous damage by He, producing more stabilized vacancylike defects distributed through the damage region of the sample. Electron microscopy shows the transformation of the extended defects from platelets to blisters and cavities
Dispelling the Myths Behind First-author Citation Counts
No full textWe 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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