1,720,968 research outputs found
Performance of an Alpha-IPEM
No full textThe ion photon emission microscope, or IPEM, is the first device that allows scientists to microscopically study the effects of single ions in air on semiconductors, microchips and even biological cells without having to focus the beam. Reported here is a prototype, the size of a conventional optical microscope, developed at Sandia. The alpha-IPEM, that employs alpha particles from a radioactive source, represents the first example of IBA imaging without an accelerator. The IPEM resolution is currently limited to similar to 10 um, but we also report a gridded-phosphor approach that could improve this resolution to that of the optical microscope, or similar to 1 um. Finally, we propose that a simple adaptation of the alpha-IPEM could be the only way to maintain the high utility of radiation effects microscopy into the future
Ion Beam Induced Luminescence of Doped Yttrium Compunds
No full textRare earth doped yttrium oxide (yttria) and silicate, Y2O3:Eu and Y2SiO5:Tb, are the most promising phosphors for advanced devices such as flat panel field-emission-displays. However, their light yield for electron excitation has proven to be lower than that predicted by early models. New experimental data are needed to improve the theoretical understanding of the cathodoluminescence (CL) that will, in turn, lead to materials that are significantly brighter. Beside the existing CL and photo luminescence (PL) measurements, one can provide new information by studying ion-induced luminescence (IL). Ions penetrate substantially deeper than electrons and their light yield should therefore not depend on surface effects. Moreover, the energy density released by ions can be much higher than that of electrons and photons, which results in possible saturation effects, further testing the adequacy of models.
We exposed the above yttrium compounds to three ion beams, H (3 MeV), C (20 MeV), Cu (50 MeV), which have substantially different electronic stopping powers. H was selected to provide an excitation close to CL, but without surface effects. The C and Cu allowed an evaluation of saturation effects because of their higher stopping powers. The IL experiments involved measuring the transient light intensity signal radiating from thin phosphor layers following their exposure to similar to 200 ns ion beam pulses. We present the transient yield curves for the two materials and discuss a general model for this behavior
Phosphors' Lifetime Measurement Employing the Time Between Photons Method
No full textThe Time Between Photons theory (hereafter TBP) is applied to the evaluation of the lifetime of phosphors employed in the Ion Photon Emission Microscope (IPEM). IPEM allows Radiation Effects Microscopy (REM) without focused ion beams and appears to be the best tool for the radiation hardness assessment of modern integrated circuit at cyclotron energies. IPEM determines the impact point of a single ion onto the sample by measuring the light spot produced on a thin phosphor layer placed on the sample surface. The spot is imaged by an optical microscope and projected at high magnification onto a Position Sensitive Detector (PSD). Phosphors, when excited by an ion, emit photons with a particular lifetime, which is important to evaluate. We measured the statistical distribution of the Time Between consecutive detected Photons (TBP) for several phosphors and have been able to link it to their lifetime employing a theory that is derived in this paper. The single-photon signals are provided by the IPEM-PSD, or faster photomultipliers when high-speed materials had to be assessed
Ion-Luminescence properties of GaN films being developed for IPEM
No full textRadiation effects microscopy (REM) for the next generation integrated circuits (ICs) will require GeV ions both to provide high ionization and to penetrate the thick overlayers in present day ICs. These ion beams can be provided by only a few cyclotrons in the world. Since it is extremely hard to focus these higher-energy ions, we have proposed the ion photon emission microscope (IPEM) that allows the determination of the ion hits by focusing the emitted photons to a position sensitive detector. The IPEM needs a thin luminescent foil that has high brightness, good spatial resolution and does not change the incident ion's energy and direction significantly. Available organic-phosphor foils require a large thickness to produce enough photons, which results in poor spatial resolution. To solve this problem, we have developed thin, lightly doped n-type GaN films that are extremely bright. We have grown high quality GaN films on sapphire using metal organic chemical vapor deposition (MOCVD), detached the films from the substrate using laser ablation, and made them self-supporting. The smallest foils have 1 mm(2) area and 1 um thickness. The optical properties, such as light yield, spectrum and decay times were measured and compared to those of conventional phosphors, by using both alpha particles from a radioactive source and 250 keV ions from an implanter. We found that the GaN performance strongly depends on composition and doping levels. The conclusion is that 1-2 um GaN film of a 1 mm(2) area may become an ideal ion position detector
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
- …
