1,721,304 research outputs found
Gamma ray emission imaging in the medical and nuclear safeguards fields
No full textGamma rays emitted from within an object can reveal information about that object in a non-destructive way, i.e. without physically opening the object and looking inside. This makes gamma ray emission imaging very useful in widely varying applications. In these notes, we highlight its application to the medical field, where we discuss molecular imaging in nuclear medicine and in vivo dose delivery verification in particle beam radiotherapy, and nuclear safeguards field, where imaging of spent nuclear fuel assemblies is part of monitoring the non-proliferation of nuclear weapons. The purpose and basic principles of gamma ray emission imaging are discussed as the foundation to look in more detail into the essential instrument design considerations and the iterative image reconstruction procedures. These notes are not intended to be a comprehensive review; their purpose is to introduce gamma ray emission imaging to those that are new to this technique. The examples of implementation that are presented were thus chosen in order to introduce the reader to a fairly wide range of applications and practical implementations
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
Fuel rod classification from Passive Gamma Emission Tomography (PGET) of spent nuclear fuel assemblies
No full textSafeguarding the disposal of spent nuclear fuel in
a geological repository needs an effective, efficient,
reliable and robust non-destructive assay (NDA) system to
ensure the integrity of the fuel prior to disposal. In the
context of the Finnish geological repository, Passive
Gamma Emission Tomography (PGET) will be a part of
such an NDA system. We report here on the results of
PGET measurements at the Finnish nuclear power plants
during the years 2017-2020. The PGET prototype device
developed by IAEA and partners was used during 2017-
2019, whereas an updated device was used in 2020. The
PGET device contains two linear arrays of collimated
CdZnTe (CZT) gamma ray detectors installed opposite
each other inside a torus. Gamma activity profiles are
recorded from all angles by rotating the detector arrays
around the fuel assembly that has been inserted into the
center of the torus. Image reconstruction from the resulting
tomographic data is defined as a constrained minimization
problem with the function being minimized containing
a data fidelity term and regularization terms. The activity
and attenuation maps, as well as detector sensitivity
corrections, are the variables in the minimization process.
The regularization terms ensure that prior information on
the (possible) locations of fuel rods and their diameter are
taken into account. Fuel rod classification, the main
purpose of the PGET method, is based on the difference
of the activity of a fuel rod from its immediate neighbors,
taking into account its distance from the assembly center.
The classification is carried out by a support vector
machine. We report on the results for 10 different fuel
types with burnups between 5.72 and 55.0 GWd/tU,
cooling times between 1.87 and 34.6 years and initial
enrichments between 1.9 and 4.4%. For the 77 fuel
assemblies measured, the total misclassification rate
including misclassifications of missing fuel rods, present
rods and water channels, was 0.94% for the Olkiluoto
campaigns and 0.66% for the Loviisa campaigns. Further
development of the image reconstruction method is
discussed. We conclude that the combination of the PGET
device and our image reconstruction method provides
a reliable base for fuel rod classification. The method is
well-suited for nuclear safeguards verification of BWR fuel
assemblies in Finland prior to geological disposal. For
VVER-440 assemblies, some further work is needed to
investigate the ability to detect missing rods near the
center of the assembly
In-air and in-water performance comparison of Passive Gamma Emission Tomography with activated Co-60 rods
No full textA first‐of‐a‐kind geological repository for spent nuclear fuel is being built in Finland and will soon start operations. To make sure all nuclear material stays in peaceful use, the fuel is measured with two complementary non‐destructive methods to verify the integrity and the fissile content of the fuel prior to disposal. For pin‐wise identification of active fuel material, a Passive Gamma Emission Tomography (PGET) device is used. Gamma radiation emitted by the fuel is assayed from 360 angles around the assembly with highly collimated CdZnTe detectors, and a 2D cross‐sectional image is reconstructed from the data. At the encapsulation plant in Finland, there will be the possibility to measure in air. Since the performance of the method has only been studied in water, measurements with mock‐up fuel were conducted at the Atominstitut in Vienna, Austria. Four different arrangements of activated Co‐60 rods, steel rods and empty positions were investigated both in air and in water to confirm the functionality of the method. The measurement medium was not observed to affect the ability of the method to distinguish modified rod positions from filled rod positions. More extended conclusions about the method performance with real spent nuclear fuel cannot be drawn from the mock‐up studies, since the gamma energies, activities, material attenuations and assembly dimensions are different, but full‐scale measurements with spent nuclear fuel are planned for 2023
Simultaneous emission and attenuation reconstruction in passive gamma emission tomography of spent nuclear fuel
No full textIn the context of international nuclear safeguards, the International Atomic Energy Agency (IAEA) has recently approved passive gamma emission tomography (PGET) as a method for inspecting spent nuclear fuel assemblies (SFAs). The PGET instrument is essentially a single photon emission computed tomography (SPECT) system that allows the reconstruction of axial cross-sections of the emission map of an SFA. The fuel material heavily self-attenuates its gamma-ray emissions, so that correctly accounting for the attenuation is a critical factor in producing accurate images. Due to the nature of the inspections, it is desirable to use as little a priori information as possible about the fuel, including the attenuation map, in the reconstruction process. Current reconstruction methods either do not correct for attenuation, assume a uniform attenuation throughout the fuel assembly, or assume an attenuation map based on an initial filtered back-projection reconstruction. We propose a method to simultaneously reconstruct the emission and attenuation maps by formulating the reconstruction as a constrained minimization problem with a least squares data fidelity term and regularization terms. Using simulated data, we show that our approach produces clear reconstructions which allow for a highly reliable classification of spent, missing, and fresh fuel rods
Improved Passive Gamma Emission Tomography image quality in the central region of spent nuclear fuel
No full textReliable non-destructive methods for verifying spent nuclear fuel are essential to draw credible nuclear safeguards conclusions from spent fuel. In Finland, spent fuel items are verified prior to the soon starting disposal in a geological repository with Passive Gamma Emission Tomography (PGET), a uniquely accurate method capable of rod-level detection of missing active material. The PGET device consists of two highly collimated detector banks, collecting gamma emission data from a 360° rotation around a fuel assembly. 2D cross-sectional activity and attenuation images are simultaneously computed. We present methods for improving reconstructed image quality in the central parts of the fuel. The results are based on data collected from 2017 to 2021 at the Finnish nuclear power plants with 10 fuel assembly types of varying characteristics, for example burnups from 5.7 to 55 GWd/tU and cooling times from 1.9 to 37 years. Data is acquired in different gamma energy windows, capturing the peaks of Cs-137 (at 662 keV) and Eu-154 (at 1274 keV), abundant isotopes in long-cooled spent nuclear fuel. Data from these gamma energy windows at well-chosen angles are used for higher-quality images, resulting in more accurate detection of empty rod positions. The method is shown to detect partial diversion of nuclear material also in the axial direction, demonstrated with a novel measurement series scanning over the edge of partial-length rods
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
- …
