1,720,983 research outputs found
Cosmogenic production rates and recoil loss effects in micrometeorites and interplanetary dust particles
No full textWe present a purely physical model to determine cosmogenic production rates for noble gases and radionuclides in micrometeorites (MMs) and interplanetary dust particles (IDPs) by solar cosmic-rays (SCR) and galactic cosmic-rays (GCR) fully considering recoil loss effects. Our model is based on various nuclear model codes to calculate recoil cross sections, recoil ranges, and finally the percentages of the cosmogenic nuclides that are lost as a function of grain size, chemical composition of the grain, and the spectral distribution of the projectiles. The main advantage of our new model compared with earlier approaches is that we consider the entire SCR particle spectrum up to 240 MeV and not only single energy points. Recoil losses for GCR-produced nuclides are assumed to be equal to recoil losses for SCR-produced nuclides. Combining the model predictions with Poynting-Robertson orbital lifetimes, we calculate cosmic-ray exposure ages for recently studied MMs, cosmic spherules, and IDPs. The ages for MMs and the cosmic-spherule are in the range <2.2–233 Ma, which corresponds, according to the Poynting-Robertson drag, to orbital distances in the range 4.0–34 AU. For two IDPs, we determine exposure ages of longer than 900 Ma, which corresponds to orbital distances larger than 150 AU. The orbital distance in the range 4–6 AU for one MM and the cosmic spherule indicate an origin either in the asteroid belt or release from comets coming either from the Kuiper Belt or the Oort Cloud. Three of the studied MMs have orbital distances in the range 23–34 AU, clearly indicating a cometary origin, either from short-period comets from the Kuiper Belt or from the Oort Cloud. The two IDPs have orbital distances of more than 150 AU, indicating an origin from Oort Cloud comets
Production and recoil loss of cosmogenic nuclides in presolar grains
Full text linkPresolar grains are small particles that condensed in the vicinity of dying stars. Some of these grains survived the voyage through the interstellar medium (ISM) and were incorporated into meteorite parent bodies at the formation of the Solar System. An important question is when these stellar processes happened, i. e., how long presolar grains were drifting through the ISM. While conventional radiometric dating of such small grains is very difficult, presolar grains are irradiated with galactic cosmic rays (GCRs) in the ISM, which induce the production of cosmogenic nuclides. This opens the possibility to determine cosmic-ray exposure (CRE) ages, i. e., how long presolar grains were irradiated in the ISM. Here, we present a new model for the production and loss of cosmogenic He-3, Li-6,Li-7, and Ne-21,Ne-22 in presolar SiC grains. The cosmogenic production rates are calculated using a state-of-the-art nuclear cross-section database and a GCR spectrum in the ISM consistent with recent Voyager data. Our findings are that previously measured He-3 and Ne-21 CRE ages agree within the (sometimes large) 2 sigma uncertainties and that the CRE ages for most presolar grains are smaller than the predicted survival times. The obtained results are relatively robust since interferences from implanted low-energy GCRs into the presolar SiC grains and/ or from cosmogenic production within the meteoroid can be neglected
CEREsFit
No full text<p>Added support and testing for Python 3.12.</p>
<p>Development environment moved to <a href="https://rye-up.com/">Rye</a>.</p>If you use this software, please cite it as below
Recommended from our members
Deciphering Galactic Chemical Evolution: Iron and Nickel Isotopes and Cosmic Ray Exposure Ages of Presolar Silicon Carbide Grains
Full text linkThe universe we live in originated in the Big Bang, which also produced the most abundant elements that are around today, namely hydrogen and helium. However, looking around on Earth today, many more elements exist around and inside of us. For example, humans are a carbon-based life form surviving by breathing nitrogen and oxygen and eating complex molecules. Elements heavier than lithium were not formed in the Big Bang. Most of them were synthesized in evolved stars, except for lithium, beryllium and boron, which were mostly formed by spallation reactions of cosmic rays with interstellar matter. This means that the composition that we find in the Solar System today represents the galactic chemical evolution of the elements and isotopes as it was 4.567 billion years ago. This composition can be studied by analyzing meteorites, which are rocks from space that regularly fall to Earth. Some of these meteorites have not significantly been altered throughout the history of the Solar System and therefore preserved a record of the original composition of the Solar System. In addition, these meteorites contain tiny, micrometer-sized, dust grains that did not form in the Solar System itself. These dust grains - presolar grains - formed in the outflow of dying stars, therefore allowing us to study how a specific star forms elements.
In this work we are looking at two aspects of presolar grain research. We first describe the newly built Chicago Instrument for Laser Ionization (CHILI) and compare it to the previous generation instruments at Argonne National Laboratory. We used CHILI to study the iron and nickel isotopic composition of presolar silicon carbide grains. While the neutron-rich isotopes Fe58 and Ni64 are mostly influenced by the parent star from which the grains formed, the neutron-poor isotopes are minimally altered and represent the composition of what went into the star in the first place. Therefore, iron and nickel isotopes are valuable in tracing nucleosynthesis in the parent star as well as to study galactic chemical evolution. Our study finds a good agreement with stellar models in terms of isotope anomalies in the neutron-rich isotopes and a good agreement with galactic chemical evolution models in the neutron-poor isotopes. It however remains a puzzle, how and why the galactic chemical evolution dominated isotopes in presolar silicon carbide grains show such a broad variety in isotopic composition compared to the Solar System.
The second part of this work focuses on the age of presolar silicon carbide grains. While these grains were transported through the interstellar medium, they were irradiated with galactic cosmic rays that induced nuclear reactions and yielded the production of so-called cosmogenic nuclides. These nuclides can be used in order to determine the time a presolar grain was exposed to the galactic cosmic ray flux. Here, we present a new model for cosmogenic production rates and discuss in detail the uncertainties that go into the model. We find that Ne21 is the most reliable cosmogenic nuclide to determine a cosmic ray exposure age of a presolar silicon carbide grain. Most presolar silicon carbide grains have ages between 10 and 200 Ma, with a clear peak in the distribution at around 20 Ma.
Our new iron and nickel isotopic measurements give tighter constraints on current galactic chemical evolution models. These measurements represent some of the first analyses with the Chicago Instrument for Laser Ionization. With this instrument, many more interesting galactic chemical evolution-dominated elements will be measured in the future, e.g., chromium and titanium, since these elements have only minimal contributions from the grain's parent star. In addition, correlated cosmogenic helium, lithium, and neon studies of individual presolar silicon carbide grains are currently in progress. Such studies will help to better understand current limitations of the cosmic-ray-induced production rates and recoil loss models
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
- …
