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Separation of Argon from atmospheric air and Measurements of 37Ar for CTBT purposes
No full textThe verification of the CTBT (Comprehensive Nuclear-Test-Ban Treaty) relies on analytical tools for the identification and localisation of clandestine nuclear test explosions. Subsurface tests produce radioactive noble gas isotopes that may migrate in soil gases away from the detonation site and to the surface. Measurement of these isotopes in quantities occurring above background levels are therefore strong indicators of a nuclear test. For on-site inspections (OSI) under the CTBT, measurement of the noble gas 37Ar is considered an important technique:
The radioactive noble gas isotope 37Ar is a definitive and unambiguous indicator of an underground nuclear explosion (UNE) because the anthropogenic background is very low. It is also noted that earthquakes do not produce neutrons, hence a natural triggering event produces no 37Ar. 37Ar is produced in the underground due to neutron activation of Calcium by the 40Ca(n,α)37Ar reaction.
It is therefore crucial to measure the natural 37Ar activity in soils and rocks at different locations and determine the factors that control the 37Ar activity. The probability to detect this isotope in air or soil gas samples collected during an OSI depends on many parameters. These include production mechanisms, transport in the gas phase and the properties of the soil or rock, such as the effective diffusion coefficient, the gas-filled porosity and the specific surface area.
The measurement for CTBT purposes requires that 37Ar can be measured on short notice and that several gas samples can be measured in a few days or weeks. 37Ar has a half live of 35 days. Separation time of Argon from atmospheric air is therefore at a premium especially for low 37Ar activities close to the detection limit.
In this dissertation a new separation line for Argon from atmospheric air is presented in Part I. In chapter 2 the physico-chemical principles for the new separation line are discussed. The construction of the separation line is described in chapter 4. Argon is separated by cryogenic adsorption. Nine parallelly operated GC columns are packed with a low silica form of X-type zeolites, exchanged with Li-cations (Li-LSX). The columns are operated at -120 °C and are cooled with N2(`) in the gas phase. An electric current is applied directly to the columns, which are manufactured from stainless steel, for counterheating. In chapter 5 the theory for the separation of Argon from atmospheric air is compared with experimental results. Nitrogen and Oxyxen are adsorbed more strongly than Argon due to the experimentally determined heat of adsorption in the range 22–28, 13–15 and 11–12 kJ/mol, respectively. And in chapter 6 the results and performance of the new separation line are presented and discussed. The Argon purity after the separation and purfication in a getter is > 99.9. The Argon recovery is typically > 94 %. A complete separation of Argon from atmospheric air requires 3–4.5 hours. The comissioning of the new separation line was a necessary requirement for the measurements of 37Ar in Part II.
Part II covers measurements of natural 37Ar activities. In chapter 2 the counting statistics of measured 37Ar activities in proportional counters is discussed. For the Minimum Detectable Activity a short lag time is three times as important as a low background, a long measurement time and a large Argon volume, respectively. In chapter 3 the calculation of the 37Ar activity is described.
Chapter 4 covers the production mechanisms of 37Ar. 37Ar is mainly produced by the reaction 40Ca(n,α)37Ar. The dominant neutron source are cosmic rays. We also present a map of global scalings of 37Ar production in 70 cm soil depth. In general, the highest scalings (< 1000 mBq/m3 air) are found in 30°–60° northern latitute and are almost always a combination of high altitude areas and high average CaCO3 soil content.
Results of atmospheric 37Ar activities are given in 5. In general, the 37Ar activities agree with the range of values measured since 2003 which have an average of ∼ 1.2 mBq/m3 air. Measurements of the 37Ar activity in atmsopheric air in 16 cc counters have been shown to be challenging.
37Ar depth profiles are discussed in chapter 6. Measured 37Ar profiles have been compared with a simplyfied diffusion model. Advection-dispersion has been included by increasing the diffusion coefficient. Both simple diffusion and diffusion-dispersion models agree qualitatively with the measured 37Ar activity. Natural 37Ar activities are largely determined (i) by depth-dependent 37Ar production from neutron activation of Calcium with cosmic ray neutrons, (ii) by radio-active decay and (iii) by diffusion (or dispersion) in the shallow soil-column.
In chapter 7 measurements of 37Ar in soils are presented. The highest 37Ar activities are generally found in 1.8–2.5 m depth. A variance analysis of 37Ar activities correlating with the parameters altitude, sampling depth, Calcium content of the soil matrix and CO2 concentration was done. At depths < 1.75 m CO2 is the most important parameter, confirming the importance of gas transport. The most important environmental parameters for the 37Ar activities are precipitation and possibly pressure fluctuations. Radon is not suitable to predict the 37Ar activity in soil. However, a possible proxy for 37Ar using the CO2 concentration and the Calcium content was developed. The deviation of calculated and measured 37Ar activities is 12 % for the samples presented in this work. However, a general applicability of the proposed proxy has to verified.
Exposure of Ca-rich materials to cosmic rays and the resulting 37Ar activities are described in chapter 8. 37Ar activities were measured in closed vessel where Ca-rich materials were exposed to cosmic rays. The largest 37Ar activity measured was ∼ 410 mBq/m3 air. In absence of gas transort out of the system, the 37Ar activity reflects the cosmogenic production by spallation of Calcium.
The emanation coefficient of 37Ar from Ca-rich materials is discussed in chapter 9. The emanation coefficients of two materials (dry Portland cement and Ca-rich gravel) exposed to cosmic ray can be explained by recoil emanation of quasi-spherical grains, their simple packing and average pore widths between adjacent grians, and implantation of recoiling 37Ar ions. The emanation coefficient of natural soils, however, are an order of magnitude larger than those determined from the dry Portland cement and Ca-rich gravel.
In chapter 10 some general conclusions of natural 37Ar activities and in chapter 11 implications for CTBT are given.
A scientific paper of the main results presented in chapters 2, 4, 6 and 7 in Part II has been submitted in April 2011 to the Environmental Science & Technology Journal, with the title >
Vacuum pressure swing adsorption-based Ar separation from air on a single adsorbent
No full textDue to their inert nature, radioactive noble gases are key for verifying the nuclear character of underground explosions. 37Ar sampling and detection is considered as an important technique for on-site inspections (OSI) for CTBT verification. Compared to radioxenon, 37Ar has a longer half-life and remains detectable for a longer time. For the detection of 37Ar, high purity Ar needs to be recovered from subsoil or atmospheric air. Less energy intensive alternatives, than current systems using at least a cryocooled adsorbent, for recovering Ar from air are sought for. In this context, research on Ar separation in adsorbents operating near room temperature for OSI is led by CTBTO. SCK CEN was contracted to verify the potential of a silver-exchanged molecular sieve (Ag ETS 10) for separating Ar from air by Vacuum Pressure Swing Adsorption (VPSA) at room temperature. In this project, we confirmed for instance its preferential adsorption of Ar over O2, with a 1.4 selectivity, through air-breakthrough measurements. Ar purity against yield was determined from separations in VPSA mode. Additional research was performed outside the project to investigate Ar separation in a larger set of conditions. These results will be discussed in the framework of Ar separation for CTBT verification
Vacuum pressure swing adsorption-based Ar separation from air on a single adsorbent
No full textDue to their inert nature, radioactive noble gases are key for verifying the nuclear character of underground explosions. 37Ar sampling and detection is considered as an important technique for on-site inspections (OSI) for CTBT verification. Compared to radioxenon, 37Ar has a longer half-life and remains detectable for a longer time. For the detection of 37Ar, high purity Ar needs to be recovered from subsoil or atmospheric air. Less energy intensive alternatives, than current systems using at least a cryocooled adsorbent, for recovering Ar from air are sought for. In this context, research on Ar separation in adsorbents operating near room temperature for OSI is led by CTBTO. SCK CEN was contracted to verify the potential of a silver-exchanged molecular sieve (Ag ETS 10) for separating Ar from air by Vacuum Pressure Swing Adsorption (VPSA) at room temperature. In this project, we confirmed for instance its preferential adsorption of Ar over O2, with a 1.4 selectivity, through air-breakthrough measurements. Ar purity against yield was determined from separations in VPSA mode. Additional research was performed outside the project to investigate Ar separation in a larger set of conditions. These results will be discussed in the framework of Ar separation for CTBT verification
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
Natural 37Ar Concentrations in Soil Air: Implications for Monitoring Underground Nuclear Explosions
No full textFor on-site inspections (OSI) under the Comprehensive Nuclear-Test-Ban Treaty (CTBT) measurement of the noble gas 37Ar is considered an important technique. 37Ar is produced underground by neutron activation of Calcium by the reaction 40Ca(n,α)37Ar. The naturally occurring equilibrium 37Ar concentration balance in soil air is a function of an exponentially decreasing production rate from cosmic ray neutrons with increasing soil depth, diffusive transport in the soil air, and radioactive decay (T1/2: 35 days). In this paper for the first time, measurements of natural 37Ar activities in soil air are presented. The highest activities of ∼100 mBq m–3 air are 2 orders of magnitude larger than in the atmosphere and are found in 1.5–2.5 m depth. At depths >8 m 37Ar activities are <20 mBq m–3 air. After identifying the main 37Ar production and gas transport factors the expected global activity range distribution of 37Ar in shallow subsoil (0.7 m below the surface) was estimated. In high altitude soils, with large amounts of Calcium and with low gas permeability, 37Ar activities may reach values up to 1 Bq m–3
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
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