Separation of Argon from atmospheric air and Measurements of 37Ar for CTBT purposes

Riedmann, Robin A. (2011). Separation of Argon from atmospheric air and Measurements of 37Ar for CTBT purposes (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The 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 << 37Ar activities in soil air >>.

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