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Concept# Procédé de séparation

Résumé

En chimie et physique, un procédé de séparation est une technique ou une technologie permettant de transformer un mélange de substances en deux ou plusieurs composants distincts. Il est mis en œuvre à des fins soit analytiques, lorsque l'objectif est d'obtenir une information (qualitative ou quantitative), soit préparatives, lorsque l'objectif est d'obtenir une substance (à un degré de pureté donné). Les buts de ce type de procédé peuvent être divers :

- purification : des impuretés doivent être extraites du composé d'intérêt ;
- concentration : élimination d'une partie du solvant. Voir aussi Dessiccation ;
- fractionnement : séparation d'un mélange complexe en plusieurs mélanges différents.

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The predicted climate changes on Earth will significantly impact the environment and human society. The climate patterns observed during the past centuries led to a better understanding of the driving forces for such changes. However, more studies using new, innovative techniques are necessary to broaden the knowledge on the involved processes and accomplish a reliable scientific basis for decision-makers.On this subject, radiometric dating is a well-established contemporary technique whereby various time periods (from a few years to millions of years) can be covered with this method. Notably, a dating gap exists so that the period from 100 to 1000 years (before today) is currently not covered. However, this period is of great interest, as it allows for studying and understanding environmental processes such as glacier dynamics, ocean, and atmospheric circulation. Interestingly, a potential nuclide that could fill this dating gap is available: Silicon-32 (Si-32). The problem, however, is the currently very inaccurately determined T1/2 of around 150 years so that a use for radiometric dating is hindered.Hence, the aim of the SNSF-funded SINCHRON (Si-32: a new chronometer) project, and therefore the primary goal of this thesis, concerns the re-determination of the Si-32 half-life. Previous half-life determinations were limited to using samples with low activities. To overcome the issue with the small amounts, Si-32 was artificially produced at the Paul Scherrer Institut (PSI). Subsequently, an innovative wet chemical separation system was developed to allow for the selective removal of Si-32 from an irradiated vanadium matrix. As a result, 20 mL of an ultra-pure Si-32 solution could be produced, fulfilling the desired parameters related to the half-life re-determination.Within the framework of this work, the determination of the half-life via the direct method was applied; i.e., both the determination of the number of atoms (N), in combination with the activity (A), are required. Within the SINCHRON-collaboration, several independent measurements were performed between various multinational metrological institutes. Based on the different requirements, the Si-32 solution was manufactured accordingly: (I) the solution's activity concentration was confirmed to be greater than 100 kBq/g, (II) a chemically very stable Si species was chosen, and (III) ultra-traces of S-32 were removed. Besides, (IV) solid samples for accelerator mass spectrometry (AMS) could be prepared from the stock solution, too. As a result, we are getting close to providing a new, recommended value with low uncertainty (less than 5%). Within the scope of this work, a preliminary T1/2 for Si-32 of 125 +/- 5 years has been determined. Additionally, we studied vanadium as a target material and determined the production cross-sections of Ti-44, Ca-41, and Al-26. These radionuclides are produced as by-products during the irradiation process. For the latter two nuclides, the presented data describe the very first experimental determination of the cross-section for vanadium as a target. In this context, two independent gamma spectrometric measurement systems were used for the activity determination of Ti-44, and no prior chemical separation of Ti-44 from the matrix was required. Contrarily, Ca-41 and Al-26 were successfully separated using a selective and robust chromatographic wet chemical separation scheme. The activity of these nuclides could then be determined using AMS.

The accurate investigation of many geophysical phenomena via direct numerical simulations is computationally not possible nowadays due to the huge range of spatial and temporal scales to be resolved. Therefore advances in this field rely on the development of new theoretical tools and numerical algorithms. In this work we investigate a new mathematical formalism that exploits the property of quasi-linear systems to self-tune towards marginally stable states. The inspiration for this study comes from the asymptotic analysis of strongly stratified flows, performed by Chini et al.. The application of multi-scale analysis to this problem, justified by the presence of scale separation, yields to a simplified quasi-linear model. In this reduced description small-scale instabilities evolve linearly about a large-scale hydrostatic field (whose evolution is fully non-linear) and modify it via a feedback term. From the only assumption of scale separation, two extremely interesting features of this model arise. First the presence of the coupling term between the two dynamics and second the quasi-linearity of the dynamics. The first aspect, generally not present in the hydrostatic approximation, can capture the non-local energy transfer between the small and the large scales, which is key for the quantification of the mixing efficiency in the deep ocean. The second aspect, namely the quasi-linearity, is suggestive of the self-organisation of the dynamics about marginally-stable states. This results in a coupled evolution where the fast dynamics adapts (is slaved) to the mean field, maintaining the marginal stability of the latter. The low-dimensional evolution that arises, enables the integration of the reduced system on temporal scales comparable to the characteristic time scale of the slow dynamics, making this novel approach highly suited to the investigation of the stratified flow problem.Building upon the results obtained by Chini et al. in the present work we extend this methodology addressing three different aspects of the reduced model.As a first case we investigate the twofold nature of the fluctuation feedback, which is not sign-definite and might lead to intense bursting events where the fluctuations exhibit positive growth rates on a fast time scale. In this scenario the scale separation is temporarily lost and the two dynamics have to be co-evolved until a new marginally stable manifold can be approached. Here we propose three different co-evolution techniques and test their efficacy on a one-dimensional model problem.The second aspect we address is the presence of a finite scale separation between the two dynamics. We develop an algorithm that carefully identifies the validity regions of the quasi-linear reduction and determines the relevance of the fluctuation feedback w.r.t. the characteristic time scale of the slow dynamics and the growth rate of the fluctuations.As a third case we derive an efficient extension of the original methodology to two-dimensional model problems, deriving an evolution equation for the wavenumber of the fastest growing mode, which then get slaved to the mean dynamics.Eventually the methodologies derived in the context of the two model problems are applied and discussed for the strongly stratified flow problem.

We have analyzed data from a multi-site campaign to observe oscillations in the F5 star Procyon. The data consist of high-precision velocities that we obtained over more than three weeks with 11 telescopes. A new method for adjusting the data weights allows us to suppress the sidelobes in the power spectrum. Stacking the power spectrum in a so-called echelle diagram reveals two clear ridges, which we identify with even and odd values of the angular degree (l = 0 and 2, and l = 1 and 3, respectively). We interpret a strong, narrow peak at 446 mu Hz that lies close to the l = 1 ridge as a mode with mixed character. We show that the frequencies of the ridge centroids and their separations are useful diagnostics for asteroseismology. In particular, variations in the large separation appear to indicate a glitch in the sound-speed profile at an acoustic depth of similar to 1000 s. We list frequencies for 55 modes extracted from the data spanning 20 radial orders, a range comparable to the best solar data, which will provide valuable constraints for theoretical models. A preliminary comparison with published models shows that the offset between observed and calculated frequencies for the radial modes is very different for Procyon than for the Sun and other cool stars. We find the mean lifetime of the modes in Procyon to be 1.29(-0.49)(+0.55) days, which is significantly shorter than the 2-4 days seen in the Sun.

2010