A Bayesian method with empirically fitted priors for the evaluation of environmental radioactivity: application to low-level radioxenon measurements

International audienceThe decision that a given detection level corresponds to the effective presence of a radionuclide is still widely made on the basis of a classic hypothesis test. However, the classic framework suffers several drawbacks, such as the conceptual and practical impossibility to provide a probability of zero radioactivity, and confidence intervals for the true activity level that are likely to contain negative and hence meaningless values. The Bayesian framework being potentially able to overcome these drawbacks, several attempts have recently been made to apply it to this decision problem. Here, we present a new Bayesian method that, unlike the previous ones, presents two major advantages together. First, it provides an estimate of the probability of no radioactivity, as well as physically meaningful point and interval estimates for the true radioactivity level. Second, whereas Bayesian approaches are often controversial because of the arbitrary choice of the priors they use, the proposed method permits to estimate the parameters of the prior density of radioactivity by fitting its marginal distribution to previously recorded activity data. The new scheme is first mathematically developed. Then, it is applied to the detection of radioxenon isotopes in noble gas measurement stations of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty

Mechanisms of formation and reactivity of imogolite types material

Reactivity of nanopar8cles represents a central issue for many laboratories around the world. Among many supported efforts the control of the morphology of nanopar8cles is mo8vated by the fact that morphology strongly influence the proper8es of the final products. Among the vast family of available nanopar8cles, imogolite is a clay nanotube for which perfect control of the diameter is possible. Imogolites were first observed in volcanic soils[1]. They are natural aluminosilicate nanotubes having the general formula (OH)3Al2O3SiOH with a 2 nm external diameter and up to micrometers in length. The impressive monodispersity in imogolite nanotube diameter has mo8vated research on their forma8on mechanism. Synthesis protocols to produce imogolite were quickly developed. Farmer et al. were the first to obtain synthe8c imogolite using low concentra8ons of AlCl3 and SiO 2 monomers as star8ng materials (millimolar concentra8ons of the reagents) [2]. However, the produc8on of large amount of imogolite or imogolite type materials remained challenging for long 8me. We will present our most recent results concerning the possibility to produce imogolite type materials from highly concentrated stock solu8ons. We will also detail the possibility to form double wall Al- Ge nanotubes and the different stages of their forma8on [3-7]. We will then detail the surface reac8vity of these nanotubes toward metals at he lab scale as well as in natural soil. (Résumé d'auteur

New findings on the structure of natural and synthetic aluminosilicates nanoparticles

Abundance within andosols of highly reactive aluminosilicate nanoparticles makes of these an important factor affecting soil dynamics (carbon sequestration1, trapping pollutants2 ...). Gaining knowledge of the structural characteristics of such nanoparticles is of fundamental importance to understand their interactions with the different soil compartments. Aluminosilicate nanoparticles can adopt two main structures. Imogolites (Al2SiO3(OH)4), natural aluminosilicate nanotubes that have been well characterized since their discovery in 19723; And allophanes, aluminosilicates with identical chemical composition but with a different structure. allophanes have been described as hollow nanospheres with a diameter ranging from 3 to 5 nm and their structure depends on the Al/Si ratio: (i) Al-rich allophanes (Al/Si=2, Imogolite type local structure); (ii) Si-rich allophanes (Al/Si<2). Nonetheless, the only evidence for allophanes spherical nature up to date has solely come from TEM observations. The actual morphological structure of allophanes still needs to be further investigated. In the present work, Aluminosilicate samples obtained from soils collected in La Reunion (a French volcanic island in the Indian Ocean region) are studied using an array of diverse characterization techniques. While XRD and FTIR results are consistent with the characteristic allophane fingerprint, NMR analysis reveals an imogolite-type local environment of silicon and aluminium, pointing to a type i Al-rich structure. However, no spherical objects could be observed using TEM. In view of such observations, we propose that the structure of these type i Allophane is not consistent with that of a hollow sphere geometry. To obtain further insight into this matter, we synthesised aluminosilicate nanoparticles (both allophane and imogolite), and thoroughly characterized them using a wide variety of high specificity techniques ranging from the macro crystalline structure (TEM, XRD) to the atomic scale (XAFS, PDF). Our findings point to a structure consistent with that of a short imogolite nanotube type structure, rather than a hollow sphere. (Résumé d'auteur

Location of largest earthquake slip and fast rupture controlled by along-strike change in fault structural maturity due to fault growth

Earthquake slip distributions are asymmetric along strike, but the reasons for the asymmetry are unknown. We address this question by establishing empirical relations between earthquake slip profiles and fault properties. We analyze the slip distributions of 27 large continental earthquakes in the context of available information on their causative faults, in particular on the directions of their long-term lengthening. We find that the largest slips during each earthquake systematically occurred on that half of the ruptured fault sections most distant from the long-term fault propagating tips, i.e., on the most mature half of the broken fault sections. Meanwhile, slip decreased linearly over most of the rupture length in the direction of long-term fault propagation, i.e., of decreasing structural maturity along strike. We suggest that this earthquake slip asymmetry is governed by along-strike changes in fault properties, including fault zone compliance and fault strength, induced by the evolution of off-fault damage, fault segmentation, and fault planarity with increasing structural maturity. We also find higher rupture speeds in more mature rupture sections, consistent with predicted effects of low-velocity damage zones on rupture dynamics. Since the direction(s) of long-term fault propagation can be determined from geological evidence, it might be possible to anticipate in which direction earthquake slip, once nucleated, may increase, accelerate, and possibly lead to a large earthquake. Our results could thus contribute to earthquake hazard assessment and Earthquake Early Warning

In situ starch localisation in Cocos nucifera L. and Elaeis guineensis Jacq.: The major reserve carbohydrate in these Arecaceae or not ?

Carbohydrates accumulated in plants can later be mobilized to support biosynthesis for metabolism and growth particularly during periods of low assimilation (dry season), or after biotic or abiotic stresses. So, they play an important role in plants functioning. Starch is considered as the most important reserve carbohydrate in plants. Its content has been often used as sole indicator of a carbohydrate surplus over current needs, more generally, of carbohydrate status of plants. The subject is well documented for temperate and fruit-bearing species, little for tropical perennial species and almost none for Arecaceae. The distribution of starch was investigated on the basis of histological studies in all vegetative organs of adult coconuts (20-years old) and oil palms (12-years old) and compared. The anatomy of the studied organs has been described. Except for roots and leaflets, coconut and oil palms were characterized by anatomical structure uniformity. In the stem, many vascular bundles and groups of fibres were distributed in a parenchyma with small cells. Vascular bundles density decreased from the outer periphery of stem to the inner part, whereas their size showed reverse gradient. Starch grains were absent in roots, leaflets and lower parts of stem. They were found primarily at mid-height of stem, sometimes at its top and in leaf petioles and rachis, although these observations varied considerably among plants. Starch accumulates mainly in the parenchyma of bark and wood of trees. In coconut and oil palm, starch grains were located mainly in the reserves parenchyma cells and very little in the vascular bundles. In parallel, quantitative biochemical analyses confirmed our anatomical observations. Starch appeared in coconut and oil palms as not a major storage carbohydrate compared with soluble sugars like sucrose and glucose. Coconut and oil palms are characterized under optimal conditions by continuous vegetative growth and fruits production with a high caloric content (68-80% of stored lipids in the mature fruit). In order to meet quickly and continuously these very consuming needs for energy, storage in the form of soluble sugars seems well to be adapted to the functioning of coconut and oil palms (with a rapid adjustment of the mobilisable carbon level according to the needs). However, the starch pool varied clearly (in time) according our initial experimental conditions (e.g. a recovery after a pathological stress on coconut palm). For this reason, starch, even if there was little, could be a good indicator of a temporary carbohydrate surplus exceeding plant demand like numerous temperate woody species and, in our case, a good indicator of phytosanitary status. After hydrolysis, starch would seem to help occasionally in achieving optimum productivity again. (Texte intégral

A biomechanical model of swallowing for understanding the influence of saliva and food bolus viscosity on flavour release

International audienceAfter swallowing a liquid or a semi-liquid food product, a thin film responsible for the dynamic profile of aroma release coats the pharyngeal mucosa. The objective of the present article was to understand and quantify physical mechanisms explaining pharyngeal mucosa coating. An elastohydrodynamic model of swallowing was developed for Newtonian liquids that focused on the most occluded region of the pharyngeal peristaltic wave. The model took lubrication by a saliva film and mucosa deformability into account. Food bolus flow rate and generated load were predicted as functions of three dimensionless variables: the dimensionless saliva flow rate, the viscosity ratio between saliva and the food bolus, and the elasticity number. Considering physiological conditions, the results were applied to predict aroma release kinetics. Two sets of conditions were distinguished. The first one was obtained when the saliva film is thin, in which case food bolus viscosity has a strong impact on mucosa coating and on flavour release. More importantly, we demonstrated the existence of a second set of conditions. It was obtained when the saliva film is thick and the food bolus coating the mucosa is very diluted by saliva during the swallowing process and the impact of its viscosity on flavour release is weak. This last phenomenon explains physically in vivo observations for Newtonian food products found in the literature. Moreover, in this case, the predicted thickness of the mix of food bolus with saliva coating the mucosa is approximately of 20 µm; value in agreement with orders of magnitude found in the literature

Remote sensing indicators to monitor forest degradation trough time in the Brazilian Amazon

Recently, several remote sensing methods have been developed to quantify the degradation of tropical forests. However, it still lacks finest spatial and temporal analysis to define trajectories of forest degradation i.e. a temporal analysis of the impacts on forest integrity. This communication aims to explore this issue and proposes a set of operational indicators to monitor forest degradation, which can constitutes a decision tool to support forestry managers and policy makers. We studied the trajectories of forest degradation in the municipality of Paragominas – PA in the eastern Brazilian Amazon between 1995 and 2009, with a focus on the forestry company Cikel (400 000 ha certified by FSC since 2001). First, we developed a semi-automatic remote sensing methodology to detect forest degradation using multi-temporal Landsat images (spatial resolution of 30m) covering the 1995-2009 period. This method included two steps: 1) Identification of logging tracks and log landings using an algorithm of Bourbier et al. (2013). This algorithm uses spectral indices and morphological filters to strengthen the spectral contrasts between bare soil and forest cover. 2) Identification of logging gaps - which are characterised by senescent vegetation due to trees fall - using a Spectral Mixture Analysis carried out in CLASlite (Asner et al., 2009) and a fraction index (Souza et al., 2013). So, we obtained annual maps identifying these three major impacts. Secondly, we calculated annual landscape metrics of forest degradation using the R package "SpatialEco". Then, we calculated indicators which synthetize information about logging impacts and logging frequencies over the period from these annual degradation metrics. Finally, we selected a set of 6 indicators and statistically analysed the trajectories of degradation occurring in Paragominas using ACP and CAH. Our results emphasize four major degradation trajectories from well managed forests to highly-logged forests. They clearly show a difference between legal and illegal logging in terms of forest degradation. Moreover, they indicate that impacts of FSC certification on forest degradation was positive. Degradation was statistically lower in the certified logged plots compared to the uncertified plots. These set of indicators are adequate to monitor forest degradation through space and provide guidance to policy-makers for a better management of forest resources. (Texte intégral