Reduction of deuterium content in carbon targets for 12C+12C reaction studies of astrophysical interest

The 12C(12C,p)23Na and 12C(12C,$\alpha$)20Ne fusion reactions are among the most important in stellar evolution since they determine the destiny of massive ($M \simeq 8-10 M_{\odot}$) stars. However, experimental low-energy investigations of such reactions are significantly hampered by ubiquitous natural hydrogen and deuterium contaminants in the carbon targets. The associated beam-induced background completely masks the reaction products of interest thus preventing cross-section measurements at the relevant energies of astrophysical interest, $E_{\mathrm{cm}} < 2$ MeV. In this work, we report about an investigation aimed at assessing possible deuterium reductions on both natural graphite and Highly Ordered Pyrolytic Graphite targets as a function of target temperature. Our results indicate that reductions up to about 80% can be attained on both targets in the temperature range investigated, $T \simeq 200-1200 {}^{\circ}$C. A further reduction by a factor of 2.5 in absolute deuterium content is observed when the scattering chamber is surrounded by a dry nitrogen atmosphere so as to minimise light-particles uptake within the chamber rest gas (and thus on target) through air leaks. The results from this study will inform the choice of optimal experimental conditions and procedures for improved measurements of the 12C + 12C reactions cross-sections at the low energies of astrophysical interest

The qualitative stage of building Bayesian belief networks in a focus group setting: Decision-making under uncertainty among Vietnamese rice farmers

An innovative mixed-methods approach to exploratory focus group design is presented using a case study conducted with smallholder rice farmers in Vietnam. Understanding human decision-making under the uncertainties of a complex and changing social and environmental context requires a flexible yet structured and theoretically grounded approach. Using Bayesian belief networks as the architecture of our model allows the study to incorporate both qualitative and quantitative data, the former gathered at this stage in a participatory focus group setting and the latter to be collected in a subsequent survey. This framework further lends itself well to incorporating systematic behavioral approaches to decision-making analysis using Ajzen’s theory of planned behavior framework, a symbiosis that remains underexplored in the literature. The visual nature of the networks makes them easily accessible to participants, and the proposed technical solutions to field implementation are flexible, inexpensive, and shown in practice to mitigate issues of co-moderating discussion across language barriers. The tools and methods described are transparent, reproducible in comparative contexts, and transferable to a range of research topics and questions

The qualitative stage of building Bayesian belief networks in a focus group setting: Decision-making under uncertainty among Vietnamese rice farmers

An innovative mixed-methods approach to exploratory focus group design is presented using a case study conducted with smallholder rice farmers in Vietnam. Understanding human decision-making under the uncertainties of a complex and changing social and environmental context requires a flexible yet structured and theoretically grounded approach. Using Bayesian belief networks as the architecture of our model allows the study to incorporate both qualitative and quantitative data, the former gathered at this stage in a participatory focus group setting and the latter to be collected in a subsequent survey. This framework further lends itself well to incorporating systematic behavioral approaches to decision-making analysis using Ajzen’s theory of planned behavior framework, a symbiosis that remains underexplored in the literature. The visual nature of the networks makes them easily accessible to participants, and the proposed technical solutions to field implementation are flexible, inexpensive, and shown in practice to mitigate issues of co-moderating discussion across language barriers. The tools and methods described are transparent, reproducible in comparative contexts, and transferable to a range of research topics and questions

Effect of the Coriolis force on the macrosegregation of aluminum in the centrifugal casting of Ti-Al alloys

International audienceWithin the framework of the ESA GRADECET project, experiments of directional solidification of cylindrical Ti-Al samples were conducted in hypergravity. The experiments were performed in a centrifuge with the apparent gravity (sum of centrifugal and terrestrial gravity) aligned along the cylinder centerline. 3D numerical simulations of aluminum macrosegregation in these samples are presented. A volume-averaging solidification model is used that accounts for centrifugal and Coriolis accelerations in a non-inertial rotating reference system. We compare the melt flow pattern and the macrosegregation formation under terrestrial gravity and under 20 g centrifugation. The results show that the Coriolis acceleration, although very weak, breaks the symmetry of the thermosolutal convection, having an important impact on the final macrosegregation pattern. The macrosegregation is entirely modified in comparison with a sample solidified under terrestrial gravity conditions. Besides the aluminum segregation intensity increases with the centrifugation level