Total reaction cross sections from 141^{141}141Pr(α\alpha,α\alpha)141^{141}Pr elastic scattering and α\alpha-induced reaction cross sections at low energies

Elastic scattering data for $^{141}$Pr($\alpha$,$\alpha$)$^{141}$Pr have been analyzed to derive a new energy-dependent local potential for the $^{141}$Pr-$\alpha$ system. This potential is used successfully to predict the cross section of the $^{141}$Pr($\alpha$,n)$^{144}$Pm reaction at low energies where new experimental data have become available very recently. Contrary to various global potentials, this new potential is able to reproduce simultaneously elastic scattering data around and above the Coulomb barrier and reaction data below the Coulomb barrier for the $^{141}$Pr-$\alpha$ system. Reasons for the partial failure of the global potentials are explained by intrinsic properties of the scattering matrix and their variation with energy. The new local potential may become the basis for the construction of a new global $\alpha$-nucleus potential.Comment: 12 pages, 8 figures, Phys. Rev. C, accepte

Relativistic Mean Field in A≈A\approx80 nuclei and low energy proton reactions

Relativistic Mean Field calculations have been performed for a number of nuclei in mass $A\approx$80 region. Ground state binding energy, charge radius and charge density values have been compared with experiment. Optical potential have been generated folding the nuclear density with the microscopic nuclear interaction DDM3Y. S-factors for low energy ($p,\gamma$) and ($p,n$) reactions have been calculated and compared with experiment.Comment: To appear in Physical Review

Alpha-particle optical potential proofs at astrophysically relevant energies

$(\alpha,\gamma)$ and $(\alpha$,n) reaction cross sections recently measured close to the reaction thresholds are rather well described by a previously developed regional optical potential. Thus, particular features of the $\alpha$-particle optical potential at energies below the Coulomb barrier, besides parameters describing $\alpha$-particle elastic scattering at higher energies are confirmed. Additional limitations of similar statistical model calculations for minor reaction channels are shown to be most likely due to an overlooked process or critical values of statistical model parameters around closed nuclear shells.Comment: 4 pages, 3 figure

Optimization of Low Reynolds Number Airfoils for Martian Rotor Applications Using an Evolutionary Algorithm

The Mars Helicopter (MH) will be flying on the NASA Mars 2020 rover mission scheduled to launch in July of 2020. Research is being performed at the Jet Propulsion Laboratory (JPL) and NASA Ames Research Center to extend the current capabilities and develop the Mars Science Helicopter (MSH) as the next possible step for Martian rotorcraft. The low atmospheric density and the relatively small-scale rotors result in very low chord-based Reynolds number flows over the rotor airfoils. The low Reynolds number regime results in rapid performance degradation for conventional airfoils due to laminar separation without reattachment. Unconventional airfoil shapes with sharp leading edges are explored and optimized for aerodynamic performance at representative Reynolds-Mach combinations for a concept rotor. Sharp leading edges initiate immediate flow separation, and the occurrence of large-scale vortex shedding is found to contribute to the relative performance increase of the optimized airfoils, compared to conventional airfoil shapes. The oscillations are shown to occur independent from laminar-turbulent transition and therefore result in sustainable performance at lower Reynolds numbers. Comparisons are presented to conventional airfoil shapes and peak lift-to-drag ratio increases between 17% and 41% are observed for similar section lift

Spin-mediated dissipation and frequency shifts of a cantilever at milliKelvin temperatures

We measure the dissipation and frequency shift of a magnetically coupled cantilever in the vicinity of a silicon chip, down to $25$ mK. The dissipation and frequency shift originates from the interaction with the unpaired electrons, associated with the dangling bonds in the native oxide layer of the silicon, which form a two dimensional system of electron spins. We approach the sample with a $3.43$ $\mu$m-diameter magnetic particle attached to an ultrasoft cantilever, and measure the frequency shift and quality factor as a function of temperature and the distance. Using a recent theoretical analysis [J. M. de Voogd et al., arXiv:1508.07972 (2015)] of the dynamics of a system consisting of a spin and a magnetic resonator, we are able to fit the data and extract the relaxation time $T_1=0.39\pm0.08$ ms and spin density $\sigma=0.14\pm0.01$ spins per nm$^2$. Our analysis shows that at temperatures $\leq500$ mK magnetic dissipation is an important source of non-contact friction.Comment: 5 pages, 3 figure

Finite temperature molecular dynamics study of unstable stacking fault free energies in silicon

We calculate the free energies of unstable stacking fault (USF) configurations on the glide and shuffle slip planes in silicon as a function of temperature, using the recently developed Environment Dependent Interatomic Potential (EDIP). We employ the molecular dynamics (MD) adiabatic switching method with appropriate periodic boundary conditions and restrictions to atomic motion that guarantee stability and include volume relaxation of the USF configurations perpendicular to the slip plane. Our MD results using the EDIP model agree fairly well with earlier first-principles estimates for the transition from shuffle to glide plane dominance as a function of temperature. We use these results to make contact to brittle-ductile transition models.Comment: 6 pages revtex, 4 figs, 16 refs, to appear in Phys. Rev.