Dynamical approach to heavy-ion induced fission using actinide target nuclei at energies around the Coulomb barrier

In order to describe heavy-ion fusion reactions around the Coulomb barrier with an actinide target nucleus, we propose a model which combines the coupled-channels approach and a fluctuation-dissipation model for dynamical calculations. This model takes into account couplings to the collective states of the interacting nuclei in the penetration of the Coulomb barrier and the subsequent dynamical evolution of a nuclear shape from the contact configuration. In the fluctuation-dissipation model with a Langevin equation, the effect of nuclear orientation at the initial impact on the prolately deformed target nucleus is considered. Fusion-fission, quasi-fission and deep quasi-fission are separated as different Langevin trajectories on the potential energy surface. Using this model, we analyze the experimental data for the mass distribution of fission fragments (MDFF) in the reactions of $^{34,36}$S+$^{238}$U and $^{30}$Si+$^{238}$U at several incident energies around the Coulomb barrier. We find that the time scale in the quasi-fission as well as the deformation of fission fragments at the scission point are different between the $^{30}$Si+$^{238}$U and $^{36}$S+$^{238}$U systems, causing different mass asymmetries of the quasi-fission.Comment: 11 figure

Efficient implementation of the nonequilibrium Green function method for electronic transport calculations

An efficient implementation of the nonequilibrium Green function (NEGF) method combined with the density functional theory (DFT) using localized pseudo-atomic orbitals (PAOs) is presented for electronic transport calculations of a system connected with two leads under a finite bias voltage. In the implementation, accurate and efficient methods are developed especially for evaluation of the density matrix and treatment of boundaries between the scattering region and the leads. Equilibrium and nonequilibrium contributions in the density matrix are evaluated with very high precision by a contour integration with a continued fraction representation of the Fermi-Dirac function and by a simple quadratureon the real axis with a small imaginary part, respectively. The Hartree potential is computed efficiently by a combination of the two dimensional fast Fourier transform (FFT) and a finite difference method, and the charge density near the boundaries is constructed with a careful treatment to avoid the spurious scattering at the boundaries. The efficiency of the implementation is demonstrated by rapid convergence properties of the density matrix. In addition, as an illustration, our method is applied for zigzag graphene nanoribbons, a Fe/MgO/Fe tunneling junction, and a LaMnO$_3/$SrMnO$_3$ superlattice, demonstrating its applicability to a wide variety of systems.Comment: 20 pages, 11 figure

Electric-field-induced lifting of the valley degeneracy in alpha-(BEDT-TTF)_2I_3 Dirac-like Landau levels

The relativistic Landau levels in the layered organic material alpha-(BEDT-TTF)_2I_3 [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] are sensitive to the tilt of the Dirac cones, which, as in the case of graphene, determine the low-energy electronic properties under appropriate pressure. We show that an applied inplane electric field, which happens to be in competition with the tilt of the cones, lifts the twofold valley degeneracy due to a different level spacing. The scenario may be tested in infrared transmission spectroscopy.Comment: 4 pages, 1 figure; version with minor corrections published in EP

A dynamical model of surrogate reactions

A new dynamical model is developed to describe the whole process of surrogate reactions; transfer of several nucleons at an initial stage, thermal equilibration of residues leading to washing out of shell effects and decay of populated compound nuclei are treated in a unified framework. Multi-dimensional Langevin equations are employed to describe time-evolution of collective coordinates with a time-dependent potential energy surface corresponding to different stages of surrogate reactions. The new model is capable of calculating spin distributions of the compound nuclei, one of the most important quantity in the surrogate technique. Furthermore, various observables of surrogate reactions can be calculated, e.g., energy and angular distribution of ejectile, and mass distributions of fission fragments. These features are important to assess validity of the proposed model itself, to understand mechanisms of the surrogate reactions and to determine unknown parameters of the model. It is found that spin distributions of compound nuclei produced in $^{18}$O+$^{238}$U $\rightarrow ^{16}$O+$^{240*}$U and $^{18}$O+$^{236}$U $\rightarrow ^{16}$O+$^{238*}$U reactions are equivalent and much less than 10$\hbar$, therefore satisfy conditions proposed by Chiba and Iwamoto (PRC 81, 044604(2010)) if they are used as a pair in the surrogate ratio method.Comment: 17 pages, 5 figure

Imaging characteristics and treatment of a penetrating brain injury caused by an oropharyngeal foreign body in a dog

A 4-year-old Border collie was presented with one episode of collapse, altered mentation, and a suspected pharyngeal stick injury. Magnetic resonance imaging (MRI) and computed tomography showed a linear foreign body penetrating the right oropharynx, through the foramen ovale and the brain parenchyma. The foreign body was surgically removed and medical treatment initiated. Complete resolution of clinical signs was noted at recheck 8 weeks later. Repeat MRI showed chronic secondary changes in the brain parenchyma. To the authors' knowledge, this is the first report of the advanced imaging findings and successful treatment of a penetrating oropharyngeal intracranial foreign body in a dog

High--Energy Photon--Hadron Scattering in Holographic QCD

This article provides an in-depth look at hadron high energy scattering by using gravity dual descriptions of strongly coupled gauge theories. Just like deeply inelastic scattering (DIS) and deeply virtual Compton scattering (DVCS) serve as clean experimental probes into non-perturbative internal structure of hadrons, elastic scattering amplitude of a hadron and a (virtual) "photon" in gravity dual can be exploited as a theoretical probe. Since the scattering amplitude at sufficiently high energy (small Bjorken x) is dominated by parton contributions (= Pomeron contributions) even in strong coupling regime, there is a chance to learn a lesson for generalized parton distribution (GPD) by using gravity dual models. We begin with refining derivation of Brower-Polchinski-Strassler-Tan (BPST) Pomeron kernel in gravity dual, paying particular attention to the role played by complex spin variable j. The BPST Pomeron on warped spacetime consists of a Kaluza-Klein tower of 4D Pomerons with non-linear trajectories, and we clarify the relation between Pomeron couplings and Pomeron form factor. We emphasize that the saddle point value j^* of the scattering amplitude in the complex j-plane representation is a very important concept in understanding qualitative behavior of the scattering amplitude. The total Pomeron contribution to the scattering is decomposed into the saddle point contribution and at most a finite number of pole contributions, and when the pole contributions are absent (which we call saddle point phase), kinematical variable (q,x,t) dependence of ln (1/q) evolution and ln(1/x) evolution parameters gamma_eff. and lambda_eff. in DIS and t-slope parameter B of DVCS in HERA experiment are all reproduced qualitatively in gravity dual

Soft X-ray analysis of a loop flare on the Sun

We present the results of an analysis of soft X-ray images for a solar flare which occurred on 1992 July 11. This flare, as seen in Yohkoh Soft X-ray Telescope (SXT) images was of comparatively simple geometry, consisting of two bright footpoints early in the flare with a bright loop seen later in the flare. We examine how closely this flare compares with the supposed paradigm of a confined simple-loop flare. Closer examination of the SXT images reveals that the flare structure consisted of at least two adjacent loops, one much fainter than the other. We examine the brighter of the two soft X-ray loops. The SXT images reveal an apparent slow, northward motion of this loop (roughly transverse to its major axis). Examination of derived emission measure and temperature images also indicate an apparent northward motion. In addition, we find an increase in the cross-sectional width at the top of the loop with time. Emission measure maps derived from the SXT images also indicates an apparent broadening of the loop-top region. We infer that the apparent northward motion and the apparent broadening of the soft X-ray emission can be explained in a reconnection scenario where successive magnetic field structures do not lie in a plane but are tilted to the south of the line of sight but with successively brightening loops oriented at less tilted angles. Halpha images for this flare reveal an evolution from a few brilliant points to a short two- ribbon-like appearance. Comparison of the SXT images with the Halpha images shows that the Halpha patches are aligned with the footpoints of the soft X-ray loops, suggesting the presence of a small arcade structure. There is no clear evidence for an eruptive signature in our observations nor in reports from other observations. The lack of an eruptive signature could suggest that the flare may have been a confined simple-loop flare, but this is not compelling due to a gap in the coronal observations prior to and early in the event. Analysis of our observations indicate that the flare exhibited characteristics suggesting that it may be better understood as a mini-arcade flare. These results casts doubt on the validity of the supposed paradigm of a confined simple-loop flare, at least for this flare. They indicate that even an apparently simple-loop flare may be considered to be a variety of arcade flare. We also find an effect which, to our knowledge, has not been reported before: the hot flaring regions later become cooler than the surrounding quiescent corona. That is, the flare loops do not evolve into bright active region loops, but into cooler loops. This may indicate an increase in the efficiency of the cooling mechanism or a transformed equilibrium state within the flaring loops

Vitrification of a monatomic 2D simple liquid

A monatomic simple liquid in two dimensions, where atoms interact isotropically through the Lennard-Jones-Gauss potential [M. Engel and H.-R. Trebin, Phys. Rev. Lett. 98, 225505 (2007)], is vitrified by the use of a rapid cooling technique in a molecular dynamics simulation. Transformation to a crystalline state is investigated at various temperatures and the time-temperature-transformation (TTT) curve is determined. It is found that the transformation time to a crystalline state is the shortest at a temerature 14% below the melting temperature Tm and that at temperatures below Tv = 0.6 Tm the transformation time is much longer than the available CPU time. This indicates that a long-lived glassy state is realized for T < Tv.Comment: 5pages,5figures,accepted for publication in CEJ

Baikal mud volcanoes: thermal features of dynamic gas hydrate systems

In Lake Baikal shallow gas hydrates have already been identified in five mud volcano/seep structures through joint Russian, Japanese and Belgian research. These mud volcano/seep structures are found at different water depths (from 1380 m to as shallow as 440 m) and contain shallow hydrates of both structure I and II. Bottom Seismic Reflections (BSRs), indicative for the presence of deep-seated hydrates, has been observed on nearby seismic profiles. We will report on detailed thermal investigations in association with gravity coring performed over the last three years in the following gas hydrate containing mud volcanoes: “K-2”, “Malenkiy” and “Bolshoy”.The “K-2” mud volcano is located on the flanks of the Kukuy Canyon at a water depth of 900 m water depth. This oval structure of 60 m in height and 800 m in diameter consists of two separate mud volcanoes corresponding to two culminations. Sediment cores have been retrieved in more than 75 sites (15 contained hydrates), with temperature sensors attached to the corer in 22 occasions. Shallow hydrates were only found in two zones of not more 50-100 m diameter: on the top and between the two culminations. These zones also stand out by anomalous low (30-43 mK/m) and high (90-113 mK/m) thermal gradients in comparison to what is measured outside the mud volcano (60-70 mK/m). Cores with hydrates were directly correlated to low thermal gradient and large non-linearity in the temperature-depth profiles. This can be explained in three ways: (1) heat absorption by hydrate dissociation; (2) topographic effect combined with a dynamic hydrate system; and (3) infiltration of cold lake water, possibly induced by local convection and/or water segregation. The localized occurrence of hydrates within the mud volcanoes and a close relation to thermal anomalies was also observed in the mud volcanoes “Malenkiy” and “Bolshoy”, located at a water depth of about 1380m. More than 30 gravity cores in both structures indicate zones with shallow hydrates in local depressions and on culminations. Thermal stations show the presence of anomalous thermal gradients, up to 180 mK/m, at short distances of background values.The mud volcanoes in Lake Baikal do not display a strong activity in terms of acoustic flaring in the water column (almost absent) and large-scale temperature anomalies (< 1 °C). However, they comprise local shallow hydrate systems in close association with anomalous low and high thermal gradients. A dynamic nature of the hydrate system in “K-2” mud volcano has been supported by small shifts of the hydrate occurrence zone within the three year period of investigation