2,771 research outputs found
Intermediate energy Coulomb excitation as a probe of nuclear structure at radioactive beam facilities
The effects of retardation in the Coulomb excitation of radioactive nuclei in
intermediate energy collisions (Elab ~100 MeV/nucleon) are investigated. We
show that the excitation cross sections of low-lying states in 11Be,
{38,40,42}S and {44,46}Ar projectiles incident on gold and lead targets are
modified by as much as 20% due to these effects. The angular distributions of
decaying gamma-rays are also appreciably modified.Comment: 21 pages, 3 figures, Phys. Rev. C, in pres
Determining the number of clusters in a mixture by iterative model space refinement with application to free-swimming fish detection
We present a clustering algorithm for use when the number of clusters is unknown. We first show that the EM algorithm for mixture modeling can be considered as an alternating minimization between the data space and the model space. We then show how data cleaning can be performed by alternating between the data space and two model spaces. Finally, we develop a mixture model approach that iteratively refines the model spaces, beginning with a coarse model and selecting finer models as indicated by the consistent Akaike information criterion
Effect of boundary conditions on diffusion in two-dimensional granular gases
We analyze the influence of boundary conditions on numerical simulations of
the diffusive properties of a two dimensional granular gas. We show in
particular that periodic boundary conditions introduce unphysical correlations
in time which cause the coefficient of diffusion to be strongly dependent on
the system size. On the other hand, in large enough systems with hard walls at
the boundaries, diffusion is found to be independent of the system size. We
compare the results obtained in this case with Langevin theory for an elastic
gas. Good agreement is found. We then calculate the relaxation time and the
influence of the mass for a particle of radius in a sea of particles of
radius . As granular gases are dissipative, we also study the influence of
an external random force on the diffusion process in a forced dissipative
system. In particular, we analyze differences in the mean square velocity and
displacement between the elastic and inelastic cases.Comment: 15 figures eps figures, include
Semiclassical treatment of fusion processes in collisions of weakly bound nuclei
We describe a semiclassical treatment of nuclear fusion reactions involving
weakly bound nuclei. In this treatment, the complete fusion probabilities are
approximated by products of two factors: a tunneling probability and the
probability that the system is in its ground state at the strong absorption
radius. We investigate the validity of the method in a schematic two-channel
application, where the channels in the continuum are represented by a single
resonant state. Comparisons with full coupled-channels calculations are
performed. The agreement between semiclassical and quantal calculations isquite
good, suggesting that the procedure may be extended to more sophisticated
discretizations of the continuum.Comment: 11 pages, 5 figure
Condensation of Hard Spheres Under Gravity
Starting from Enskog equation of hard spheres of mass m and diameter D under
the gravity g, we first derive the exact equation of motion for the equilibrium
density profile at a temperature T and examine its solutions via the gradient
expansion. The solutions exist only when \beta\mu \le \mu_o \approx 21.756 in 2
dimensions and \mu_o\approx 15.299 in 3 dimensions, where \mu is the
dimensionless initial layer thickness and \beta=mgD/T. When this inequality
breaks down, a fraction of particles condense from the bottom up to the Fermi
surface.Comment: 9 pages, one figur
On ‘Organized Crime’ in the illicit antiquities trade: moving beyond the definitional debate
The extent to which ‘organized crime’ is involved in illicit antiquities trafficking is unknown and frequently debated. This paper explores the significance and scale of the illicit antiquities trade as a unique transnational criminal phenomenon that is often said to be perpetrated by and exhibit traits of so-called ‘organized crime.’ The definitional debate behind the term ‘organized crime’ is considered as a potential problem impeding our understanding of its existence or extent in illicit antiquities trafficking, and a basic progression-based model is then suggested as a new tool to move beyond the definitional debate for future research that may help to elucidate the actors, processes and criminal dynamics taking place within the illicit antiquities trade from source to market. The paper concludes that researchers should focus not on the question of whether organized criminals- particularly in a traditionally conceived, mafia-type stereotypical sense- are involved in the illicit antiquities trade, but instead on the structure and progression of antiquities trafficking itself that embody both organized and criminal dynamics
Long-Time Behavior of Velocity Autocorrelation Function for Interacting Particles in a Two-Dimensional Disordered System
The long-time behavior of the velocity autocorrelation function (VACF) is
investigated by the molecular dynamics simulation of a two-dimensional system
which has both a many-body interaction and a random potential. With
strengthening the random potential by increasing the density of impurities, a
crossover behavior of the VACF is observed from a positive tail, which is
proportional to t^{-1}, to a negative tail, proportional to -t^{-2}. The latter
tail exists even when the density of particles is the same order as the density
of impurities. The behavior of the VACF in a nonequilibrium steady state is
also studied. In the linear response regime the behavior is similar to that in
the equilibrium state, whereas it changes drastically in the nonlinear response
regime.Comment: 12 pages, 5 figure
Velocity Correlations in Driven Two-Dimensional Granular Media
Simulations of volumetrically forced granular media in two dimensions produce
s tates with nearly homogeneous density. In these states, long-range velocity
correlations with a characteristic vortex structure develop; given sufficient
time, the correlations fill the entire simulated area. These velocity
correlations reduce the rate and violence of collisions, so that pressure is
smaller for driven inelastic particles than for undriven elastic particles in
the same thermodynamic state. As the simulation box size increases, the effects
of veloc ity correlations on the pressure are enhanced rather than reduced.Comment: 12 pages, 6 figures, 21 reference
Stanford Aerospace Research Laboratory research overview
Over the last ten years, the Stanford Aerospace Robotics Laboratory (ARL) has developed a hardware facility in which a number of space robotics issues have been, and continue to be, addressed. This paper reviews two of the current ARL research areas: navigation and control of free flying space robots, and modelling and control of extremely flexible space structures. The ARL has designed and built several semi-autonomous free-flying robots that perform numerous tasks in a zero-gravity, drag-free, two-dimensional environment. It is envisioned that future generations of these robots will be part of a human-robot team, in which the robots will operate under the task-level commands of astronauts. To make this possible, the ARL has developed a graphical user interface (GUI) with an intuitive object-level motion-direction capability. Using this interface, the ARL has demonstrated autonomous navigation, intercept and capture of moving and spinning objects, object transport, multiple-robot cooperative manipulation, and simple assemblies from both free-flying and fixed bases. The ARL has also built a number of experimental test beds on which the modelling and control of flexible manipulators has been studied. Early ARL experiments in this arena demonstrated for the first time the capability to control the end-point position of both single-link and multi-link flexible manipulators using end-point sensing. Building on these accomplishments, the ARL has been able to control payloads with unknown dynamics at the end of a flexible manipulator, and to achieve high-performance control of a multi-link flexible manipulator
Circular No. 39 - A Day at the Utah Agricultural Experiment Station
From the days when Indian chieftains with their hostile bands roamed the sagebrush areas of Utah down to the present, nature\u27s forces have been subdued by many a daring and successful conquest thru the intelligence and unyielding perseverance of Utah\u27s pioneers. Since its establishment, the energies of the Agricultural Experiment Station have been directed toward a continuance of this conquest
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