2,308 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
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
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
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
Liquid-Solid Phase Transition of the System with Two particles in a Rectangular Box
We study the statistical properties of two hard spheres in a two dimensional
rectangular box. In this system, the relation like Van der Waals equation loop
is obtained between the width of the box and the pressure working on side
walls. The auto-correlation function of each particle's position is calculated
numerically. By this calculation near the critical width, the time at which the
correlation become zero gets longer according to the increase of the height of
the box. Moreover, fast and slow relaxation processes like and
relaxations observed in supper cooled liquid are observed when the height of
the box is sufficiently large. These relaxation processes are discussed with
the probability distribution of relative position of two particles.Comment: 6 figure
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
Coulomb corrected eikonal description of the breakup of halo nuclei
The eikonal description of breakup reactions diverges because of the Coulomb
interaction between the projectile and the target. This divergence is due to
the adiabatic, or sudden, approximation usually made, which is incompatible
with the infinite range of the Coulomb interaction. A correction for this
divergence is analysed by comparison with the Dynamical Eikonal Approximation,
which is derived without the adiabatic approximation. The correction consists
in replacing the first-order term of the eikonal Coulomb phase by the
first-order of the perturbation theory. This allows taking into account both
nuclear and Coulomb interactions on the same footing within the computationally
efficient eikonal model. Excellent results are found for the dissociation of
11Be on lead at 69 MeV/nucleon. This Coulomb Corrected Eikonal approximation
provides a competitive alternative to more elaborate reaction models for
investigating breakup of three-body projectiles at intermediate and high
energies.Comment: 19 pages, 9 figures, accepted for publication in Phys. Rev.
Structure Effects on Coulomb Dissociation of B
Coulomb Dissociation provides an alternative method for determining the
radiative capture cross sections at astrophysically relevant low relative
energies. For the breakup of B on Ni, we calculate the total Coulomb
Dissociation cross section and the angular distribution for E1, E2 and M1. Our
calculations are performed first within the standard first order semiclassical
theory of Coulomb Excitation, including the correct three body kinematics, and
later including the projectile-target nuclear interactions.Comment: 6 pages, proceedings from International Workshop on RNB, Puri, India,
January 1998 - to be published in J. Phys.
Global Equation of State of two-dimensional hard sphere systems
Hard sphere systems in two dimensions are examined for arbitrary density.
Simulation results are compared to the theoretical predictions for both the low
and the high density limit, where the system is either disordered or ordered,
respectively. The pressure in the system increases with the density, except for
an intermediate range of volume fractions , where a
disorder-order phase transition occurs. The proposed {\em global equation of
state} (which describes the pressure {\em for all densities}) is applied to the
situation of an extremely dense hard sphere gas in a gravitational field and
shows reasonable agreement with both experimental and numerical data.Comment: 4 pages, 2 figure
Colors of Inner Disk Classical Kuiper Belt Objects
We present new optical broadband colors, obtained with the Keck 1 and Vatican
Advanced Technology telescopes, for six objects in the inner classical Kuiper
Belt. Objects in the inner classical Kuiper Belt are of interest as they may
represent the surviving members of the primordial Kuiper Belt that formed
interior to the current position of the 3:2 resonance with Neptune, the current
position of the plutinos, or, alternatively, they may be objects formed at a
different heliocentric distance that were then moved to their present
locations. The six new colors, combined with four previously published, show
that the 10 inner belt objects with known colors form a neutral clump and a
reddish clump in B-R color. Nonparametric statistical tests show no significant
difference between the B-R color distribution of the inner disk objects
compared to the color distributions of Centaurs, plutinos, or scattered disk
objects. However, the B-R color distribution of the inner classical Kuiper belt
objects does differ significantly from the distribution of colors in the cold
(low inclination) main classical Kuiper belt. The cold main classical objects
are predominately red, while the inner classical belt objects are a mixture of
neutral and red. The color difference may reveal the existence of a gradient in
the composition and /or surface processing history in the primordial Kuiper
Belt, or indicate that the inner disk objects are not dynamically analogous to
the cold main classical belt objects.Comment: Accepted for publication in A
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