595 research outputs found
Large Area Crop Inventory Experiment (LACIE). Intensive test site assessment report
There are no author-identified significant results in this report
On the construction of high-order force gradient algorithms for integration of motion in classical and quantum systems
A consequent approach is proposed to construct symplectic force-gradient
algorithms of arbitrarily high orders in the time step for precise integration
of motion in classical and quantum mechanics simulations. Within this approach
the basic algorithms are first derived up to the eighth order by direct
decompositions of exponential propagators and further collected using an
advanced composition scheme to obtain the algorithms of higher orders. Contrary
to the scheme by Chin and Kidwell [Phys. Rev. E 62, 8746 (2000)], where
high-order algorithms are introduced by standard iterations of a force-gradient
integrator of order four, the present method allows to reduce the total number
of expensive force and its gradient evaluations to a minimum. At the same time,
the precision of the integration increases significantly, especially with
increasing the order of the generated schemes. The algorithms are tested in
molecular dynamics and celestial mechanics simulations. It is shown, in
particular, that the efficiency of the new fourth-order-based algorithms is
better approximately in factors 5 to 1000 for orders 4 to 12, respectively. The
results corresponding to sixth- and eighth-order-based composition schemes are
also presented up to the sixteenth order. For orders 14 and 16, such highly
precise schemes, at considerably smaller computational costs, allow to reduce
unphysical deviations in the total energy up in 100 000 times with respect to
those of the standard fourth-order-based iteration approach.Comment: 23 pages, 2 figures; submitted to Phys. Rev.
Dynamical Properties and Plasmon Dispersion of a Weakly Degenerate Correlated One-Component Plasma
Classical Molecular Dynamics (MD) simulations for a one-component plasma
(OCP) are presented. Quantum effects are included in the form of the Kelbg
potential. Results for the dynamical structure factor are compared with the
Vlasov and RPA (random phase approximation) theories. The influence of the
coupling parameter , degeneracy parameter and the form
of the pair interaction on the optical plasmon dispersion is investigated. An
improved analytical approximation for the dispersion of Langmuir waves is
presented.Comment: 23 pages, includes 7 ps/eps-figures and 2 table
Spontaneous alloying in binary metal microclusters - A molecular dynamics study -
Microcanonical molecular dynamics study of the spontaneous alloying(SA),
which is a manifestation of fast atomic diffusion in a nano-sized metal
cluster, is done in terms of a simple two dimensional binary Morse model.
Important features observed by Yasuda and Mori are well reproduced in our
simulation. The temperature dependence and size dependence of the SA phenomena
are extensively explored by examining long time dynamics. The dominant role of
negative heat of solution in completing the SA is also discussed. We point out
that a presence of melting surface induces the diffusion of core atoms even if
they are solid-like. In other words, the {\it surface melting} at substantially
low temperature plays a key role in attaining the SA.Comment: 15 pages, 12 fgures, Submitted to Phys.Rev.
Discovery of Bright Galactic R Coronae Borealis and DY Persei Variables: Rare Gems Mined from ACVS
We present the results of a machine-learning (ML) based search for new R
Coronae Borealis (RCB) stars and DY Persei-like stars (DYPers) in the Galaxy
using cataloged light curves from the All-Sky Automated Survey (ASAS) Catalog
of Variable Stars (ACVS). RCB stars - a rare class of hydrogen-deficient
carbon-rich supergiants - are of great interest owing to the insights they can
provide on the late stages of stellar evolution. DYPers are possibly the
low-temperature, low-luminosity analogs to the RCB phenomenon, though
additional examples are needed to fully establish this connection. While RCB
stars and DYPers are traditionally identified by epochs of extreme dimming that
occur without regularity, the ML search framework more fully captures the
richness and diversity of their photometric behavior. We demonstrate that our
ML method can use newly discovered RCB stars to identify additional candidates
within the same data set. Our search yields 15 candidates that we consider
likely RCB stars/DYPers: new spectroscopic observations confirm that four of
these candidates are RCB stars and four are DYPers. Our discovery of four new
DYPers increases the number of known Galactic DYPers from two to six;
noteworthy is that one of the new DYPers has a measured parallax and is m ~ 7
mag, making it the brightest known DYPer to date. Future observations of these
new DYPers should prove instrumental in establishing the RCB connection. We
consider these results, derived from a machine-learned probabilistic
classification catalog, as an important proof-of-concept for the efficient
discovery of rare sources with time-domain surveys.Comment: 18 pages, 2 new figures, accepted for publication in Ap
An orbital-free molecular dynamics study of melting in K_20, K_55, K_92, K_142, Rb_55 and Cs_55 clusters
The melting-like transition in potasium clusters K_N, with N=20, 55, 92 and
142, is studied by using an orbital-free density-functional constant-energy
molecular dynamics simulation method, and compared to previous theoretical
results on the melting-like transition in sodium clusters of the same sizes.
Melting in potasium and sodium clusters proceeds in a similar way: a surface
melting stage develops upon heating before the homogeneous melting temperature
is reached. Premelting effects are nevertheless more important and more easily
established in potasium clusters, and the transition regions spread over
temperature intervals which are wider than in the case of sodium. For all the
sizes considered, the percentage melting temperature reduction when passing
from Na to K clusters is substantially larger than in the bulk. Once those two
materials have been compared for a number of different cluster sizes, we study
the melting-like transition in Rb_55 and Cs_55 clusters and make a comparison
with the melting behavior of Na_55 and K_55. As the atomic number increases,
the height of the specific heat peaks decreases, their width increases, and the
melting temperature decreases as in bulk melting, but in a more pronounced way.Comment: LaTeX file. 6 pages with 17 pictures. Final version with minor
change
Micro Structured Sensors for Neutron Detection
The shortage of 3He gas, identified as a problem several years ago, initiated research into alternative neutron detectors for various applications. One such technology is the microstructured semiconductor neutron detector (MSND). These compact detectors have microstructures etched deeply into the substrates that are subsequently backfilled with neutron reactive material. Single sided devices typically have thermal neutron detection efficiencies exceeding 30%, while double sided microstructured semiconductor neutron detectors (DS-MSND) have yielded \u3e69% thermal neutron detection efficiency. Both MSNDs and DS-MSNDs have been integrated into compact low-noise and low-power electronics modules. Dosimetry calculations indicate that these detectors can be used as active wearable neutron dosimeters. A discussion on the physics, performance and instrumentation of these MSNDs will be presented.
The radiation environment in a nuclear reactor precludes the use of semiconductor detectors for in-core sensors, leading to the invention of another miniaturized neutron detector, the micro- pocket fission detector (MPFD). The detectors were developed for real time reactor power monitoring and also for pulse tracking for power excursion experiments. These miniaturized fission chambers have gas pockets on the order of 1 mm3 with a small concentration of uranium electrodeposited inside the gas chamber. The detectors are composed of radiation hard materials and assembled without adhesives. The small geometries can be assembled in arrays to transmit reactor power at various locations. Stable device operation was confirmed by testing under steady-state reactor conditions. Reactor power transients were observed in real-time. Design details and performance of MPFDs will be presented
Exploring the Leo II dSph I.: The Variable Star Content
We present the first comprehensive catalogue of variable stars in the Leo II
dwarf spheroidal galaxy. We have identified 148 RR Lyrae type variables, of
which 140 were amenable to derivation of variability parameters with our data.
We have also confirmed the existence of four anomalous Cepheids as identified
in previous studies.
The average period of the RR Lyrae ab variables (0.62 days), the fraction of
c variables (0.24) and the minimum period of the RR Lyrae ab variables (0.51
days) all define Leo II as an "Oosterhoff intermediate" galaxy. We have used
the properties of these variables to derive a metallicity for Leo II of
approximately [Fe/H]=-1.9. The presence of longer period, higher amplitude RR
Lyrae variable implies a metallicity distribution that extends to as poor as
[Fe/H]=-2.3.
Leo II's location on the period-metallicity relation of clusters, like that
of other ``Ootershoff intermediate'' objects, falls between the Oosterhoff
Class I and Oosterhoff Class II clusters. The properties of the variable
populations of these objects are consistent with the idea that the Oosterhoff
"dichotomy" is a continuum. The gap between the classes seems to be explained
by the horizontal branch of Galactic globular clusters shifting away from the
instability strip at at intermediate metallicities. However, Leo II, as well as
other Oosterhoff intermediate objects, has a second parameter effect strong
enough to leave horizontal branch stars in the instability strip.Comment: 44 pages, 8 figures, 3 tables, accepted in Astronomical Journa
Structure and relaxations in liquid and amorphous Selenium
We report a molecular dynamics simulation of selenium, described by a
three-body interaction. The temperatures T_g and T_c and the structural
properties are in agreement with experiment. The mean nearest neighbor
coordination number is 2.1. A small pre-peak at about 1 AA^-1 can be explained
in terms of void correlations. In the intermediate self-scattering function,
i.e. the density fluctuation correlation, classical behavior, alpha- and
beta-regimes, is found. We also observe the plateau in the beta-regime below
T_g. In a second step, we investigated the heterogeneous and/or homogeneous
behavior of the relaxations. At both short and long times the relaxations are
homogeneous (or weakly heterogeneous). In the intermediate time scale, lowering
the temperature increases the heterogeneity. We connect these different domains
to the vibrational (ballistic), beta- and alpha-regimes. We have also shown
that the increase in heterogeneity can be understood in terms of relaxations
Atomic-scale modeling of the deformation of nanocrystalline metals
Nanocrystalline metals, i.e. metals with grain sizes from 5 to 50 nm, display
technologically interesting properties, such as dramatically increased
hardness, increasing with decreasing grain size. Due to the small grain size,
direct atomic-scale simulations of plastic deformation of these materials are
possible, as such a polycrystalline system can be modeled with the
computational resources available today.
We present molecular dynamics simulations of nanocrystalline copper with
grain sizes up to 13 nm. Two different deformation mechanisms are active, one
is deformation through the motion of dislocations, the other is sliding in the
grain boundaries. At the grain sizes studied here the latter dominates, leading
to a softening as the grain size is reduced. This implies that there is an
``optimal'' grain size, where the hardness is maximal.
Since the grain boundaries participate actively in the deformation, it is
interesting to study the effects of introducing impurity atoms in the grain
boundaries. We study how silver atoms in the grain boundaries influence the
mechanical properties of nanocrystalline copper.Comment: 10 pages, LaTeX2e, PS figures and sty files included. To appear in
Mater. Res. Soc. Symp. Proc. vol 538 (invited paper). For related papers, see
http://www.fysik.dtu.dk/~schiotz/publist.htm
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