65,840 research outputs found
Visualizing Basic Nuclear Reactions
There are few instructional tools available to teach basic nuclear reactions to beginning students. The activity described in this paper can be used to help students visualize and write basic nuclear reactions such as alpha, beta, and positron decay, as well as electron capture. These reactions are represented using the technology of thermochromic paints, which either change color or turn colorless depending upon the temperature. By using a special thermochromic paint that turns colorless upon heating, students are able to visualize nuclear interactions. For instance, when positron decay occurs, the object depicting a proton will decay into a neutron by the application of heat. In order to avoid confusion, the heating instrument is referred to as a time gun. This paper includes the details of preparing and incorporating the activity into the classroom environment
Interstellar absorption lines in the spectrum of sigma Sco using Copernicus observations
Since the launch of Copernicus in 1972, studies have been made of the depletion of gas-phase elements onto dust grains. A few stars have been studied in detail, resulting in a standard depletion pattern which has since been used for comparison. Recent developments, however, have suggested that this standard pattern may need to be re-examined. Some weak, semi-forbidden lines were detected recently which may be able to resolve some of the ambiguities. Studies of single elements have shown that depletion of carbon and oxgyen are much smaller than previously determined. The high resolution ultraviolet spectral scans of sigma Sco were originally made in 1973, but have only recently been analyzed. All these stars are bright and moderately reddened. All four stars will be analyzed in detail, but sigma Sco is the first one completed. The data has broad coverage of ions, making these stars excellent candidates for determination of accurate depletions. A profile-fitting analysis was used rather than curves-of-growth in order to determine separate abundances and depletions in components separated by several km/sec
Novel crystal phase in suspensions of hard ellipsoids
We present a computer simulation study on the crystalline phases of hard
ellipsoids of revolution. For aspect ratios greater than or equal to 3 the
previously suggested stretched-fcc phase [D. Frenkel and B. M. Mulder, Mol.
Phys. 55, 1171 (1985)] is replaced by a novel crystalline phase. Its unit cell
contains two ellipsoids with unequal orientations. The lattice is simple
monoclinic. The angle of inclination of the lattice, beta, is a very soft
degree of freedom, while the two right angles are stiff. For one particular
value of beta, the close-packed version of this crystal is a specimen of the
family of superdense packings recently reported [Donev et al., Phys. Rev. Lett.
92, 255506 (2004)]. These results are relevant for studies of nucleation and
glassy dynamics of colloidal suspensions of ellipsoids.Comment: 4 pages, 4 figure
Time allocation among three technical information channels by R and D engineers
Dependance of research project efficiency on information channels - operations researc
Liquid crystal director fluctuations and surface anchoring by molecular simulation
We propose a simple and reliable method to measure the liquid crystal surface
anchoring strength by molecular simulation. The method is based on the
measurement of the long-range fluctuation modes of the director in confined
geometry. As an example, molecular simulations of a liquid crystal in slab
geometry between parallel walls with homeotropic anchoring have been carried
out using the Monte Carlo technique. By studying different slab thicknesses, we
are able to calculate separately the position of the elastic boundary
condition, and the extrapolation length
The Transition between Nonorthogonal Polarization Modes in PSR B2016+28 at 1404 MHz
Polarization observations of the radio emission from PSR B2016+28 at 1404 MHz
reveal properties that are consistent with two, very different, interpretations
of the pulsar's viewing geometry. The pulsar's average polarization properties
show a rapid change in position angle (PA) near the pulse center, suggesting
that the observer's sightline nearly intersects the star's magnetic pole. But
single pulse, polarization observations of the pulsar show nearly orthogonal
modes of polarization following relatively flat and parallel PA trajectories
across the pulse, suggesting that the sightline is far from the pole.
Additionally, PA histograms reveal a "modal connecting bridge", of unknown
origin, joining the modal PA trajectories over much of the pulse and following
the rapid PA change shown in the average data. The nonorthogonality of
polarization modes is incorporated in a statistical model of radio polarization
to account for the deviations from mode orthogonality that are observed in the
pulsar. The model is used to interpret the rapid PA change and modal connecting
bridge as a longitudinally-resolved transition between modes of nonorthogonal
polarization. Thus, the modal PA trajectories are argued to reflect the
pulsar's true viewing geometry. This interpretation is consistent with the
pulsar's morphological classification, preserves the Radhakrishnan & Cooke
model of pulsar radio emission, and avoids the complication that the modal
connecting bridge might be produced by some other emission mechanism. The
statistical model's ability to simulate the rich variety of polarization
properties observed in the emission lends additional support to the model's
applicability and its underlying assumption that the polarization modes occur
simultaneously.Comment: Accepted for publication in Ap
Compressing nearly hard sphere fluids increases glass fragility
We use molecular dynamics to investigate the glass transition occurring at
large volume fraction, phi, and low temperature, T, in assemblies of soft
repulsive particles. We find that equilibrium dynamics in the (phi, T) plane
obey a form of dynamic scaling in the proximity of a critical point at T=0 and
phi=phi_0, which should correspond to the ideal glass transition of hard
spheres. This glass point, `point G', is distinct from athermal jamming
thresholds. A remarkable consequence of scaling behaviour is that the dynamics
at fixed phi passes smoothly from that of a strong glass to that of a very
fragile glass as phi increases beyond phi_0. Correlations between fragility and
various physical properties are explored.Comment: 5 pages, 3 figures; Version accepted at Europhys. Let
- …