5,828 research outputs found
Chemical Identification of the Radioelements Produced from Carbon and Boron by Deuteron Bombardment
Chemical experiments were made on the radioactive substances resulting from the bombardment of carbon and boron by deuterons. Carbon is shown to yield an isotope of nitrogen and boron an isotope of carbon. The nitrogen so formed has a half-life of 10.5 minutes while that found by Curie and Joliot on bombardment of boron with alpha-particles has a half-life of 14 minutes. These facts are discussed
Electron Acceleration by Multi-Island Coalescence
Energetic electrons of up to tens of MeV are created during explosive
phenomena in the solar corona. While many theoretical models consider magnetic
reconnection as a possible way of generating energetic electrons, the precise
roles of magnetic reconnection during acceleration and heating of electrons
still remain unclear. Here we show from 2D particle-in-cell simulations that
coalescence of magnetic islands that naturally form as a consequence of tearing
mode instability and associated magnetic reconnection leads to efficient
energization of electrons. The key process is the secondary magnetic
reconnection at the merging points, or the `anti-reconnection', which is, in a
sense, driven by the converging outflows from the initial magnetic reconnection
regions. By following the trajectories of the most energetic electrons, we
found a variety of different acceleration mechanisms but the energization at
the anti-reconnection is found to be the most important process. We discuss
possible applications to the energetic electrons observed in the solar flares.
We anticipate our results to be a starting point for more sophisticated models
of particle acceleration during the explosive energy release phenomena.Comment: 14 pages, 12 figures (degraded figure quality), 1 table. Accepted for
publication in ApJ
Reading the Number of Extra Dimensions in the Spectrum of Hawking Radiation
After a brief review of the production and decay of Schwarzschild-like
(4+n)-dimensional black holes in the framework of theories with Large Extra
Dimensions, we proceed to derive the greybody factors and emission rates for
scalars, fermions and gauge bosons on the brane. We present and discuss
analytic and numerical methods for obtaining the above results, and demonstrate
that both the amount and type of Hawking radiation emitted by the black hole
can help us to determine the number of spacelike dimensions that exist in
nature.Comment: 8 pages, Latex file, 1 figure, to appear in the proceedings of the
String Phenomenology 2003 Conference, Durham, UK, 29th July-4th August, 200
`Island Surfing' Mechanism of Electron Acceleration During Magnetic Reconnection
One of the key unresolved problems in the study of space plasmas is to
explain the production of energetic electrons as magnetic field lines
`reconnect' and release energy in a exposive manner. Recent observations
suggest possible roles played by small scale magnetic islands in the
reconnection region, but their precise roles and the exact mechanism of
electron energization have remained unclear. Here we show that secondary
islands generated in the reconnection region are indeed efficient electron
accelerators. We found that, when electrons are trapped inside the islands,
they are energized continuously by the reconnection electric field prevalent in
the reconnection diffusion region. The size and the propagation speed of the
secondary islands are similar to those of islands observed in the magnetotail
containing energertic electrons.Comment: 5 pages, 4 figures, submitted to J. Geophys. Res
Phenotypic Plasticity of Mouse Spermatogonial Stem Cells
BACKGROUND:Spermatogonial stem cells (SSCs) continuously undergo self-renewal division to support spermatogenesis. SSCs are thought to have a fixed phenotype, and development of a germ cell transplantation technique facilitated their characterization and prospective isolation in a deterministic manner; however, our in vitro SSC culture experiments indicated heterogeneity of cultured cells and suggested that they might not follow deterministic fate commitment in vitro. METHODOLOGY AND PRINCIPAL FINDINGS:In this study, we report phenotypic plasticity of SSCs. Although c-kit tyrosine kinase receptor (Kit) is not expressed in SSCs in vivo, it was upregulated when SSCs were cultured on laminin in vitro. Both Kit(-) and Kit(+) cells in culture showed comparable levels of SSC activity after germ cell transplantation. Unlike differentiating spermatogonia that depend on Kit for survival and proliferation, Kit expressed on SSCs did not play any role in SSC self-renewal. Moreover, Kit expression on SSCs changed dynamically once proliferation began after germ cell transplantation in vivo. CONCLUSIONS/SIGNIFICANCE:These results indicate that SSCs can change their phenotype according to their microenvironment and stochastically express Kit. Our results also suggest that activated and non-activated SSCs show distinct phenotypes
Soft Null Hypotheses: A Case Study of Image Enhancement Detection in Brain Lesions
This work is motivated by a study of a population of multiple sclerosis (MS)
patients using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI)
to identify active brain lesions. At each visit, a contrast agent is
administered intravenously to a subject and a series of images is acquired to
reveal the location and activity of MS lesions within the brain. Our goal is to
identify and quantify lesion enhancement location at the subject level and
lesion enhancement patterns at the population level. With this example, we aim
to address the difficult problem of transforming a qualitative scientific null
hypothesis, such as "this voxel does not enhance", to a well-defined and
numerically testable null hypothesis based on existing data. We call the
procedure "soft null hypothesis" testing as opposed to the standard "hard null
hypothesis" testing. This problem is fundamentally different from: 1) testing
when a quantitative null hypothesis is given; 2) clustering using a mixture
distribution; or 3) identifying a reasonable threshold with a parametric null
assumption. We analyze a total of 20 subjects scanned at 63 visits (~30Gb), the
largest population of such clinical brain images
Light emission patterns from stadium-shaped semiconductor microcavity lasers
We study light emission patterns from stadium-shaped semiconductor (GaAs)
microcavity lasers theoretically and experimentally. Performing systematic wave
calculations for passive cavity modes, we demonstrate that the averaging by
low-loss modes, such as those realized in multi-mode lasing, generates an
emission pattern in good agreement with the ray model's prediction. In
addition, we show that the dependence of experimental far-field emission
patterns on the aspect ratio of the stadium cavity is well reproduced by the
ray model.Comment: 5 pages, 4 figure
Relativistic Particle Acceleration in a Folded Current Sheet
Two-dimensional particle simulations of a relativistic Harris current sheet
of pair plasmashave demonstrated that the system is unstable to the
relativistic drift kink instability (RDKI) and that a new kind of acceleration
process takes place in the deformed current sheet. This process contributes to
the generation of non-thermal particles and contributes to the fast magnetic
dissipation in the current sheet structure. The acceleration mechanism and a
brief comparison with relativistic magnetic reconnection are presented.Comment: 11 preprint pages, including 3 .eps figure
Supply-Chain Culture Clashes in Europe. Pitfalls in Japanese Service Operations
Japanese companies value service and quality highly and they put much effort in realising this. However, survey research carried out in 2001 among senior managers of Japanese logistics companies in the Netherlands, indicated that these efforts do not result in significant performance differences compared to western companies. In this exploratory paper, we report of company visits and interviews with managers of Japanese logistics companies in Western Europe. They described a clash of cultures underlying their operations, prohibiting them from achieving performance excellence. The causes focus around two key factors: the unique concept of Japanese service, based on future rewards which are absent in Western Europe; different employment circumstances in Western Europe, which make Japanese human resource management ineffective, and the Japanese career development system which makes that Japanese managers do not always have the right focus in their job abroad. We conclude that Japanese subsidiaries in Western Europe should keep on nourishing their uneq
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