29,751 research outputs found
Neutrino-Induced Fission and r-Process Nucleosynthesis
An r-process scenario with fission but no fission cycling is considered to
account for the observed abundance patterns of neutron-capture elements in
ultra-metal-poor stars. It is proposed that neutrino reactions play a crucial
role in inducing the fission of the progenitor nuclei after the r-process
freezes out in Type II Supernovae. To facilitate neutrino-induced fission, the
proposed r-process scenario is restricted to occur in a low-density environment
such as the neutrino-driven wind from the neutron star. Further studies to
develop this scenario are emphasized.Comment: 11 pages, 2 figures, to appear in ApJ
An efficient hybrid model and dynamic performance analysis for multihop wireless networks
Multihop wireless networks can be subjected to nonstationary phenomena due to a dynamic network topology and time varying traffic. However, the simulation techniques used to study multihop wireless networks focus on the steady-state performance even though transient or nonstationary periods will often occur. Moreover, the majority of the simulators suffer from poor scalability. In this paper, we develop an efficient performance modeling technique for analyzing the time varying queueing behavior of multihop wireless networks. The one-hop packet transmission (service) time is assumed to be deterministic, which could be achieved by contention-free transmission, or approximated in sparse or lightly loaded multihop wireless networks. Our model is a hybrid of time varying adjacency matrix and fluid flow based differential equations, which represent dynamic topology changes and nonstationary network queues, respectively. Numerical experiments show that the hybrid fluid based model can provide reasonably accurate results much more efficiently than standard simulators. Also an example application of the modeling technique is given showing the nonstationary network performance as a function of node mobility, traffic load and wireless link quality. © 2013 IEEE
Probing r-Process Production of Nuclei Beyond Bi209 with Gamma Rays
We estimate gamma-ray fluxes due to the decay of nuclei beyond Bi209 from a
supernova or a supernova remnant assuming that the r-process occurs in
supernovae. We find that a detector with a sensitivity of about 10**(-7)
photons/cm**2/s at energies of 40 keV to 3 MeV may detect fluxes due to the
decay of Ra226, Th229, Am241, Am243, Cf249, and Cf251 in the newly discovered
supernova remnant near Vela. In addition, such a detector may detect fluxes due
to the decay of Ac227 and Ra228 produced in a future supernova at a distance of
about 1 kpc. As nuclei with mass numbers A > 209 are produced solely by the
r-process, such detections are the best proof for a supernova r-process site.
Further, they provide the most direct information on yields of progenitor
nuclei with A > 209 at r-process freeze-out. Finally, detection of fluxes due
to the decay of r-process nuclei over a range of masses from a supernova or a
supernova remnant provides the opportunity to compare yields in a single
supernova event with the solar r-process abundance pattern.Comment: 24 pages, 3 figures, to appear in the October 10, 1999 issue of Ap
Supernovae as the Site of the r-Process: Implications for Gamma-Ray Astronomy
We discuss how detection of gamma-ray emission from the decay of r-process
nuclei can improve our understanding of r-process nucleosynthesis. We find that
a gamma-ray detector with a sensitivity of 10**(-7)/cm**2/s at 100-700 keV may
detect the emission from the decay of Sb125, Cs137, Ce144, Eu155, and Os194
produced in a future Galactic supernova. In addition, such a detector may
detect the emission from the decay of Sn126 in the Vela supernova remnant and
the diffuse emission from the decay of Sn126 produced by past supernovae in our
Galaxy. The required detector sensitivity is similar to what is projected for
the proposed Advanced Telescope for High Energy Nuclear Astrophysics (ATHENA).
Both the detection of gamma-ray emission from the decay of several r-process
nuclei (e.g., Sb125 and Os194) produced in future Galactic supernovae and the
detection of emission from the decay of Sn126 in the Vela supernova remnant
would prove that supernovae are a site of the r-process. Furthermore, the
former detection would allow us to determine whether or not the r-process
nuclei are produced in relative proportions specified by the solar r-process
abundance pattern in supernova r-process events. Finally, detection of diffuse
emission from the decay of Sn126 in our Galaxy would eliminate neutron
star/neutron star mergers as the main source for the r-process nuclei near mass
number A=126.Comment: 14 pages, AASTeX, submitted to the Astrophysical Journa
A new 2-D model of a thin annular disk using a modified assumption
The work describes an improved 2-D model for a thin annulus by using a modified assumption with regard to coupled vibration. With this approach, the impedance spectrum and displacements due to radial modes, both in radial and thickness direction of a thin ring, are obtained. Bending displacement is investigated by finite element analysis (FEA) and matches our model. The bending in the thickness direction is coupled to radial modes and shows several node circles in the high radial overtone frequency range. The model is validated by FEA with excellent agreement between the new theory and FEA result
Angle-resolved photoemission studies of the superconducting gap symmetry in Fe-based superconductors
The superconducting gap is the fundamental parameter that characterizes the
superconducting state, and its symmetry is a direct consequence of the
mechanism responsible for Cooper pairing. Here we discuss about angle-resolved
photoemission spectroscopy measurements of the superconducting gap in the
Fe-based high-temperature superconductors. We show that the superconducting gap
is Fermi surface dependent and nodeless with small anisotropy, or more
precisely, a function of momentum. We show that while this observation is
inconsistent with weak coupling approaches for superconductivity in these
materials, it is well supported by strong coupling models and global
superconducting gaps. We also suggest that the strong anisotropies measured by
other probes sensitive to the residual density of states are not related to the
pairing interaction itself, but rather emerge naturally from the smaller
lifetime of the superconducting Cooper pairs that is a direct consequence of
the momentum dependent interband scattering inherent to these materials.Comment: 7 pages, 5 figure
A time dependent performance model for multihop wireless networks with CBR traffic
In this paper, we develop a performance modeling technique for analyzing the time varying network layer queueing behavior of multihop wireless networks with constant bit rate traffic. Our approach is a hybrid of fluid flow queueing modeling and a time varying connectivity matrix. Network queues are modeled using fluid-flow based differential equation models which are solved using numerical methods, while node mobility is modeled using deterministic or stochastic modeling of adjacency matrix elements. Numerical and simulation experiments show that the new approach can provide reasonably accurate results with significant improvements in the computation time compared to standard simulation tools. © 2010 IEEE
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