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Transient Physical Effects in Electron Beam Sintering
The extensive use of the electron beam in manufacturing processes like welding or perforating
revealed the high potentials for also using it for solid freeform fabrication. First approaches like
feeding wire into a melt pool have successfully shown the technical feasibility. Among other
features, the electron beam exhibits high scanning speed, high power output, and beam density.
While in laser-based machines the fabrication is working in a stable way, transient physical
effects in the electron beam process can be observed, which still restrict process stability. For
instance, a high power input of the electron beam can result in sudden scattering of the metal
powder. The authors have developed an electron beam freeform fabrication system and examined
the above mentioned effects. Thus, the paper provides methods in order to identify, isolate and
avoid these effects, and to finally realize a reproducible process.Mechanical Engineerin
Numerical Toy-Model Calculation of the Nucleon Spin Autocorrelation Function in a Supernova Core
We develop a simple model for the evolution of a nucleon spin in a hot and
dense nuclear medium. A given nucleon is limited to one-dimensional motion in a
distribution of external, spin-dependent scattering potentials. We calculate
the nucleon spin autocorrelation function numerically for a variety of
potential densities and distributions which are meant to bracket realistic
conditions in a supernova core. For all plausible configurations the width of
the spin-density structure function is found to be less than the temperature.
This is in contrast with a naive perturbative calculation based on the one-pion
exchange potential which overestimates the width and thus suggests a large
suppression of the neutrino opacities by nucleon spin fluctuations. Our results
suggest that it may be justified to neglect the collisional broadening of the
spin-density structure function for the purpose of estimating the neutrino
opacities in the deep inner core of a supernova. On the other hand, we find no
indication that processes such as axion or neutrino pair emission, which depend
on nucleon spin fluctuations, are substantially suppressed beyond the
multiple-scattering effect already discussed in the literature. Aside from
these practical conclusions, our model reveals a number of interesting and
unexpected insights. For example, the spin-relaxation rate saturates with
increasing potential strength only if bound states are not allowed to form by
including a repulsive core. There is no saturation with increasing density of
scattering potentials until localized eigenstates of energy begin to form.Comment: 14 latex pages in two-column format, 15 postscript figures included,
uses revtex.sty and epsf.sty. Submitted to Physical Review
Gravitational wave background from neutron star phase transition for a new class of equation of state
We study the generation of a stochastic gravitational wave (GW) background
produced by a population of neutron stars (NSs) which go over a hadron-quark
phase transition in its inner shells. We obtain, for example, that the NS phase
transition, in cold dark matter scenarios, could generate a stochastic GW
background with a maximum amplitude of , in the
frequency band for stars forming at redshifts of up
to We study the possibility of detection of this isotropic GW
background by correlating signals of a pair of `advanced' LIGO observatories.Comment: 7 pages, 1 figur
Neutralino spectrum in top-down models of UHECR
We calculate the cosmic ray spectrum of ultra high energy neutralinos that
one should expect provided that the observed ultra high energy cosmic rays are
produced by the decay of superheavy particles X, M_X>10^{12} GeV, in
supersymmetric models. Our calculation uses an extended DGLAP formalism.
Forthcoming cosmic ray observatories should be able to detect these
neutralinos.Comment: 10 pages, revtex, 3 eps figures. Difference between the present work
and Montecarlo simulations clarifie
On The Origin of Very High Energy Cosmic Rays
We discuss the most recent developments in our understanding of the
acceleration and propagation of cosmic rays up to the highest energies. In
particular we specialize our discussion to three issues: 1) developments in the
theory of particle acceleration at shock waves; 2) the transition from galactic
to extragalactic cosmic rays; 3) implications of up-to-date observations for
the origin of ultra high energy cosmic rays (UHECRs).Comment: Invited Review Article to appear in Modern Physics Letters A, Review
Sectio
Non-extensivity Effects and the Highest Energy Cosmic Ray Affair
Recent measurements of the cosmic microwave background confirm that it is
described by a Planckian distribution with high precision. It is
non-extensivity bounded to be less than some parts in , or to some parts
in at most. This deviation may appear minuscule, but may have a
non-negligible effect on a particle propagating through this background over
the course of millions of years. In this paper we analyze the possible
influence of such a slight deviation upon the propagation of nuclei and protons
of ultra-high energy. These particles interact via photopion and
photodisintegration processes which we examine taking into account a slight
non-extensive background. We show that such a deviation does not exhibit a
significant difference in the energy attenuation length of extremely high
energy cosmic rays.Comment: Revised version, improvements per referee's suggestion
Ultra-high energy cosmic rays from Quark Novae
We explore acceleration of ions in the Quark Nova (QN) scenario, where a
neutron star experiences an explosive phase transition into a quark star (born
in the propeller regime). In this picture, two cosmic ray components are
isolated: one related to the randomized pulsar wind and the other to the
propelled wind, both boosted by the ultra-relativistic Quark Nova shock. The
latter component acquires energies while
the former, boosted pulsar wind, achieves ultra-high energies
eV. The composition is dominated by ions present in the pulsar wind in the
energy range above eV, while at energies below eV the
propelled ejecta, consisting of the fall-back neutron star crust material from
the explosion, is the dominant one. Added to these two components, the
propeller injects relativistic particles with Lorentz factors , later to be accelerated by galactic supernova shocks. The
QN model appears to be able to account for the extragalactic cosmic rays above
the ankle and to contribute a few percent of the galactic cosmic rays below the
ankle. We predict few hundred ultra-high energy cosmic ray events above
eV for the Pierre Auger detector per distant QN, while some thousands
are predicted for the proposed EUSO and OWL detectors.Comment: 20 pages, 1 figure. Major revisions in the text. Accepted for
publication in the Astrophysical Journa
IceCube-Plus: An Ultra-High Energy Neutrino Telescope
While the first kilometer-scale neutrino telescope, IceCube, is under
construction, alternative plans exist to build even larger detectors that will,
however, b e limited by a much higher neutrino energy threshold of 10 PeV or
higher rather than 10 to 100 GeV. These future projects detect radio and
acoustic pulses as w ell as air showers initiated by ultra-high energy
neutrinos. As an alternative, we here propose an expansion of IceCube, using
the same strings, placed on a gri d with a spacing of order 500 m. Unlike other
proposals, the expanded detector uses methods that are understood and
calibrated on atmospheric neutrinos. Atmosp heric neutrinos represent the only
background at the energies under consideratio n and is totally negligible.
Also, the cost of such a detector is understood. We conclude that supplementing
the 81 IceCube strings with a modest number of addi tional strings spaced at
large distances can almost double the effective volume of the detector.
Doubling the number of strings on a 800 m grid can deliver a d etector that
this a factor of 5 larger for horizontal muons at modest cost.Comment: Version to be published in JCA
Ultra-High Energy Cosmic Ray Nuclei from Individual Magnetized Sources
We investigate the dependence of composition, spectrum and angular
distributions of ultra-high energy cosmic rays above 10^19 eV from individual
sources on their magnetization. We find that, especially for sources within a
few megaparsecs from the observer, observable spectra and composition are
severely modified if the source is surrounded by fields of ~ 10^-7 Gauss on
scales of a few megaparsecs. Low energy particles diffuse over larger distances
during their energy loss time. This leads to considerable hardening of the
spectrum up to the energy where the loss distance becomes comparable to the
source distance. Magnetized sources thus have very important consequences for
observations, even if cosmic rays arrive within a few degrees from the source
direction. At the same time, details in spectra and chemical composition may be
intrinsically unpredictable because they depend on the unknown magnetic field
structure. If primaries are predominantly nuclei of atomic mass A accelerated
up to a maximum energy E_max with spectra not much softer than E^-2, secondary
protons from photo-disintegration can produce a conspicuous peak in the
spectrum at energy ~ E_max/A. A related feature appears in the average mass
dependence on energy.Comment: 15 pages, 16 ps figures, published version with minor changes, see
http://stacks.iop.org/1475-7516/2004/i=08/a=01
SimProp: a Simulation Code for Ultra High Energy Cosmic Ray Propagation
A new Monte Carlo simulation code for the propagation of Ultra High Energy
Cosmic Rays is presented. The results of this simulation scheme are tested by
comparison with results of another Monte Carlo computation as well as with the
results obtained by directly solving the kinetic equation for the propagation
of Ultra High Energy Cosmic Rays. A short comparison with the latest flux
published by the Pierre Auger collaboration is also presented.Comment: 19 pages, 12 eps figures, version accepted for publication in JCA
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