3,165 research outputs found

### Reaching silicon-based NEMS performances with 3D printed nanomechanical resonators

The extreme miniaturization in NEMS resonators offers the possibility to reach an unprecedented resolution in high-performance mass sensing. These very low limits of detection are related to the combination of two factors: a small resonator mass and a high quality factor. The main drawback of NEMS is represented by the highly complex, multi-steps, and expensive fabrication processes. Several alternatives fabrication processes have been exploited, but they are still limited to MEMS range and very low-quality factor. Here we report the fabrication of rigid NEMS resonators with high-quality factors by a 3D printing approach. After a thermal step, we reach complex geometry printed devices composed of ceramic structures with high Youngâ€™s modulus and low damping showing performances in line with silicon-based NEMS resonators ones. We demonstrate the possibility of rapid fabrication of NEMS devices that present an effective alternative to semiconducting resonators as highly sensitive mass and force sensors

### Electron-positron energy deposition rate from neutrino pair annihilation in the equatorial plane of rapidly rotating neutron and quark stars

The neutrino-antineutrino annihilation into electron-positron pairs near the
surface of compact general relativistic stars could play an important role in
supernova explosions, neutron star collapse, or for close neutron star binaries
near their last stable orbit. General relativistic effects increase the energy
deposition rates due to the annihilation process. We investigate the deposition
of energy and momentum due to the annihilations of neutrinos and antineutrinos
in the equatorial plane of the rapidly rotating neutron and quark stars,
respectively. We analyze the influence of general relativistic effects, and we
obtain the general relativistic corrections to the energy and momentum
deposition rates for arbitrary stationary and axisymmetric space-times. We
obtain the energy and momentum deposition rates for several classes of rapidly
rotating neutron stars, described by different equations of state of the
neutron matter, and for quark stars, described by the MIT bag model equation of
state and in the CFL (Color-Flavor-Locked) phase, respectively. Compared to the
Newtonian calculations, rotation and general relativistic effects increase the
total annihilation rate measured by an observer at infinity. The differences in
the equations of state for neutron and quark matter also have important effects
on the spatial distribution of the energy deposition rate by
neutrino-antineutrino annihilation.Comment: 29 pages, 6 figures, accepted for publication in MNRA

### Neutrino Trapping in a Supernova and Ion Screening

Neutrino-nucleus elastic scattering is reduced in dense matter because of
correlations between ions. The static structure factor for a plasma of
electrons and ions is calculated from Monte Carlo simulations and parameterized
with a least squares fit. Our results imply a large increase in the neutrino
mean free path. This strongly limits the trapping of neutrinos in a supernova
by coherent neutral current interactions.Comment: 9 pages, 1 postscript figure using epsf.st

### Signal for supernova $\nu_\mu$ and $\nu_\tau$ neutrinos in water \v{C}erenkov detectors

We suggest that photons with energies between 5 and 10 MeV, generated by the
($\nu,\nu'p\gamma$) and ($\nu,\nu'n\gamma$) reactions on $^{16}$O, constitute a
signal which allows a unique identification of supernova $\nu_\mu$ and
$\nu_\tau$ neutrinos in water \v{C}erenkov detectors. We calculate the yield of
such $\gamma$ events and estimate that a few hundred of them would be detected
in Superkamiokande for a supernova at 10 kpc distance.Comment: 8 pages, RevTex 3.0, figures and text available at
http://www.krl.caltech.edu/preprints/MAP.htm

### Neutrino Interactions in Hot and Dense Matter

We study the charged and neutral current weak interaction rates relevant for
the determination of neutrino opacities in dense matter found in supernovae and
neutron stars. We establish an efficient formalism for calculating differential
cross sections and mean free paths for interacting, asymmetric nuclear matter
at arbitrary degeneracy. The formalism is valid for both charged and neutral
current reactions. Strong interaction corrections are incorporated through the
in-medium single particle energies at the relevant density and temperature. The
effects of strong interactions on the weak interaction rates are investigated
using both potential and effective field-theoretical models of matter. We
investigate the relative importance of charged and neutral currents for
different astrophysical situations, and also examine the influence of
strangeness-bearing hyperons. Our findings show that the mean free paths are
significantly altered by the effects of strong interactions and the
multi-component nature of dense matter. The opacities are then discussed in the
context of the evolution of the core of a protoneutron star.Comment: 41 pages, 25 figure

### Electron capture on iron group nuclei

We present Gamow-Teller strength distributions from shell model Monte Carlo
studies of fp-shell nuclei that may play an important role in the pre-collapse
evolution of supernovae. We then use these strength distributions to calculate
the electron-capture cross sections and rates in the zero-momentum transfer
limit. We also discuss the thermal behavior of the cross sections. We find
large differences in these cross sections and rates when compared to the naive
single-particle estimates. These differences need to be taken into account for
improved modeling of the early stages of type II supernova evolution

### Neutrino-electron scattering in dense magnetized plasma

We derive exact expressions for the cross section of neutrino scattering on
electrons in dense, hot stellar matter, in the presence of strong magnetic
fields. Numerical calculations of the scattering cross sections at various
densities, temperatures and magnetic fields, are performed. Strong, quantizing
magnetic fields modify significantly the angular and energy dependence of the
scattering cross section.Comment: Physical Review D, to be published, 18 pages, using REVTEX, without
figures. Figures (hardcopy) available upon request from one of the authors
([email protected]

### Neutrino Spectroscopy of the Early Phase of Nearby Supernovae

Neutrinos emitted during stellar core collapse up to their trapping phase
carry information about the stage from which the Supernova explosion process
initiates. The dominant $\nu_e$ emission mechanism is by electron capture on
free protons and f-p shell nuclei and the spectrum of these neutrinos is a
function of the ambient physical conditions within the core as well as the
nuclear equation of state. The number of collapse phase $\nu_e$ which can be
detected by Super-Kamioka and Sudbury Neutrino Observatory from a Supernova
within 1 kpc, and their generic energy spectra are given.Comment: 9 pages of text and tables plus 2 pages of figures. Accepted for
publication in Phys. Rev. Lett. on 11th Jul., 1997. Please e-mail Comments
etc. to [email protected]

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