20,437 research outputs found
Refraction of shear zones in granular materials
We study strain localization in slow shear flow focusing on layered granular
materials. A heretofore unknown effect is presented here. We show that shear
zones are refracted at material interfaces in analogy with refraction of light
beams in optics. This phenomenon can be obtained as a consequence of a recent
variational model of shear zones. The predictions of the model are tested and
confirmed by 3D discrete element simulations. We found that shear zones follow
Snell's law of light refraction.Comment: 4 pages, 3 figures, minor changes, jounal ref. adde
Reconstruction of supernova {\nu}_{\mu}, {\nu}_{\tau}, anti-{\nu}_{\mu}, and anti-{\nu}_{\tau} neutrino spectra at scintillator detectors
We present a new technique to directly reconstruct the spectra of mu/tau
neutrinos and antineutrinos from a supernova, using neutrino-proton elastic
scattering events (nu+p to nu+p) at scintillator detectors. These neutrinos,
unlike electron neutrinos and antineutrinos, have only neutral current
interactions, which makes it very challenging, with any reaction, to detect
them and measure their energies. With updated inputs from theory and
experiments, we show that this channel provides a robust and sensitive measure
of their spectra. Given the low yields and lack of spectral information in
other neutral current channels, this is perhaps the only realistic way to
extract such information. This will be indispensable for understanding flavor
oscillations of SN neutrinos, as it is likely to be impossible to disentangle
neutrino mixing from astrophysical uncertainties in a SN without adequate
spectral coverage of all flavors. We emphasize that scintillator detectors,
e.g., Borexino, KamLAND, and SNO+, have the capability to observe these events,
but they must be adequately prepared with a trigger for a burst of low-energy
events. We also highlight the capabilities of a larger detector like LENA.Comment: v3: Typo corrected in Eq.14, and metadata edits. Matches PRD version.
14 pages, 9 figures, 1 tabl
The Full Re-Ionization of Helium
Observations of resolved HeII Lyman alpha absorption in spectra of two QSO's
suggest that the epoch of helium ionization occurred at z~3. Proximity zones in
the spectra of the quasars (z=3.18, 3.285) at 304 A resemble Stromgren spheres,
suggesting that the intergalactic medium is only singly ionized in helium. We
present models of the proximity effect which include the full physics of the
ionization, heating and cooling and an accurately simulated inhomogeneous gas
distribution. In these models the underdense intergalactic medium is heated to
at least 10,000-20,000 K after cooling to as low as a few 1000 K due to
cosmological expansion, with higher temperatures achieved farther away from the
quasar due to absorption-hardened ionizing spectra. The quasars turn on for a
few times 10^7 years with a fairly steady flux output at 228 A comparable to
the 304 A flux output directly observed with HST. The recoveries in the spectra
occur naturally due to voids in the IGM and may provide a fairly
model-independent probe of the baryon density.Comment: 5 pages, 3 figures, to appear in the proceedings of "After the Dark
Ages: When Galaxies were Young (the Universe at 2<z<5)", 9th Annual October
Astrophysics Conference in Marylan
Thermodynamic phase-field model for microstructure with multiple components and phases: the possibility of metastable phases
A diffuse-interface model for microstructure with an arbitrary number of
components and phases was developed from basic thermodynamic and kinetic
principles and formalized within a variational framework. The model includes a
composition gradient energy to capture solute trapping, and is therefore suited
for studying phenomena where the width of the interface plays an important
role. Derivation of the inhomogeneous free energy functional from a Taylor
expansion of homogeneous free energy reveals how the interfacial properties of
each component and phase may be specified under a mass constraint. A diffusion
potential for components was defined away from the dilute solution limit, and a
multi-obstacle barrier function was used to constrain phase fractions. The
model was used to simulate solidification via nucleation, premelting at phase
boundaries and triple junctions, the intrinsic instability of small particles,
and solutal melting resulting from differing diffusivities in solid and liquid.
The shape of metastable free energy surfaces is found to play an important role
in microstructure evolution and may explain why some systems premelt at phase
boundaries and phase triple junctions while others do not.Comment: 14 pages, 8 figure
EXIST: Mission Design Concept and Technology Program
The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed very large
area coded aperture telescope array, incorporating 8m^2 of pixellated Cd-Zn-Te
(CZT) detectors, to conduct a full-sky imaging and temporal hard x-ray (10-600
keV) survey each 95min orbit. With a sensitivity (5sigma, 1yr) of ~0.05mCrab
(10-150 keV), it will extend the ROSAT soft x-ray (0.5-2.5keV) and proposed
ROSITA medium x-ray (2-10 keV) surveys into the hard x-ray band and enable
identification and study of sources ~10-20X fainter than with the ~15-100keV
survey planned for the upcoming Swift mission. At ~100-600 keV, the ~1mCrab
sensitivity is 300X that achieved in the only previous (HEAO-A4, non-imaging)
all-sky survey. EXIST will address a broad range of key science objectives:
from obscured AGN and surveys for black holes on all scales, which constrain
the accretion history of the universe, to the highest sensitivity and
resolution studies of gamma-ray bursts it will conduct as the Next Generation
Gamma-Ray Burst mission. We summarize the science objectives and mission
drivers, and the results of a mission design study for implementation as a free
flyer mission, with Delta IV launch. Key issues affecting the telescope and
detector design are discussed, and a summary of some of the current design
concepts being studied in support of EXIST is presented for the wide-field but
high resolution coded aperture imaging and very large area array of imaging CZT
detectors. Overall mission design is summarized, and technology development
needs and a development program are outlined which would enable the launch of
EXIST by the end of the decade, as recommended by the NAS/NRC Decadal Survey.Comment: 14 pages, 8 figures, 2 tables. PDF file only. Presented at SPIE (Aug.
2002) and to appear in Proc. SPIE, vol. 485
Mapping Low-Density Intergalactic Gas: a Third Helium Lyman-alpha Forest
We present a new HST/STIS spectrum of the z=3.18 quasar PKS 1935-692 and
summarize the spectral features shortwards of 304A in the rest frame likely to
be caused by foreground HeII Lyman-alpha absorption. In accord with previous
results on two other quasars at similar redshifts, we demonstrate a correlation
with the HI Lyman-alpha forest absorption, and show that much of the helium
absorption is caused by a comparable quantity of more diffuse gas with
Omega~0.01, that is not detected in HI. The helium ionization zone around the
quasar is detected as well as a void seen in both HI and HeII. The properties
of the absorption are in broad agreement with those of the other quasars and
with models of the protogalactic gas distribution and ionization at this
redshift.Comment: 17 pages including 5 figures. As accepted for publication in The
Astronomical Journal (minor revisions
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