595 research outputs found
Terrestrial Effects Of Nearby Supernovae In The Early Pleistocene
Recent results have strongly confirmed that multiple supernovae happened at
distances ~100 pc consisting of two main events: one at 1.7 to 3.2 million
years ago, and the other at 6.5 to 8.7 million years ago. These events are said
to be responsible for excavating the Local Bubble in the interstellar medium
and depositing 60Fe on Earth and the Moon. Other events are indicated by
effects in the local cosmic ray (CR) spectrum. Given this updated and refined
picture, we ask whether such supernovae are expected to have had substantial
effects on the terrestrial atmosphere and biota. In a first cut at the most
probable cases, combining photon and cosmic ray effects, we find that a
supernova at 100 pc can have only a small effect on terrestrial organisms from
visible light and that chemical changes such as ozone depletion are weak.
However, tropospheric ionization right down to the ground due to the
penetration of TeV cosmic rays will increase by nearly an order of
magnitude for thousands of years, and irradiation by muons on the ground and in
the upper ocean will increase 20-fold, which will approximately triple the
overall radiation load on terrestrial organisms. Such irradiation has been
linked to possible changes in climate and increased cancer and mutation rates.
This may be related to a minor mass extinction around the Pliocene-Pleistocene
boundary, and further research on the effects is needed.Comment: Revised version accepted at ApJ
Strong anisotropy in surface kinetic roughening: analysis and experiments
We report an experimental assessment of surface kinetic roughening properties
that are anisotropic in space. Working for two specific instances of silicon
surfaces irradiated by ion-beam sputtering under diverse conditions (with and
without concurrent metallic impurity codeposition), we verify the predictions
and consistency of a recently proposed scaling Ansatz for surface observables
like the two-dimensional (2D) height Power Spectral Density (PSD). In contrast
with other formulations, this Ansatz is naturally tailored to the study of
two-dimensional surfaces, and allows to readily explore the implications of
anisotropic scaling for other observables, such as real-space correlation
functions and PSD functions for 1D profiles of the surface. Our results confirm
that there are indeed actual experimental systems whose kinetic roughening is
strongly anisotropic, as consistently described by this scaling analysis. In
the light of our work, some types of experimental measurements are seen to be
more affected by issues like finite space resolution effects, etc. that may
hinder a clear-cut assessment of strongly anisotropic scaling in the present
and other practical contexts
Discovery of the Barium Isotopes
Thirty-eight barium isotopes have so far been observed; the discovery of
these isotopes is discussed. For each isotope a brief summary of the first
refereed publication, including the production and identification method, is
presented.Comment: to be published in At. Data Nucl. Data Table
PEG–Polypeptide Block Copolymers as pH-Responsive Endosome-Solubilizing Drug Nanocarriers
Herein we report the potential of click chemistry-modified polypeptide-based block copolymers for the facile fabrication of pH-sensitive nanoscale drug delivery systems. PEG–polypeptide copolymers with pendant amine chains were synthesized by combining N-carboxyanhydride-based ring-opening polymerization with post-functionalization using azide–alkyne cycloaddition. The synthesized block copolymers contain a polypeptide block with amine-functional side groups and were found to self-assemble into stable polymersomes and disassemble in a pH-responsive manner under a range of biologically relevant conditions. The self-assembly of these block copolymers yields nanometer-scale vesicular structures that are able to encapsulate hydrophilic cytotoxic agents like doxorubicin at physiological pH but that fall apart spontaneously at endosomal pH levels after cellular uptake. When drug-encapsulated copolymer assemblies were delivered systemically, significant levels of tumor accumulation were achieved, with efficacy against the triple-negative breast cancer cell line, MDA-MB-468, and suppression of tumor growth in an in vivo mouse model.Novartis Institutes of Biomedical ResearchNational Institutes of Health (U.S.) (Centers for Cancer Nanotechnology Excellence Grant P30 CA14051)National Institutes of Health (U.S.) (Centers for Cancer Nanotechnology Excellence Grant 5 U54 CA151884-02)National Science Foundation (U.S.). Graduate Research FellowshipNatural Sciences and Engineering Research Council of Canada (Postdoctoral Fellowship
JWST Near-Infrared Detectors: Latest Test Results
The James Webb Space Telescope, an infrared-optimized space telescope being developed by NASA for launch in 2013, will utilize cutting-edge detector technology in its investigation of fundamental questions in astrophysics. JWST's near infrared spectrograph, NIRSpec utilizes two 2048 x 2048 HdCdTe arrays with Sidecar ASIC readout electronics developed by Teledyne to provide spectral coverage from 0.6 microns to 5 microns. We present recent test and calibration results for the NIRSpec flight arrays as well as data processing routines for noise reduction and cosmic ray rejection
Search for Branons at LEP
We search, in the context of extra-dimension scenarios, for the possible
existence of brane fluctuations, called branons. Events with a single photon or
a single Z-boson and missing energy and momentum collected with the L3 detector
in e^+ e^- collisions at centre-of-mass energies sqrt{s}=189-209$ GeV are
analysed. No excess over the Standard Model expectations is found and a lower
limit at 95% confidence level of 103 GeV is derived for the mass of branons,
for a scenario with small brane tensions. Alternatively, under the assumption
of a light branon, brane tensions below 180 GeV are excluded
Direct Observation of Longitudinally Polarised W Bosons
The three different helicity states of W bosons, produced in the reaction
e+e- -> W+W- -> l nu q q~ are studied using leptonic and hadronic W decays at
sqrt{s}=183GeV and 189GeV. The W polarisation is also measured as a function of
the scattering angle between the W- and the direction of the e- beam. The
analysis demonstrates that W bosons are produced with all three helicities, the
longitudinal and the two transverse states. Combining the results from the two
center-of-mass energies and with leptonic and hadronic W decays, the fraction
of longitudinally polarised W bosons is measured to be 0.261 +/- 0.051(stat.)
+/- 0.016(syst.) in agreement with the expectation from the Standard Model
Search for Charginos with a Small Mass Difference with the Lightest Supersymmetric Particle at \sqrt{s} = 189 GeV
A search for charginos nearly mass-degenerate with the lightest
supersymmetric particle is performed using the 176 pb^-1 of data collected at
189 GeV in 1998 with the L3 detector. Mass differences between the chargino and
the lightest supersymmetric particle below 4 GeV are considered. The presence
of a high transverse momentum photon is required to single out the signal from
the photon-photon interaction background. No evidence for charginos is found
and upper limits on the cross section for chargino pair production are set. For
the first time, in the case of heavy scalar leptons, chargino mass limits are
obtained for any \tilde{\chi}^{+-}_1 - \tilde{\chi}^0_1 mass difference
K0s K0s Final State in Two-Photon Collisions and Implications for Glueballs
The K0s K0s final state in two-photon collisions is studied with the L3
detector at LEP. The mass spectrum is dominated by the formation of the
f_2'(1525) tensor meson in the helicity-two state with a two-photon width times
the branching ratio into K Kbar of 76 +- 6 +- 11 eV. A clear signal for the
formation of the f_J(1710) is observed and it is found to be dominated by the
spin-two helicity-two state. No resonance is observed in the mass region around
2.2 GeV and an upper limit of 1.4 eV at 95% C.L. is derived for the two-photon
width times the branching ratio into K0s K0s for the glueball candidate
xi(2230)
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