606 research outputs found
A study of local approximation for polarization potentials
We discuss the derivation of an equivalent \textit{l}-independent
polarization potential for use in the optical Schr\"{o}dinger equation that
describes the elastic scattering of heavy ions. Three diffferent methods are
used for this purpose. Application of our theory to the low energy scattering
of the halo nucleus Li from a C target is made. It is found that
the notion of \textit{l}-independent polarization potential has some validity
but can not be a good substitute for the \textit{l}-dependent local equivalent
Feshbach polarization potential.Comment: 8 pages, 4 figure
Interplay of static and dynamic effects in 6He+ 238U Fusion
We investigate the influence of the neutron halo and the breakup channel in
6He + 238U fusion at near-barrier energies. To include static effects of the
2n-halo in 6He nuclei, we use a single-folding potential obtained from an
appropriate nucleon-238U interaction and a realistic 6He density. Dynamical
effects arising from the breakup process are then included through
coupled-channel calculations. These calculations suggest that static effects
dominate the cross section at energies above the Coulomb barrier, while the
sub-barrier fusion cross section appears to be determined by coupling to the
breakup channel. This last conclusion is uncertain due to the procedure
employed to measure the fusion cross-section.Comment: 13 pages, 4 figure
Tuning the electrical conductance of metalloporphyrin supramolecular wires
In contrast with conventional single-molecule junctions, in which the current flows parallel to the long axis or plane of a molecule, we investigate the transport properties of M(II)-5,15-diphenylporphyrin (M-DPP) single-molecule junctions (M=Co, Ni, Cu, or Zn divalent metal ions), in which the current flows perpendicular to the plane of the porphyrin. Novel STM-based conductance measurements combined with quantum transport calculations demonstrate that current-perpendicular-to-the-plane (CPP) junctions have three-orders-of-magnitude higher electrical conductanc than their current in-plane (CIP) counterparts, ranging from 2.10−2 G0 for Ni-DPP up to 8.10−2 G0 for Zn-DPP. The metal ion in the center of the DPP skeletons is strongly coordinated with the nitrogens of the pyridyl coated electrodes, with a binding energy that is sensitive to the choice of metal ion. We find that the binding energies of Zn-DPP and Co-DPP are significantly higher than those of Ni-DPP and Cu-DPP. Therefore when combined with its higher conductance, we identify Zn-DPP as the favoured candidate for high conductance CPP single-molecule devices
The Pierre Auger Observatory III: Other Astrophysical Observations
Astrophysical observations of ultra-high-energy cosmic rays with the Pierre
Auger ObservatoryComment: Contributions to the 32nd International Cosmic Ray Conference,
Beijing, China, August 201
Neutron reflection study of the adsorption of the phosphate surfactant NaDEHP onto alumina from water.
The adsorption of a phosphorus analogue of the surfactant AOT, sodium bis(2-ethylhexyl) phosphate (NaDEHP), at the water/alumina interface is described. The material is found to adsorb as an essentially water-free bilayer from neutron reflection measurements. This is similar to the behavior of AOT under comparable conditions, although AOT forms a thicker, more hydrated layer. The NaDEHP shows rather little variation with added salt, but a small thickening of the layer on increasing the pH, in contrast to the behavior of AOT.We thank BP plc and EPSRC for financial support for this work as well as the ISIS and ILL staff and scientists for the allocation of beam time and technical assistance with NR measurements. We also appreciate Chris Sporikou at Department of Chemistry, University of Cambridge, for help with the surfactant synthesis.This is the final version of the article. It first appeared at http://dx.doi.org/10.1021/la504837
A search for point sources of EeV photons
Measurements of air showers made using the hybrid technique developed with
the fluorescence and surface detectors of the Pierre Auger Observatory allow a
sensitive search for point sources of EeV photons anywhere in the exposed sky.
A multivariate analysis reduces the background of hadronic cosmic rays. The
search is sensitive to a declination band from -85{\deg} to +20{\deg}, in an
energy range from 10^17.3 eV to 10^18.5 eV. No photon point source has been
detected. An upper limit on the photon flux has been derived for every
direction. The mean value of the energy flux limit that results from this,
assuming a photon spectral index of -2, is 0.06 eV cm^-2 s^-1, and no celestial
direction exceeds 0.25 eV cm^-2 s^-1. These upper limits constrain scenarios in
which EeV cosmic ray protons are emitted by non-transient sources in the
Galaxy.Comment: 28 pages, 10 figures, accepted for publication in The Astrophysical
Journa
Azimuthal asymmetry in the risetime of the surface detector signals of the Pierre Auger Observatory
The azimuthal asymmetry in the risetime of signals in Auger surface detector
stations is a source of information on shower development. The azimuthal
asymmetry is due to a combination of the longitudinal evolution of the shower
and geometrical effects related to the angles of incidence of the particles
into the detectors. The magnitude of the effect depends upon the zenith angle
and state of development of the shower and thus provides a novel observable,
, sensitive to the mass composition of cosmic rays
above eV. By comparing measurements with predictions from
shower simulations, we find for both of our adopted models of hadronic physics
(QGSJETII-04 and EPOS-LHC) an indication that the mean cosmic-ray mass
increases slowly with energy, as has been inferred from other studies. However,
the mass estimates are dependent on the shower model and on the range of
distance from the shower core selected. Thus the method has uncovered further
deficiencies in our understanding of shower modelling that must be resolved
before the mass composition can be inferred from .Comment: Replaced with published version. Added journal reference and DO
Reconstruction of inclined air showers detected with the Pierre Auger Observatory
We describe the method devised to reconstruct inclined cosmic-ray air showers
with zenith angles greater than detected with the surface array of
the Pierre Auger Observatory. The measured signals at the ground level are
fitted to muon density distributions predicted with atmospheric cascade models
to obtain the relative shower size as an overall normalization parameter. The
method is evaluated using simulated showers to test its performance. The energy
of the cosmic rays is calibrated using a sub-sample of events reconstructed
with both the fluorescence and surface array techniques. The reconstruction
method described here provides the basis of complementary analyses including an
independent measurement of the energy spectrum of ultra-high energy cosmic rays
using very inclined events collected by the Pierre Auger Observatory.Comment: 27 pages, 19 figures, accepted for publication in Journal of
Cosmology and Astroparticle Physics (JCAP
Ultrahigh-energy neutrino follow-up of Gravitational Wave events GW150914 and GW151226 with the Pierre Auger Observatory
On September 14, 2015 the Advanced LIGO detectors observed their first
gravitational-wave (GW) transient GW150914. This was followed by a second GW
event observed on December 26, 2015. Both events were inferred to have arisen
from the merger of black holes in binary systems. Such a system may emit
neutrinos if there are magnetic fields and disk debris remaining from the
formation of the two black holes. With the surface detector array of the Pierre
Auger Observatory we can search for neutrinos with energy above 100 PeV from
point-like sources across the sky with equatorial declination from about -65
deg. to +60 deg., and in particular from a fraction of the 90% confidence-level
(CL) inferred positions in the sky of GW150914 and GW151226. A targeted search
for highly-inclined extensive air showers, produced either by interactions of
downward-going neutrinos of all flavors in the atmosphere or by the decays of
tau leptons originating from tau-neutrino interactions in the Earth's crust
(Earth-skimming neutrinos), yielded no candidates in the Auger data collected
within s around or 1 day after the coordinated universal time (UTC)
of GW150914 and GW151226, as well as in the same search periods relative to the
UTC time of the GW candidate event LVT151012. From the non-observation we
constrain the amount of energy radiated in ultrahigh-energy neutrinos from such
remarkable events.Comment: Published version. Added journal reference and DOI. Added Report
Numbe
- …