989 research outputs found
Bird Migration Through A Mountain Pass Studied With High Resolution Radar, Ceilometers, And Census
Autumnal migration was studied with high-resolution radar, ceilometer, and daily census in the area of Franconia Notch, a major pass in the northern Appalachian Mountains. Under synoptic conditions favorable for migration, broadfront movements of migrants toward the south passed over the mountains, often above a temperature inversion. Birds at lower elevations appeared to be influenced by local topography. Birds moving southwest were concentrated along the face of the mountain range. Birds appeared to deviate their flights to follow local topography through the pass. Specific migratory behavior was not associated with species or species groups. Under synoptic conditions unfavorable for southward migration, multimodal movements probably associated with local flights were as dense as the southward migrations described above. Avian migrants reacting to local terrain may result in concentrations of migrants over ridge summits or other topographic features
A Systematic Investigation of Light Heavy-Ion Reactions
We introduce a novel coupling potential for the scattering of deformed light
heavy-ion reactions. This new approach is based on replacing the usual
first-derivative coupling potential by a new, second derivative coupling
potential in the coupled-channels formalism. The new approach has been
successfully applied to the study of the C+C, C+Mg,
O+Si and O+Mg systems and made major improvements
over all the previous coupled-channels calculations for these systems. This
paper also shows the limitations of the standard coupled-channels theory and
presents a global solution to the problems faced in the previous theoretical
accounts of these reactions.Comment: 7 pages with 4 figure
Barrier Distributions as a Tool to Investigate Fusion and Fission
The recent availability of precisely measured fusion cross-sections has
enabled the extraction of a representation of the distribution of barriers
encountered during fusion. These representations, obtained from a variety of
reactions, provide a direct observation of how the structure of the fusing
nuclei changes the inter-nuclear potential landscape, thus affecting the fusion
probability. Recent experiments showing the effects of static quadrupole and
hexadecapole deformation, single-- and double-phonon states, transfer of
nucleons between two nuclei, and high lying excited states are reviewed. The
application of these concepts to the explanation of the anomalous
fission-fragment anisotropies observed following reactions with actinides is
discussed.Comment: 12 pages, To be published in the Proceedings of the NN 97 Conference,
Gatlinburg, Tennessee, June 1997 (Nucl. Phys. A
Dark Matter in Nuclear Physics
This White Paper was prepared for the Nuclear Science Town Meeting on "Neutrinos, Neutrons and Fundamental Symmetries, held in Chicago on Jan. 17-19, 2007
A Global Potential Analysis of the O+Si Reaction Using a New Type of Coupling Potential
A new approach has been used to explain the experimental data for the
O+Si system over a wide energy range in the laboratory system
from 29.0 to 142.5 MeV. A number of serious problems has continued to plague
the study of this system for a couple of decades. The explanation of anomalous
large angle scattering data; the reproduction of the oscillatory structure near
the Coulomb barrier; the out-of-phase problem between theoretical predictions
and experimental data; the consistent description of angular distributions
together with excitation functions data are just some of these problems. These
are long standing problems that have persisted over the years and do represent
a challenge calling for a consistent framework to resolve these difficulties
within a unified approach. Traditional frameworks have failed to describe these
phenomena within a single model and have so far only offered different
approaches where these difficulties are investigated separately from one
another. The present work offers a plausible framework where all these
difficulties are investigated and answered. Not only it improves the
simultaneous fits to the data of these diverse observables, achieving this
within a unified approach over a wide energy range, but it departs for its
coupling potential from the standard formulation. This new feature is shown to
improve consistently the agreement with the experimental data and has made
major improvement on all the previous coupled-channels calculations for this
system.Comment: 21 pages with 12 figure
One-neutron removal reactions on light neutron-rich nuclei
A study of high energy (43--68 MeV/nucleon) one-neutron removal reactions on
a range of neutron-rich psd-shell nuclei (Z = 5--9, A = 12--25) has been
undertaken. The inclusive longitudinal and transverse momentum distributions
for the core fragments, together with the cross sections have been measured for
breakup on a carbon target. Momentum distributions for reactions on tantalum
were also measured for a subset of nuclei. An extended version of the Glauber
model incorporating second order noneikonal corrections to the JLM
parametrisation of the optical potential has been used to describe the nuclear
breakup, whilst the Coulomb dissociation is treated within first order
perturbation theory. The projectile structure has been taken into account via
shell model calculations employing the psd-interaction of Warburton and Brown.
Both the longitudinal and transverse momentum distributions, together with the
integrated cross sections were well reproduced by these calculations and
spin-parity assignments are thus proposed for B, C, N,
O, F. In addition to the large spectroscopic amplitudes for
the s intruder configuration in the N=9 isotones,B and
C, significant s admixtures appear to occur in the
ground state of the neighbouring N=10 nuclei B and C. Similarly,
crossing the N=14 subshell, the occupation of the s orbital is
observed for O, F. Analysis of the longitudinal and transverse
momentum distributions reveals that both carry spectroscopic information, often
of a complementary nature. The general utility of high energy nucleon removal
reactions as a spectroscopic tool is also examined.Comment: 50 pages, 19 figures, submitted to Phys. Rev.
Testing two nuclear physics approximations used in the standard leaky box model for the spallogenic production of LiBeB
The spallative production rates of Lithium, Beryllium and Boron (LiBeB) are a
necessary component in any calculation of the evolution of these nuclei in the
Galaxy. Previous calculations of these rates relied on two assumptions relating
to the nuclear physics aspects: the straight-ahead approximation that describes
the distribution of fragment energies and the assumption that the major
contributor to the production rate arises from single-step reactions between
primary cosmic ray projectiles and interstellar medium targets. We examine both
assumptions by using a semi-empirical description for the spall's energy
distribution and by including the reactions that proceed via intermediary
fragments. After relaxing the straight-ahead approximation we find the changes
in the production rates and emerging fluxes are small and do not warrant
rejection of this approximation. In contrast we discover that two-step
reactions can alter the production rate considerably leading to noticeable
increases in the efficiency of producing the LiBeB nuclei. Motivated by this
result we introduce a cascade technique to compute the production rates exactly
and find that the results differ only slightly from those of our two-step
calculations. We thus conclude that terminating the reaction network at the
two-step order is sufficiently accurate for current studies of spallation.Comment: accepted in Ap
Refractive elastic scattering of carbon and oxygen nuclei: The mean field analysis and Airy structures
The experimental data on the OC and OC elastic
scatterings and their optical model analysis are presented. Detailed and
complete elastic angular distributions have been measured at the Strasbourg
Vivitron accelerator at several energies covering the energy range between 5
and 10 MeV per nucleon. The elastic scattering angular distributions show the
usual diffraction pattern and also, at larger angles, refractive effects in the
form of nuclear rainbow and associated Airy structures. The optical model
analysis unambiguously shows the evolution of the refractive scattering
pattern. The observed structure, namely the Airy minima, can be consistently
described by a nucleus-nucleus potential with a deep real part and a weakly
absorptive imaginary part. The difference in absorption in the two systems is
explained by an increased imaginary (mostly surface) part of the potential in
the OC system. The relation between the obtained potentials and
those reported for the symmetrical OO and CC
systems is drawn.Comment: 10 pages, 9 figures, Phys. rev. C in pres
Quantum Tunneling in Nuclear Fusion
Recent theoretical advances in the study of heavy ion fusion reactions below
the Coulomb barrier are reviewed. Particular emphasis is given to new ways of
analyzing data, such as studying barrier distributions; new approaches to
channel coupling, such as the path integral and Green function formalisms; and
alternative methods to describe nuclear structure effects, such as those using
the Interacting Boson Model. The roles of nucleon transfer, asymmetry effects,
higher-order couplings, and shape-phase transitions are elucidated. The current
status of the fusion of unstable nuclei and very massive systems are briefly
discussed.Comment: To appear in the January 1998 issue of Reviews of Modern Physics. 13
Figures (postscript file for Figure 6 is not available; a hard copy can be
requested from the authors). Full text and figures are also available at
http://nucth.physics.wisc.edu/preprints
The Prelude to the Deep Minimum between Solar Cycles 23 and 24: Interplanetary Scintillation Signatures in the Inner Heliosphere
Extensive interplanetary scintillation (IPS) observations at 327 MHz obtained
between 1983 and 2009 clearly show a steady and significant drop in the
turbulence levels in the entire inner heliosphere starting from around ~1995.
We believe that this large-scale IPS signature, in the inner heliosphere,
coupled with the fact that solar polar fields have also been declining since
~1995, provide a consistent result showing that the buildup to the deepest
minimum in 100 years actually began more than a decade earlier.Comment: 9 pages, 4 figures, accepted for publication in Geophysical Research
Letters on 28 September 201
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