31,729 research outputs found
Total and partial cross sections of the Sn()Te reaction measured via in-beam -ray spectroscopy
An extended database of experimental data is needed to address uncertainties
of the nuclear-physics input parameters for Hauser-Feshbach calculations.
Especially +nucleus optical model potentials at low energies are not
well known. The in-beam technique with an array of high-purity germanium (HPGe)
detectors was successfully applied to the measurement of absolute cross
sections of an (,) reaction on a heavy nucleus at sub-Coulomb
energies. The total and partial cross-section values were measured by means of
in-beam -ray spectroscopy. Total and partial cross sections were
measured at four different -particle energies from
MeV to MeV. The measured total cross-section values are in
excellent agreement with previous results obtained with the activation
technique, which proves the validity of the applied method. The experimental
data was compared to Hauser-Feshbach calculations using the nuclear reaction
code TALYS. A modified version of the semi-microscopic +nucleus optical
model potential OMP 3, as well as modified proton and widths, are
needed in order to obtain a good agreement between experimental data and
theory. It is found, that a model using a local modification of the
nuclear-physics input parameters simultaneously reproduces total cross sections
of the Sn(,) and Sn(,p) reactions. The
measurement of partial cross sections turns out to be very important in this
case in order to apply the correct -ray strength function in the
Hauser-Feshbach calculations. The model also reproduces cross-section values of
-induced reactions on Cd, as well as of (,n) reactions
on Sn, hinting at a more global character of the obtained
nuclear-physics input.Comment: 8 pages, 9 figure
Gas Giant Protoplanets Formed by Disk Instability in Binary Star Systems
We present a suite of three dimensional radiative gravitational hydrodynamics
models suggesting that binary stars may be quite capable of forming planetary
systems similar to our own. The new models with binary companions do not employ
any explicit artificial viscosity, and also include the third (vertical)
dimension in the hydrodynamic calculations, allowing for transient phases of
convective cooling. The calculations of the evolution of initially marginally
gravitationally stable disks show that the presence of a binary star companion
may actually help to trigger the formation of dense clumps that could become
giant planets. We also show that in models without binary companions, which
begin their evolution as gravitationally stable disks, the disks evolve to form
dense rings, which then break-up into self-gravitating clumps. These latter
models suggest that the evolution of any self-gravitating disk with sufficient
mass to form gas giant planets is likely to lead to a period of disk
instability, even in the absence of a trigger such as a binary star companion.Comment: 52 pages, 28 figure
Erosion and deposition in interplain channels of the Maury channel system
Large turbidity currents originating on the insular margin of southern lceland have flowed clown a 2 500 km-long pathway comprising rise valleys, unchanneled plains and segments of erosional and depositional deep-sea channels that are collectively called the Maury Channel system. Two steep interplain reaches of the channel have been eut up to 100 m through volcanogenic turbidites of probable La te Pleistocene age. Near-bottom observations with side-scan sonars and profllers across the upper channels (at 59°24\u27N, 18°50\u27W, 2 750 m depth) and at the lower interplain channel (around 56°23\u27N, 24°25\u27W, 3 340 m depth) defmed their structure and morphology. The upper channels, and a tributary to the lower channel, start as broad, shallow depressions that deepen and narrow downstream. The lower channel bas a pattern of anastomosing branches that probably evolved by head ward extension of low-angle tribu taries to the original sinuous channel, and its branches are at different stages of development. Several hundred bottom photographs show well-indurated rocks on channel walls and floors, with such flysch-like characteristics as cyclic graded bedding, clastic dikes, and syndepositional deformation. The lower-channel branches have been eut by turbidity currents with speeds of 5- 12 rn/sec., and combined discharges exceeding 1 x 106 m3 /sec. Bedrock erosion in and around the channels bas proceeded by intense corrasion and fluid stressing, and is marked by such small-scale effects as rock polishing, fluting, pot-holing and ledge recession. Rockfalls have caused retreat of steep channel walls, and conglomerate or pcbbly mudstone deposits suggest that debris flows have been locally active. Sorne coarse debris delivered by these processes and clay halls torn from semi-lithifled outcrops remain in the channels, but the channel f1ll is generally thin, with a patch y veneer of pelagie mud that bas accumulated since the last major turbidity current event. The surfaces of the unconsolidated s~diment have been smoothed and lineated, or moulded into seo ur moats and occasional fields of ripples, by thermohaline currents
Microstrip resonator for microwaves with controllable polarization
In this work the authors implemented a resonator based upon microstrip
cavities that permits the generation of microwaves with arbitrary polarization.
Design, simulation, and implementation of the resonators were performed using
standard printed circuit boards. The electric field distribution was mapped
using a scanning probe cavity perturbation technique. Electron spin resonance
using a standard marker was carried out in order to verify the polarization
control from linear to circular.Comment: 3 pages, 3 figures, submitted to Appl. Phys. Let
Heterogeneous Dynamics of Coarsening Systems
We show by means of experiments, theory and simulations, that the slow
dynamics of coarsening systems displays dynamic heterogeneity similar to that
observed in glass-forming systems. We measure dynamic heterogeneity via novel
multi-point functions which quantify the emergence of dynamic, as opposed to
static, correlations of fluctuations. Experiments are performed on a coarsening
foam using Time Resolved Correlation, a recently introduced light scattering
method. Theoretically we study the Ising model, and present exact results in
one dimension, and numerical results in two dimensions. For all systems the
same dynamic scaling of fluctuations with domain size is observed.Comment: Minor changes; to be published in Phys. Rev. Let
Observable Dependent Quasi-Equilibrium in Slow Dynamics
We present examples demonstrating that quasi-equilibrium
fluctuation-dissipation behavior at short time differences is not a generic
feature of systems with slow non-equilibrium dynamics. We analyze in detail the
non-equilibrium fluctuation-dissipation ratio X(t,tw) associated with a
defect-pair observable in the Glauber-Ising spin chain. It turns out that throughout the short-time regime and in particular X(tw,tw) = 3/4 for
. The analysis is extended to observables detecting defects at a
finite distance from each other, where similar violations of quasi-equilibrium
behaviour are found. We discuss our results in the context of metastable
states, which suggests that a violation of short-time quasi-equilibrium
behavior could occur in general glassy systems for appropriately chosen
observables.Comment: 17 pages, 5 figures; substantially improved version of
cond-mat/040571
Experimental constraints on the -ray strength function in Zr using partial cross sections of the Y(p,)Zr reaction
Partial cross sections of the Y(p,)Zr reaction have
been measured to investigate the -ray strength function in the
neutron-magic nucleus Zr. For five proton energies between
MeV and MeV, partial cross sections for the population of seven
discrete states in Zr have been determined by means of in-beam
-ray spectroscopy. Since these -ray transitions are dominantly
of character, the present measurement allows an access to the low-lying
dipole strength in Zr. A -ray strength function based on the
experimental data could be extracted, which is used to describe the total and
partial cross sections of this reaction by Hauser-Feshbach calculations
successfully. Significant differences with respect to previously measured
strength functions from photoabsorption data point towards deviations from the
Brink-Axel hypothesis relating the photo-excitation and de-excitation strength
functions.Comment: 5 pages, 5 figure
Growing massive black holes through super-critical accretion of stellar-mass seeds
The rapid assembly of the massive black holes that power the luminous quasars
observed at remains a puzzle. Various direct collapse models have
been proposed to head-start black hole growth from initial seeds with masses
, which can then reach a billion solar mass while
accreting at the Eddington limit. Here we propose an alternative scenario based
on radiatively inefficient super-critical accretion of stellar-mass holes
embedded in the gaseous circum-nuclear discs (CNDs) expected to exist in the
cores of high redshift galaxies. Our sub-pc resolution hydrodynamical
simulations show that stellar-mass holes orbiting within the central 100 pc of
the CND bind to very high density gas clumps that arise from the fragmentation
of the surrounding gas. Owing to the large reservoir of dense cold gas
available, a stellar-mass black hole allowed to grow at super-Eddington rates
according to the "slim disc" solution can increase its mass by 3 orders of
magnitudes within a few million years. These findings are supported by
simulations run with two different hydro codes, RAMSES based on the Adaptive
Mesh Refinement technique and GIZMO based on a new Lagrangian Godunov-type
method, and with similar, but not identical, sub-grid recipes for star
formation, supernova feedback, black hole accretion and feedback. The low
radiative efficiency of super-critical accretion flows are instrumental to the
rapid mass growth of our black holes, as they imply modest radiative heating of
the surrounding nuclear environment.Comment: 12 pages, 8 figures, 2 tables. Accepted for publication in MNRA
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