3,564 research outputs found
Complete Fusion Enhancement and Suppression of Weakly Bound Nuclei at Near Barrier Energies
We consider the influence of breakup channels on the complete fusion of
weakly bound systems in terms of dynamic polarization potentials. It is argued
that the enhancement of the cross section at sub-barrier energies may be
consistent with recent experimental observations that nucleon transfer, often
leading to breakup, is dominant compared to direct breakup. The main trends of
the experimental complete fusion cross section for Li + Bi are
analyzed in the framework of the DPP approach.Comment: 12 pages, 2 figure
Large Scale Flows from Orion-South
Multiple optical outflows are known to exist in the vicinity of the active
star formation region called Orion-South (Orion-S). We have mapped the velocity
of low ionization features in the brightest part of the Orion Nebula, including
Orion-S, and imaged the entire nebula with the Hubble Space Telescope. These
new data, combined with recent high resolution radio maps of outflows from the
Orion-S region, allow us to trace the origin of the optical outflows. It is
confirmed that HH 625 arises from the blueshifted lobe of the CO outflow from
136-359 in Orion-S while it is likely that HH 507 arises from the blueshifted
lobe of the SiO outflow from the nearby source 135-356. It is likely that
redshifted lobes are deflected within the photon dominated region behind the
optical nebula. This leads to a possible identification of a new large shock to
the southwest from Orion-S as being driven by the redshifted CO outflow arising
from 137-408. The distant object HH 400 is seen to have two even further
components and these all are probably linked to either HH 203, HH 204, or HH
528. Distant shocks on the west side of the nebula may be related to HH 269.
The sources of multiple bright blueshifted Herbig-Haro objects (HH 202, HH 203,
HH 204, HH 269, HH 528) remain unidentified, in spite of earlier claimed
identifications. Some of this lack of identification may arise from the fact
that deflection in radial velocity can also produce a change in direction in
the plane of the sky. The best way to resolve this open question is through
improved tangential velocities of low ionization features arising where the
outflows first break out into the ionized nebula.Comment: Astronomical Journal, in press. Some figures are shown at reduced
resolution. A full-resolution version is available at
http://ifront.org/wiki/Orion_South_Outflows_Pape
The Coulomb phase shift revisited
We investigate the Coulomb phase shift, and derive and analyze new and more
precise analytical formulae. We consider next to leading order terms to the
Stirling approximation, and show that they are important at small values of the
angular momentum and other regimes. We employ the uniform approximation.
The use of our expressions in low energy scattering of charged particles is
discussed and some comparisons are made with other approximation methods.Comment: 13 pages, 5 figures, 1 tabl
Quark Delocalization, Color Screening, and Nuclear Intermediate Range Attraction
We consider the effect of including quark delocalization and color screening,
in the nonrelativistic quark cluster model, on baryon-baryon potentials and
phase shifts. We find that the inclusion of these additional effects allows a
good qualitative description of both.Comment: 10 pages, LaTeX, 4 figures in PostScript after text, LA-UR-91-215
Effective Widths and Effective Number of Phonons of Multiphonon Giant Resonances
We discuss the origin of the difference between the harmonic value of the
width of the multiphonon giant resonances and the smaller observed value.
Analytical expressions are derived for both the effective width and the average
cross-section. The contribution of the Brink-Axel mechanism in resolving the
discrepancy is pointed out.Comment: 9 pages, 4 figure
Coulomb Breakup Mechanism of Neutron-Halo Nuclei in a Time-Dependent Method
The mechanism of the Coulomb breakup reactions of the nuclei with
neutron-halo structure is investigated in detail. A time-dependent
Schr\"odinger equation for the halo neutron is numerically solved by treating
the Coulomb field of a target as an external field. The momentum distribution
and the post-acceleration effect of the final fragments are discussed in a
fully quantum mechanical way to clarify the limitation of the intuitive picture
based on the classical mechanics. The theory is applied to the Coulomb breakup
reaction of Be + Pb. The breakup mechanism is found to be
different between the channels of and
, reflecting the underlying structure of Be. The
calculated result reproduces the energy spectrum of the breakup fragments
reasonably well, but explains only about a half of the observed longitudinal
momentum difference.Comment: 15 pages,revtex, 9 figures (available upon request
Isospin structure of one- and two-phonon GDR excitations
Isospin is included in the description of Coulomb excitation of multiple
giant isovector dipole resonances. In the excitation of even-even nuclei, a
relevant portion of the excitation strength is shown to be associated with 1+
two-phonon states, which tends to be hindered or completely supressed in
calculations in which the isospin degree of freedom is not considered. We find
that the excitation cross sections is strongly dependent on the ground state
isospin.Comment: 8 pages, 2 figure
Fine Scale Temperature Fluctuations in the the Orion Nebula and the t^2 Problem
We present a high spatial resolution map of the columnar electron temperature
(Tc) of a region to the south west of the Trapezium in the Orion Nebula. This
map was derived from Hubble Space Telescope images that isolated the primary
lines of HI for determination of the local extinction and of the OIII lines for
determination of Tc. Although there is no statistically significant variation
of Tc with distance from the dominant ionizing star theta1-Ori-C, we find small
scale variations in the plane of the sky down to a few arcseconds that are
compatible with the variations inferred from comparing the value of Te derived
from forbidden and recombination lines, commonly known as the t^2 problem. We
present other evidence for fine scale variations in conditions in the nebula,
these being variations in the surface brightness of the the nebula,
fluctuations in radial velocities, and ionization changes. From our Tc map and
other considerations we estimate that t^2=0.028 +-0.006 for the Orion nebula.
Shadowed regions behind clumps close to the ionization front can make a
significant contribution to the observed temperature fluctuations, but they
cannot account for the t^2 values inferred from several methods of temperature
determination. It is shown that an anomalous broadening of nebular emission
lines appears to have the same sense of correlation as the temperature
anomalies, although a causal link is not obvious.Comment: 53 pages, 13 images, many of the images have been downgraded to be
able to fit within the astro-ph file size limit
Theory of Multiphonon Excitation in Heavy-Ion Collisions
We study the effects of channel coupling in the excitation dynamics of giant
resonances in relativistic heavy ions collisions. For this purpose, we use a
semiclassical approximation to the Coupled-Channels problem and separate the
Coulomb and the nuclear parts of the coupling into their main multipole
components. In order to assess the importance of multi-step processes, we
neglect the resonance widths and solve the set of coupled equations exactly.
Finite widths are then considered. In this case, we handle the coupling of the
ground state with the dominant Giant Dipole Resonance exactly and study the
excitation of the remaining resonances within the Coupled-Channels Born
Approximation. A comparison with recent experimental data is made.Comment: 29 pages, 7 Postscript figures available upon reques
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