387 research outputs found
Influence of the Dirac sea on proton electromagnetic knockout
We use the relativistic distorted-wave impulse approximation (RDWIA) to study
the effects of negative-energy components of Dirac wave functions on the
left-right asymmetry for (e,e'p) reactions on 16-O with 0.2 < Q^2 < 0.8 and
12-C with 0.6 < Q^2 < 1.8 (GeV/c)^2. Spinor distortion is more important for
the bound state than for the ejectile and the net effect decreases with Q^2.
Spinor distortion breaks Godon equivalence and the data favor the CC2 operator
with intermediate coupling to the sea. The left-right asymmetry for Q^2 < 1.2
(GeV/c)^2 is described well by RDWIA calcuations, but at Q^2 = 1.8 (GeV/c)^2
the observed variation with missing momentum is flatter than predicted.Comment: 12 pages, 9 figures, to be submitted to PR
Isospin dependence of 6He+p optical potential and the symmetry energy
A consistent folding analysis of the elastic p(6He,6He)p scattering and
charge exchange p(6He,6Li*)n reaction data measured at Elab=41.6A MeV has been
performed within the coupled channels formalism. We have used the isovector
coupling to link the isospin dependence of 6He+p optical potential to the cross
section of p(6He,6Li*)n reaction exciting the 0+ isobaric analog state (IAS) at
3.563 MeV in 6Li. Based on these results and the Hartree-Fock calculation of
asymmetric nuclear matter using the same isospin-dependent effective
nucleon-nucleon interaction, we were able to confirm that the most realistic
value of the symmetry energy Esym is around 31 MeV. Our analysis has also shown
that the measured charge exchange p(6He,6Li*)n data are quite sensitive to the
halo tail of the 6He density used in the folding calculation and the IAS of 6Li
is likely to have a halo structure similar to that established for the ground
state of 6He.Comment: Accepted for publication in Phys. Rev.
Higher order effects in the and transfer reactions
Full Coupled Channels Calculations were performed for the
and transfer reactions at several deuteron incident
energies from MeV up to 3.27 MeV. A strong polarization effect
between the entrance channel and the transfer channels
and was
observed. This polarization effect had to be taken into account in order to
obtain realistic spectroscopic factors from these reactions.Comment: 15 papes, 13 figures, accepted for publication in Phys. Rev.
Approximations in Fusion and Breakup reactions induced by Radioactive Beams
Some commonly used approximations for complete fusion and breakup
transmission coefficients in collisions of weakly bound projectiles at near
barrier energies are assessed. We show that they strongly depend on the adopted
classical trajectory and can be significantly improved with proper treatment of
the incident and emergent currents in the WKB approximation.Comment: 15 pages, 7 figure
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
Nucleon-nucleon cross sections in neutron-rich matter and isospin transport in heavy-ion reactions at intermediate energies
Nucleon-nucleon (NN) cross sections are evaluated in neutron-rich matter
using a scaling model according to nucleon effective masses. It is found that
the in-medium NN cross sections are not only reduced but also have a different
isospin dependence compared with the free-space ones. Because of the
neutron-proton effective mass splitting the difference between nn and pp
scattering cross sections increases with the increasing isospin asymmetry of
the medium. Within the transport model IBUU04, the in-medium NN cross sections
are found to influence significantly the isospin transport in heavy-ion
reactions. With the in-medium NN cross sections, a symmetry energy of
was found most acceptable
compared with both the MSU isospin diffusion data and the presently acceptable
neutron-skin thickness in Pb. The isospin dependent part of isobaric nuclear incompressibility was further narrowed down to
MeV. The possibility of determining simultaneously the in-medium
NN cross sections and the symmetry energy was also studied. The proton
transverse flow, or even better the combined transverse flow of neutrons and
protons, can be used as a probe of the in-medium NN cross sections without much
hindrance from the uncertainties of the symmetry energy.Comment: 32 pages including 14 figures. Submitted to Phys. Rev.
A new insight into the observation of spectroscopic strength reduction in atomic nuclei: implication for the physical meaning of spectroscopic factors
Experimental studies of one nucleon knockout from magic nuclei suggest that
their nucleon orbits are not fully occupied. This conflicts a commonly accepted
view of the shell closure associated with such nuclei. The conflict can be
reconciled if the overlap between initial and final nuclear states in a
knockout reaction are calculated by a non-standard method. The method employs
an inhomogeneous equation based on correlation-dependent effective
nucleon-nucleon (NN) interactions and allows the simplest wave functions, in
which all nucleons occupy only the lowest nuclear orbits, to be used. The
method also reproduces the recently established relation between reduction of
spectroscopic strength, observed in knockout reactions on other nuclei, and
nucleon binding energies. The implication of the inhomogeneous equation method
for the physical meaning of spectroscopic factors is discussed.Comment: 4 pages, accepted by Phys. Rev. Let
Particle decay branching ratios for states of astrophysical importance in 19Ne
We have measured proton and alpha-particle branching ratios of excited states
in 19Ne formed using the 19F(3He,t) reaction at a beam energy of 25 MeV. These
ratios have a large impact on the astrophysical reaction rates of
15O(alpha,gamma), 18F(p,gamma) and 18F(p,alpha), which are of interest in
understanding energy generation in x-ray bursts and in interpreting anticipated
gamma-ray observations of novae. We detect decay protons and alpha-particles
using a silicon detector array in coincidence with tritons measured in the
focal plane detector of our Enge split-pole spectrograph. The silicon array
consists of five strip detectors of the type used in the Louvain-Edinburgh
Detector Array, subtending angles from 130 degrees to 165 degrees with
approximately 14% lab efficiency. The correlation angular distributions give
additional confidence in some prior spin-parity assignments that were based on
gamma branchings. We measure Gamma_p/Gamma=0.387+-0.016 for the 665 keV proton
resonance, which agrees well with the direct measurement of Bardayan et al.Comment: 5 pages, 2 figures, 3 tables. Prepared using RevTex 4 and BibTex.
Further minor revisions, incl. fig. 1 font size increase, 1 table removal,
and minor changes to the tex
Fast nucleon emission as a probe of the isospin momentum dependence
In this article we investigate the structure of the non-local part of the
symmetry term, that leads to a splitting of the effective masses of protons and
neutrons in asymmetric matter. Based on microscopic transport simulations we
suggest some rather sensitive observables in collisions of neutron-rich
(unstable) ions at intermediate () energies. In particular we focus the
attention on pre-equilibrium nucleon emissions. We discuss interesting
correlations between the N/Z content of the fast emitted particles and their
rapidity or transverse momentum, that show a nice dependence on the
prescription used for the effective mass splitting.Comment: 5 pages, 6 figures, revtex
Optical-Model Description of Time-Reversal Violation
A time-reversal-violating spin-correlation coefficient in the total cross
section for polarized neutrons incident on a tensor rank-2 polarized target is
calculated by assuming a time-reversal-noninvariant, parity-conserving
``five-fold" interaction in the neutron-nucleus optical potential. Results are
presented for the system for neutron incident energies
covering the range 1--20 MeV. From existing experimental bounds, a strength of
keV is deduced for the real and imaginary parts of the five-fold
term, which implies an upper bound of order on the relative -odd
strength when compared to the central real optical potential.Comment: 11 pages (Revtex
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