47 research outputs found
Multi-electron SEFs for nuclear reactions involved in advanced stages of stellar evolution
Multi-electron screening effects encountered in laboratory astrophysical
reactions are investigated by considering the reactants Thomas-Fermi atoms. By
means of that model, previous studies are extended to derive the corresponding
screening enhancement factor (SEF), so that it takes into account ionization,
thermal, exchange and relativistic effects. The present study, by imposing a
very satisfactory constraint on the possible values of the screening energies
and the respective SEFs, corrects the current (and the future) experimental
values of the astrophysical factors associated with nuclear reactions involved
in advanced stages of stellar evolution.Comment: 13 RevTex pages+6 ps figures; Accepted for publication in Nuclear
Physics
^7Be(p,γ)^8B cross section and the properties of ^7Be
We study the nonresonant part of the ^7Be(p,γ)^8B reaction using a three-cluster resonating group model that is variationally converged and virtually complete in ^4He+^3He+p model space. The importance of using adequate nucleon-nucleon interaction is demonstrated. We find that the low-energy astrophysical S factor is linearly correlated with the quadrupole moment of ^7Be. A range of parameters is found where the most important ^8B, ^7Be, and ^7Li properties are reproduced simultaneously; the corresponding S factor at E_(c.m.)=20 keV is 24.6–26.1 eV b
Energy Loss, Electron Screening, and the Astrophysical 3He(d,p)4He cross section
We reanalyze the low-energy 3He(d,p)4He cross section measurements of
Engstler et al. using recently measured energy loss data for proton and
deuteron beams in a helium gas. Although the new 3He(d,p)4He S-factors are
significantly lower than those reported by Engstler et al. they clearly show
the presence of electron screening effects. From the new S-factors we find an
electron screening energy in agreement with the adiabatic limit.Comment: 8 Page RevTeX document, two postscript figures, now in a
self-extracting uufile type archiv
Electron screening in molecular fusion reactions
Recent laboratory experiments have measured fusion cross sections at
center-of-mass energies low enough for the effects of atomic and molecular
electrons to be important. To extract the cross section for bare nuclei from
these data (as required for astrophysical applications), it is necessary to
understand these screening effects. We study electron screening effects in the
low-energy collisions of Z=1 nuclei with hydrogen molecules. Our model is based
on a dynamical evolution of the electron wavefunctions within the TDHF scheme,
while the motion of the nuclei is treated classically. We find that at the
currently accessible energies the screening effects depend strongly on the
molecular orientation. The screening is found to be larger for molecular
targets than for atomic targets, due to the reflection symmetry in the latter.
The results agree fairly well with data measured for deuteron collisions on
molecular deuterium and tritium targets.Comment: 15 Page RevTeX document, twelve postscript figures, now in a uufile
packag
Influence of tunneling on electron screening in low energy nuclear reactions in laboratories
Using a semiclassical mean field theory, we show that the screening potential
exhibits a characteristic radial variation in the tunneling region in sharp
contrast to the assumption of the constant shift in all previous works. Also,
we show that the explicit treatment of the tunneling region gives a larger
screening energy than that in the conventional approach, which studies the time
evolution only in the classical region and estimates the screening energy from
the screening potential at the external classical turning point. This
modification becomes important if the electronic state is not a single
adiabatic state at the external turning point either by pre-tunneling
transitions of the electronic state or by the symmetry of the system even if
there is no essential change with the electronic state in the tunneling region.Comment: 3 figure
Influence of the Electronic Chaotic Motion on the Fusion Dynamics at Astrophysical Energies
We perform semi-classical molecular dynamics simulations of screening by
bound electrons in low energy nuclear reactions. In our simulations quantum
effects corresponding to the Pauli and Heisenberg principle are enforced by
constraints. In addition to the well known adiabatic and sudden limits, we
propose a new "dissipative limit" which is expected to be important not only at
high energies but in the extremely low energy region. The dissipative limit is
associated with the chaotic behavior of the electronic motion. It affects also
the magnitude of the enhancement factor. We discuss also numerical experiments
using polarized targets. The derived enhancement factors in our simulation are
in agreement with those extracted within the -matrix approach.Comment: 17 pages, 9 figure
Determining the 12C(α,γ) 16O cross section from Coulomb dissociation
We estimate the E1 and E2 contributions to the Coulomb dissociation reaction 16O+Pb→α+12C+Pb using semiclassical Coulomb excitation theory. For projectile energies below 300 MeV/nucleon and scattering angles greater than 1°, we find that the process is dominated by the E2 component. This is in contrast to the astrophysically interesting 12C(α,γ)16O cross section, which is dominated by the E1 multipole at the most effective energy of 300 keV. The E2 sensitivity of Coulomb dissociation would usefully complement forthcoming 16N β-decay data, which will constrain only the E1 component
Atomic effects in astrophysical nuclear reactions
Two models are presented for the description of the electron screening
effects that appear in laboratory nuclear reactions at astrophysical energies.
The two-electron screening energy of the first model agrees very well with the
recent LUNA experimental result for the break-up reaction , which so far defies all available theoretical models.
Moreover, multi-electron effects that enhance laboratory reactions of the CNO
cycle and other advanced nuclear burning stages, are also studied by means of
the Thomas-Fermi model, deriving analytical formulae that establish a lower and
upper limit for the associated screening energy. The results of the second
model, which show a very satisfactory compatibility with the adiabatic
approximation ones, are expected to be particularly useful in future
experiments for a more accurate determination of the CNO astrophysical factors.Comment: 14 RevTex pages + 2 ps (revised) figures. Phys.Rev.C (in production