20 research outputs found

### ^7Be(p,Î³)^8B cross section and the properties of ^7Be

We study the nonresonant part of the ^7Be(p,Î³)^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 24.6â€“26.1 eV b

### Determining the 12C(Î±,Î³) 16O cross section from Coulomb dissociation

We estimate the E1 and E2 contributions to the Coulomb dissociation reaction 16O+Pbâ†’Î±+12C+Pb using semiclassical Coulomb excitation theory. For projectile energies below 300 MeV/nucleon and scattering angles greater than 1Â°, we find that the process is dominated by the E2 component. This is in contrast to the astrophysically interesting 12C(Î±,Î³)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 Î²-decay data, which will constrain only the E1 component

### Effect of the source charge on charged-boson interferometry

We investigate quantal perturbations of the interferometric correlations of charged bosons by the Coulomb field of an instantaneous, charged source. The source charge increases the apparent source size by weakening the correlation at nonzero relative momenta. The effect is strongest for pairs with a small total momentum and is stronger for kaons than for pions of the same momenta. The low-energy data currently available are well described by this effect. A simple expression is proposed to account for the effect

### 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 $%
He3(He3,2p)He^{4}$, 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

### One- and two-electron atomic screening in fusion reactions

Recent laboratory experiments have measured fusion cross sections at center-of-mass energies low enough that the effects of atomic electrons are 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 present a model in which the evolution of the electron wave function is treated dynamically in the time-dependent Hartree-Fock scheme, while the motion of the nuclei is treated classically. We have calculated screening in the d+2H and d+3He reactions and give the effective screening energy Ue at small internuclear separations as a function of E. The resulting Ue values do not exceed the previously established adiabatic limits, and thus cannot explain the higher screening energies derived from experiment

### Astrophysical factors:Zero energy vs. Most effective energy

Effective astrophysical factors for non-resonant astrophysical nuclear
reaction are invariably calculated with respect to a zero energy limit. In the
present work that limit is shown to be very disadvantageous compared to the
more natural effective energy limit. The latter is used in order to modify the
thermonuclear reaction rate formula so that it takes into account both plasma
and laboratory screening effects.Comment: 7 RevTex pages. Accepted for publication in Phys.Rev.

### Effect of the source charge on charged-beam interferometry

We investigate quantal perturbations of the interferometric correlations of
charged bosons by the Coulomb field of an instantaneous, charged source. The
source charge increases the apparent source size by weakening the correlation
at non-zero relative momenta. The effect is strongest for pairs with a small
total momentum and is stronger for kaons than for pions of the same momenta.
The experimental data currently available are well described by this effect
without invoking Pratt's exploding source model. A simple expression is
proposed to account for the effect.Comment: 9 pages TEX, 3 Postscript figures available at
http://www.krl.caltech.edu/preprints/MAP.htm

### Extrapolation of the astrophysical S factor for ^7Be(p,gamma)^8B to solar energies

We investigate the energy dependence of the astrophysical $S$ factor for the
reaction $^7$Be$(p,\gamma)^8$B, the primary source of high-energy solar
neutrinos in the solar $pp$ chain. Using simple models we explore the model
dependence in the extrapolation of the experimental data to the region of
astrophysical interest near 20 keV. We find that below approximately 400 keV
the energy dependence is very well understood and constrained by the data for
the elastic scattering of low energy neutrons from $^7$Li. Above 400 keV
nuclear distortion of the wave function of the incident proton introduces a
significant model dependence. This is particularly important for the s-wave
contribution to the $S$ factor. The extracted value of S(0) is $19.0 \pm 1.0
\pm 0.2$~eVb. The first error is experimental while the second is an estimate
of the theoretical error in the extrapolation.Comment: 30 pages, 12 figures, 6 table