arXiv subjects

T. D. Shoppa

Publications and source records attributed to T. D. Shoppa.

8 recordsLinked to original sources

arXiv1998

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.

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arXiv1998

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$(p,γ)^8$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.

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arXiv1998

Direct capture astrophysical S factors at low energy

We investigate the energy dependence of the astrophysical S factors for the reactions 7Be(p,gamma)8B, the primary source of high-energy solar neutrinos in the solar pp chain, and 16O(p,gamma)17F, an important reaction in the CNO cycle. Both of these reactions have predicted S factors which rise at low energies; we find the source of this behavior to be a pole in the S factor at a center-of-mass energy E = -E_B, the point where the energy of the emitted photon vanishes. The pole arises from a divergence of the radial integrals.

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arXiv1996

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.

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arXiv1996

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.

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arXiv1995

Nuclear correlation and finite interaction-range effects in high-energy $(e,e'p)$ nuclear transparency

Nuclear transparency is calculated for high-energy, semi-inclusive $(e,e'p)$ reactions, by accounting for all orders of Glauber multiple-scattering and by using realistic finite-range $p N$ interaction and (dynamically and statistically) correlated nuclear wave functions. The nuclear correlation effect is reduced due to the $p N$ finite-range effect. The net effect is small, and depends sensitively on details of the nuclear correlations in finite nuclei, which are poorly known at present.

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arXiv1994

The Be7(p,gamma)B8 cross section and the properties of Be7

We study the nonresonant part of the $^7$Be($p,γ$)$^8$B reaction using a three-cluster resonating group model that is variationally converged and virtually complete in the $^4$He+$^3$He+$p$ 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 $^7$Be. A range of parameters is found where the most important $^7$Be and $^7$Li properties are reproduced simultaneously; the corresponding $S$-factor at $E_{\rm cm}=20$ keV is $24.6-26.1$ eV$\cdot$b.

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arXiv1994

The E2 contribution to the 8B -> p + 7Be Coulomb dissociation cross section

We have calculated the E1 and E2 contributions to the low-energy B-8 + Pb-208 -> p + Be-7 + Pb-208 Coulomb dissociation cross sections using the kinematics of a recent experiment at RIKEN. Using a potential model description of the Be-7 (p,gamma) B-8 reaction, we find that the E2 contributions cannot a priori be ignored in the analysis of the data. Its inclusion reduces the extracted Be-7 (p,gamma) B-8 S-factor at solar energies by about 25%.

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