The Cross Section of 3He(3He,2p)4He measured at Solar Energies

We report on the results of the \hethet\ experiment at the underground accelerator facility LUNA (Gran Sasso). For the first time the lowest projectile energies utilized for the cross section measurement correspond to energies below the center of the solar Gamow peak ($E_{\rm 0}$=22 keV). The data provide no evidence for the existence of a hypothetical resonance in the energy range investigated. Although no extrapolation is needed anymore (except for energies at the low-energy tail of the Gamow peak), the data must be corrected for the effects of electron screening, clearly observed the first time for the \hethet\ reaction. The effects are however larger than expected and not understood, leading presently to the largest uncertainty on the quoted $S_{\rm b}(E_{\rm 0})$ value for bare nuclides (=5.40 MeV b).Comment: 18 pages, 10 postscript figures, Calculations concerning hypothetical resonanz added, Submitted to Phys. Rev. C., available at this URL: HTTP://www.lngs.infn.it/lngs/htexts/luna/luna.htm

The Single-Particle Structure of Neutron-Rich Nuclei of Astrophysical Interest at the Ornl Hribf

The rapid nuetron-capture process (r process) produces roughly half of the elements heavier than iron. The path and abundances produced are uncertain, however, because of the lack of nuclear strucure information on important neutron-rich nuclei. We are studying nuclei on or near the r-process path via single-nucleon transfer reactions on neutron-rich radioactive beams at ORNL's Holifield Radioactive Ion Beam Facility (HRIBF). Owing to the difficulties in studying these reactions in inverse kinematics, a variety of experimental approaches are being developed. We present the experimental methods and initial results.Comment: Proceedings of the Third International Conference on Fission and Properties of Neutron-Rich Nucle

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

The 21Na(p,gamma)22Mg Reaction and Oxygen-Neon Novae

The 21Na(p,gamma)22Mg reaction is expected to play an important role in the nucleosynthesis of 22Na in Oxygen-Neon novae. The decay of 22Na leads to the emission of a characteristic 1.275 MeV gamma-ray line. This report provides the first direct measurement of the rate of this reaction using a radioactive 21Na beam, and discusses its astrophysical implications. The energy of the important state was measured to be E$_{c.m.}$= 205.7 $\pm$ 0.5 keV with a resonance strength $\omega\gamma = 1.03\pm0.16_{stat}\pm0.14_{sys}$ meV.Comment: Accepted for publication in Physical Review Letter

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.