The instrumentation in Hall A at the Thomas Jefferson National Accelerator Facility was designed to study electro-and photo-induced reactions at very high luminosity and good momentum and angular resolution for at least one of the reaction products. The central components of Hall A are two identical high resolution spectrometers, which allow the vertical drift chambers in the focal plane to provide a momentum resolution of better than 2 x 10(-4). A variety of Cherenkov counters, scintillators and lead-glass calorimeters provide excellent particle identification. The facility has been operated successfully at a luminosity well in excess of 10(38) CM-2 s(-1). The research program is aimed at a variety of subjects, including nucleon structure functions, nucleon form factors and properties of the nuclear medium. (C) 2003 Elsevier B.V. All rights reserved

Neutron elastic-scattering angular distributions were measured at beam energies of 11.9 and 16.9 MeV on 40,48 Ca targets. These data plus other elastic-scattering measurements, total and reaction cross sections measurements, (e, e ′ p) data, and single-particle energies for magic and doubly magic nuclei have been analyzed in the dispersive optical model (DOM) generating nucleon self-energies (optical-model potentials) which can be related, via the many-body Dyson equation, to spectroscopioc factors and occupation probabilities. It is found that for stable nuclei with N ≥ Z, the imaginary surface potential for protons exhibits a strong dependence on the neutron-proton asymmetry. This result leads to a more modest dependence of the spectroscopic factors on asymmetry. The measured data and the DOM analysis of all considered nuclei clearly demonstrates that the neutron imaginary surface potential displays very little dependence on the neutron-proton asymmetry for nuclei near stability (N ≥ Z).

We have measured the cross section for quasielastic 1p-shell proton knockout in the 16O(e,e(')p) reaction at omega = 0.439 GeV and Q2 = 0.8 (GeV/c)(2) for missing momentum P(miss)

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The photon analyzing power for the photodisintegration of the deuteron was measured for seven gamma-ray energies between 2.39 and 4.05 MeV using the linearly polarized gamma-ray beam of the High-Intensity Gamma-ray Source at the Duke Free-Electron Laser Laboratory. The data provide a stringent test of theoretical calculations for the inverse reaction, the neutron-proton radiative capture reaction at energies important for Big-Bang Nucleosynthesis. Our data are in excellent agreement with potential model and effective field theory calculations. Therefore, the uncertainty in the baryon density ΩBh 2 obtained from Big-Bang Nucleosynthesis can be reduced at least by 20%. 24.70.+s, 27.10.+h, 21.45.+v Big-Bang Nucleosynthesis (BBN) is an observational cornerstone of the hot Big-Bang (BB) cosmology. According to [1] the neutron(n)-proton(p) capture reaction p(n, γ)d with a deuteron (d) and a 2.225 MeV γ ray in the exit channel is of special interest, because the BB abundance of deuterium provides direct information on the baryon density in the early universe at times between about 0.01 and 200 seconds after the BB. Knowing accurately the n-p capture cross section in the energy range from 25 to 200 keV in the center-of-mass (c.m.) system and using the experimental value for the primeval deuterium number density (D/H) p [2, 3], would allow for an accurate determination of the baryon density Ω B h 2 (h is the Hubble constant in units of 100 km/s/Mpc). From Ω B h 2 one can predict the abundances of the three light elements 3 He, 4 He, and 7 Li. According to [1], the 10% uncertainty in the deuterium-inferred baryon density Ω B h 2 = 0.019 ± 0.002 comes in almost equal parts from the (D/H) measurements and theoretical uncertainties in predicting the deuterium abundance. For the latter, the knowledge of the n-p capture cross section is of crucial importance. Unfortunately, there is a near-complete lack of data at energies relevant to BBN. Aside from thermal energies, data exist only at n-p c.m. energies of 275 keV and above. As a consequence, the ENDF-B/VI [4] evaluation has been used [1] in the BBN energy range. This evaluation is normalized to the high-precision thermal n-p capture cross-section measurements. The 5% uncertainty that is assigned in this approach contributes a significant fraction to the uncertainty in the baryon density and consequently in the abundances of the light elements produced in BBN.Very recently, with the precision results from WMAP (Wilkinson Microwave Anisotropy Probe) for the Cosmic Microwave Background (CMB) and its anisotropies an independent and even more accurate result became available: Ω B h 2 = 0.0224 ± 0.0009 [5,6]. The comparison of the baryon density predictions from BBN and the CMB is a fundamental test of BB cosmology [7]. Any deviation points to either unknown systematics or the need for new physics. Therefore, it is of crucial importance to reduce the uncertainty in Ω B h 2 obtained from BBN. As stated above, 50% of the uncertainty is due to the uncertainty in the n-p capture cr...

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