[en] The discrepancy between low-energy nucleon-deuteron breakup cross-section data and calculations, which do not include three-nucleon forces, has been cited as a possible signature of the influence of three-nucleon forces section. The comparison between data and calculations is difficult to interpret because there are significant disagreements between the data. To help clarify the situation we have made kinematically complete cross-section measurements for n-d breakup at an incident neutron energy of 13.0 MeV. The experimental techniques and data analysis method will be described. Preliminary results will be presented in comparison to calculations and previous data

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[en] Since an accurate value for the neutron-neutron (nn) scattering length a_nn is of fundamental interest, its determination should not rely on one source of experimental information only. Besides the π d capture reaction, the nd breakup reaction has been the classical reaction used for determining a_nn. However, none of the published values for a_nn obtained from kinematically complete nd → n+n+p breakup data are based on a rigorous treatment of the three-nucleon continuum. In addition, the scale uncertainty associated with the existing nd breakup cross-section data in the region of the nn final-state interaction peak is too large to allow for a meaningful reanalysis. Therefore, a new kinematically complete nd breakup experiment is underway at TUNL at an incident neutron energy of 13 MeV. State-of-the-art three-nucleon continuum calculations will be used to analyze the data. In order to investigate the possible influence of three-nucleon force effects, a_nn will be determined from data taken at four production angles of the nn pair between 20.5 degrees and 43 degrees (lab)

[en] The excitation energies, spins, parities, total widths and neutron elastic scattering widths of seven T=1/2 levels of ¹³C at excitation energies between 11.5 and 12.5 MeV have been determined via a phase-shift analysis of n + ¹²C total cross section, elastic differential cross section and analysing power data in the energy range from 7.03 to 8.56 MeV. The proposed level structure for ¹³C is consistent with the known structure of the mirror nucleus ¹³N. Above 12 MeV excitation energy, the results of the phase-shift analysis disagree significantly with the R-matrix analysis of n-¹²C cross section data recently reported by other authors. (author)

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[en] Complementary databases were constructed from published and some new measurements of σ(θ), Ay(θ) and σ_T for n+²⁰⁸Pb and n+²⁰⁹Bi covering the energy range 1.0 to 80 MeV. A dispersive optical-model analysis was performed for both scattering systems while constraining many of the parameters to be identical. A good representation of the data is obtained with conventional geometry and spin-orbit parameters. The ²⁰⁸Pb model predicts quite well measured energies of single-particle and single-hole bound states. Occupation probabilities are calculated. A fully constrained model is presented in which the only differences between the two systems are the Fermi energy and the usual isospin term. This latter model represents all the data well

[en] Three nucleon force (3NF) effects in the three-nucleon (3N) continuum are of special concern in the determination of the nn effective range parameters from nd breakup data. However, since the np effective range parameters are accurately known from two-body np scattering, the nd → n+n+p reaction in conjunction with rigorous 3N continuum calculations is a powerful tool for determining the importance of 3NF effects in this reaction. Therefore, kinematically complete nd data for the absolute cross section for the np final-state-interactions (FSI) are being taken at TUNL at four production angles of the np pair at an incident neutron energy of 13 MeV. The data analysis will be based on rigorous 3N calculations using realistic NN potentials. Any sizable deviation from the two-body result and especially an angular dependence in the values obtained for a_np are clear signatures of 3NF effects. The data are being accumulated simultaneously with the kinematically complete measurement of the nn FSI cross section

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[en] In order to study multiple scattering effects both in the gas and particularly in the solid materials of high-pressure gas scintillators, two asymmetry experiments have been performed by scattering of 15.6 MeV polarized neutrons from helium contained in stainless steel vessels of different wall thicknesses. A monte Carlo computer code taking into account the polarization dependence of the differential scattering cross sections has been written to simulate the experiments and to calculate corrections for multiple scattering on helium, xenon and the gas containment materials. Besides the asymmetries for the various scattering processes involved, the code yields time-of-flight spectra of the scattered neutrons and pulse height spectra of the helium recoil nuclei in the gas scintillator. The agreement between experimental results and Monte Carlo calculations is satisfactory. (Auth.)