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  • 1
    ISSN: 1434-601X
    Keywords: 25.70.Ef ; 21.60.Gx ; 27.30.+t
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The12C+12C→8Be gs +116O gs reaction has been studied at c.m. energies of 27.9, 32.5 and 35.0 MeV. The energy dependence of the cross section is consistent with a resonant behaviour like the one observed for two12C nuclei excited in a very deformed state in the final channel. The angular distributions for the two reactions show also a similar oscillatory behaviour. The analysis suggests that the reaction proceeds through the formation of an intermediate state with a complex nature, involving largely deformed configurations as well as almost spherical ones.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. For improved measurements of the key astrophysical reaction 12C(α,γ)16O in inverted kinematics, a recoil separator ERNA is being developed at the 4 MV Dynamitron tandem accelerator in Bochum to detect directly the 16O recoils with about 50% efficiency. Calculations of the ion beam optics including all filtering and focusing elements of ERNA are presented. Since the 12C projectiles and the 16O recoils have essentially the same momentum, and since the 12C ion beam emerging from the accelerator passes through a momentum filter (analysing magnet), the 12C ion beam must be as free as possible from 16O contamination for ERNA to succeed. In the present work, the 16O contamination was reduced from a level of 1 × 10−11 to a level below 2 × 10−29 by the installation of Wien filters both before and after the analysing magnet. The measurement of these and other beam specifications involved other parts of the final ERNA layout – sequentially a Wien filter, a 60˚ dipole magnet, another Wien filter, and a ΔE-E telescope. The setup led to a measured suppression factor of 5 × 10−18 for the 12C ion beam. The experiments also indicate that an almost free choice of the charge state for the 16O recoils is possible in the separator.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1434-601X
    Keywords: PACS: 24.50.+g direct reactions – 24.10 nuclear reactions models- and methods – 26.20.+f hydrostatic stellar nucleosynthesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: The Trojan-horse method has been suggested as a means to study a two-body reaction at astrophysical energies via a three-body breakup reaction. In order to test this method the 6Li(12C,α12C)2H reaction was studied in a kinematically complete experiment at an incident energy of 18 MeV. Coincidence spectra show the presence of the quasi-free α-12C scattering process. The excitation function of the three-body reaction is calculated in the plane wave impulse approximation assuming quasi-free scattering and is compared with the experimental data. Also, the excitation function of the virtual α-12C elastic scattering is extracted from the three-body reaction cross section at low deuteron momenta and compared with the behaviour of the free scattering cross section.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1434-601X
    Keywords: PACS. 24.10.-i Nuclear-reaction models and methods – 25.40.-h Nucleon-induced reactions – 26.35.+c Big bang nucleosynthesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: The available astrophysical S(E) factor data for the reaction 7Li (p,α)α at 10 〈 E 〈 1000keV exhibit an exponential increase at low energies due to the effects of electron screening. A parametrisation of the data using a non-resonant, direct process and two subthreshold resonances reproduces the data at energies E≥ 100keV, while at lower energies this calculated S b(E) factor curve for bare nuclides drops below the data, which in turn represent the case of electron-shielded nuclides, i.e. the electron-shielded S s(E) factor. The comparison between S b(E) and S s(E) leads to an electron-screening potential energy U e = 350eV, which is much higher than the adiabatic limit of 175eV and not understood at present. The deduced value of S b(0) is considerably smaller than the previously adopted value of 59keV b, significantly increasing the calculated abundance of 7Li in big-bang nucleosynthesis. The Trojan-horse method was applied to the reaction 7Li (p,α)α to determine the energy dependence of the S b(E) factor for 10 〈 E 〈 370keV, free from the effects of the Coulomb barrier and electron screening. The THM results are close to the calculated S b(E) curve and suggest that the THM may become a powerful way to obtain improved information on low-energy cross-sections and associated electron-screening effects in a model-independent way.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1434-601X
    Keywords: PACS. 26.20.+f Hydrostatic steller nucleosynthesis – 34.50.Bw Energy loss and stopping power
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: The energy loss of deuterons in 3He gas was measured at Ed = 15 to 100 keV using the 3He pressure dependence of the 3He(d,p) 4He cross-section at a given incident energy. At the highest energies, the observed energy loss is in good agreement with a standard compilation. However, with decreasing energy the experimental values drop steadily below the theoretical values and near Ed = 18 keV they drop sharply (within 1 keV) reaching the domain of nuclear stopping power. This threshold behavior is due to the minimum 1s→ 2s electron excitation of the He target atoms, i.e. it is a quantum effect. Some consequences are discussed.
    Type of Medium: Electronic Resource
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