We report on the measurement of the ^{7}Be(n,p)^{7}Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n_TOF facility at CERN. This reaction plays a key role in the lithium yield of the big bang nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and they showed a large discrepancy between each other. The measurement was performed with a Si telescope and a high-purity sample produced by implantation of a ^{7}Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low energy, relative to current evaluations, in the region of BBN interest, the present results are consistent with the values inferred from the time-reversal ^{7}Li(p,n)^{7}Be reaction, thus yielding only a relatively minor improvement on the so-called cosmological lithium problem. The relevance of these results on the near-threshold neutron production in the p+^{7}Li reaction is also discussed.
At the Karlsruhe pulsed 3.75 MV Van de Graaff accelerator the thermonuclear 48 Ca(n,γ) 49 Ca(8.72 min) cross section was measured by the fast cyclic activation technique via the 3084.5 keV γ-ray line of the 49 Ca-decay. Samples of CaCO3 enriched in 48 Ca by 77.87 % were irradiated between two gold foils which served as capture standards. The capture cross-section was measured at the neutron energies 25, 151, 176, and 218 keV, respectively. Additionally, the thermal capture cross-section was measured at the reactor BR1 in Mol, Belgium, via the prompt and decay γ-ray lines using the same target material. The 48 Ca(n,γ) 49 Ca cross-section in the thermonuclear and thermal energy range has been calculated using the direct-capture model combined with folding potentials. The potential strengths are adjusted to the scattering length and the binding energies of the final states in 49 Ca. The small coherent elastic cross section of 48 Ca+n is explained through the nuclear Ramsauer effect. Spectroscopic factors of 49 Ca have been extracted from the thermal capture cross-section with better accuracy than from a recent (d,p) experiment. Within the uncertainties both results are in agreement. The non-resonant thermal and thermonuclear experimental data for this reaction can be reproduced using the direct-capture model. A possible interference with a resonant contribution is discussed. The neutron spectroscopic factors of 49 Ca determined from shell-model calculations are compared with the values extracted from the experimental cross sections for 48 Ca(d,p) 49 Ca and 48 Ca(n,γ) 49 Ca.
The neutron capture cross section of 26 Mg was measured relative to the known gold cross section at thermonuclear energies using the fast cyclic activation technique. The experiment was performed at the 3.75 MV Van-de-Graaff accelerator, Forschungszentrum Karlsruhe. The experimental capture cross section is the sum of resonant and direct contributions. For the resonance at E n,lab ϭ220 keV our new results are in disagreement with the data from Weigmann, Macklin, and Harvey ͓Phys. Rev. C 14, 1328 ͑1976͔͒. An improved Maxwellian averaged capture cross section is derived from the new experimental data taking into account sand p-wave capture and resonant contributions. The properties of so-called potential resonances which influence the p-wave neutron capture of 26 Mg are discussed in detail. ͓S0556-2813͑98͒02108-6͔
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