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
We have measured the 26 Al(n,␣ 0 ) 23 Mg and 26 Al(n, p 1 ) 26 Mg* cross sections from thermal energy to approximately 10 keV and 70 keV, respectively. These reactions are thought to be the major mechanisms for the destruction of 26 Al in many nucleosynthesis environments; hence, an accurate determination of their rates is important for understanding the observations of ␥ rays from ''live'' 26 Al in our galaxy and of ''extinct'' 26 Al in meteorites. The astrophysical rate for the 26 Al(n,␣ 0 ) 23 Mg reaction determined from our measurements is in good agreement with the rate determined via inverse measurements. On the other hand, the rate we determined for the 26 Al(n,p 1 ) 26 Mg* reaction is significantly larger than previously reported. In addition, we were able to determine this rate in the temperature range below 0.2 GK which was not covered by previous measurements. This lower temperature range may be important for understanding the production of 26 Al in Red Giant stars. Both of our rates are significantly different than the rates used in most nucleosynthesis calculations. We discuss the impact of our measurements on the nucleosynthesis of 26 Al.
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