Measurement of neutron capture on ${}^{48}$Ca at thermal and thermonuclear energies

Abstract

At the Karlsruhe pulsed 3.75\,MV Van de Graaff accelerator the thermonuclear ${}^{48}$Ca(n,$\gamma$)${}^{49}$Ca(8.72\,min) cross section was measured by the fast cyclic activation technique via the 3084.5\,keV $\gamma$-ray line of the ${}^{49}$Ca-decay. Samples of CaCO${}_3$ 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 $\gamma$-ray lines using the same target material. The ${}^{48}$Ca(n,$\gamma$)${}^{49}$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,$\gamma$)${}^{49}$Ca.

arXiv comment15 pages (uses Revtex), 7 postscript figures (uses psfig), accepted for publication in PRC, uuencoded tex-files and postscript-files also available at ftp://is1.kph.tuwien.ac.at/pub/ohu/Ca.uu

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