Measurement of neutron capture on 48Ca at thermal and thermonuclear energies.

Abstract

At the Karlsruhe pulsed 3.75 MV Van de Graaff accelerator the thermonuclear $^{48}\mathrm{Ca}$(n,\ensuremath{\gamma}${)}^{49}$Ca(8.72 min) cross section was measured by the fast cyclic activation technique via the 3084.4 keV \ensuremath{\gamma}-ray line of the $^{49}\mathrm{Ca}$ decay. Samples of ${\mathrm{CaCO}}_{3}$ enriched in $^{48}\mathrm{Ca}$ 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 \ensuremath{\gamma}-ray lines using the same target material. The $^{48}\mathrm{Ca}$(n,\ensuremath{\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 $^{49}\mathrm{Ca}$. The small coherent elastic cross section of $^{48}\mathrm{Ca}$+n is explained through the nuclear Ramsauer effect. Spectroscopic factors of $^{49}\mathrm{Ca}$ 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 nonresonant 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 $^{49}\mathrm{Ca}$ determined from shell-model calculations are compared with the values extracted from the experimental cross sections for $^{48}\mathrm{Ca}$(d,p${)}^{49}$Ca and $^{48}\mathrm{Ca}$(n,\ensuremath{\gamma}${)}^{49}$Ca. \textcopyright{} 1996 The American Physical Society.