Neutron-diffraction patterns have been measured for isotopically enriched powder samples of calcite using both steady-state and pulsed-neutron techniques. Greatly enhanced precision over previous work has been achieved for ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ and ${}_{}{}^{44}{}_{}{}^{}\mathrm{Ca}$, while the results for ${}_{}{}^{42}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{43}{}_{}{}^{}\mathrm{Ca}$, and ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$ represent new data. The coherent scattering lengths deduced from these measurements have been employed in a more definitive analysis of primary electric-dipole gamma rays from thermal-neutron capture. In three cases ${(}^{40}$Ca, ${}_{}{}^{42}{}_{}{}^{}\mathrm{Ca}$, and ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$) the estimates from a spherical optical-model formulation of the direct-capture mechanism are in agreement with the experimental cross sections. In ${}_{}{}^{44}{}_{}{}^{}\mathrm{Ca}$ the theory overestimates the measured cross sections on average by about 60%; this divergence can be explained by considering the modifications to the theory due to collective vibrations of the ${}_{}{}^{44}{}_{}{}^{}\mathrm{Ca}$ core.

• Received 5 December 1988

DOI:https://doi.org/10.1103/PhysRevC.39.1297