Thermal-neutron scattering length and capture byCa46

We have deduced the coherent scattering amplitude of ${}_{}{}^{46}{}_{}{}^{}\mathrm{Ca}$ as 3.55±0.21 fm by measuring the neutron-diffraction patterns of calcite enriched to 32% in ${}_{}{}^{46}{}_{}{}^{}\mathrm{Ca}$. We have employed this value in calculating the direct-capture cross sections of the primary electric-dipole γ transitions in ${}_{}{}^{47}{}_{}{}^{}\mathrm{Ca}$ using optical-model potentials with physically realistic parameters. The calculated cross sections agree reasonably well with measurements showing that thermal-neutron capture by ${}_{}{}^{46}{}_{}{}^{}\mathrm{Ca}$ is controlled largely by a simple direct mechanism.