Thermal-neutron scattering lengths and capture by even calcium isotopes

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 Ca40 and Ca44, while the results for Ca42, Ca43, and Ca48 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 (40Ca, Ca42, and Ca48) the estimates from a spherical optical-model formulation of the direct-capture mechanism are in agreement with the experimental cross sections. In Ca44 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 Ca44 core.