The bound coherent neutron scattering lengths of the oxygen isotopes

The technique of neutron interferometry was used to measure the bound coherent neutron scattering length b_coh of the oxygen isotopes ¹⁷O and ¹⁸O. From the measured difference in optical path between two water samples, either H₂¹⁷O or H ₂¹⁸O versus H₂^natO, where nat denotes the natural isotopic composition, we obtain b_coh, ¹⁷O = 5.867(4) fm and b_coh,¹⁸O = 6.009(5) fm, based on the accurately known value of b_coh,^natO = 5.805(4) fm which is equal to b_coh,¹⁶O within the experimental uncertainty. Our results for b_coh,¹⁷O and b_coh,¹⁸O differ appreciably from the standard tabulated values of 5.6(5) fm and 5.84(7) fm, respectively. In particular, our measured scattering-length contrast of 0.204(3) fm between ¹⁸O and ^natO is nearly a factor of 6 greater than the tabulated value, which renders feasible neutron diffraction experiments using ¹⁸O isotope substitution and thereby offers new possibilities for measuring the partial structure factors of oxygen-containing compounds, such as water.