  Multilayer neutron optics require precise control of interface morphology for
optimal performance. In this work, we investigate the effects of different
growth conditions on the interface morphology of Ni/Ti based multilayers, with
a focus on incorporating low-neutron-absorbing 11B4C and using different ion
assistance schemes. Grazing incidence small angle X-ray scattering was used to
probe the structural and morphological details of buried interfaces, revealing
that the layers become more strongly correlated and the interfaces form mounds
with increasing amounts of 11B4C. Applying high flux ion assistance during
growth can reduce mound formation but lead to interface mixing, while a high
flux modulated ion assistance scheme with an initial buffer layer grown at low
ion energy and the top layer at higher ion energy prevents intermixing. The
optimal condition was found to be adding 26.0 at.% 11B4C combined with high
flux modulated ion assistance. A multilayer with a period of 48.2 {\AA} and 100
periods was grown under these conditions, and coupled fitting to neutron and
X-ray reflectivity data revealed an average interface width of only 2.7 {\AA},
a significant improvement over the current state-of-the-art commercial Ni/Ti
multilayers. Overall, our study demonstrates that the addition of 11B4C and the
use of high flux modulated ion assistance during growth can significantly
improve the interface morphology of Ni/Ti multilayers, leading to improved
neutron optics performance.
