Nickel has a positive scattering cross section, and titanium has a negative scattering cross section, and in both elements the absorption cross section is small, which makes Ni-Ti the most efficient technology with neutrons. The number of Ni-Ti layers needed increases rapidly as ∝ ∝ m z , with z in the range 2–4, which affects the cost. This has a strong bearing on the economic strategy of neutron instrument design.