Opportunities in the use of Very Cold Neutrons in reflectometry techniques

Reflectometry techniques are especially suited for time-of-flight measurements. The use of longer wavelengths does not modify the physics probed during a reflectivity measurement. Besides, reflectometry measurements are not affected by effects such as multiple scattering or absorption. The use of longer wavelengths would permit to achieve instrumental gains. Indeed, in the scattering plane, the phase space can be used more efficiently by a geometrical factor proportional to λ 2 provided by a simple increase of the incidence angles on the sample (for a given Q range). Perpendicular to the scattering plane, the neutron flux can be increased by a factor proportional to λ due to the increased critical angle of optics used for focusing in this direction. However, to comply with a given pulse structure of the neutron source would either require to proportionally decrease the instrument length or to drop neutron frames. Both options are viable depending on the scientific goals and we show that the flux penalties are actually minimal. However, owing to the fact that the performances of reflectometry instrumentation at ESS are expected to be extremely high, it is questionable if it is worth investing in Very Cold Neutrons (VCN) production for this specific technique the more so as the implementation will not be optimal for extrinsic reasons. On the other hand, implementing VCN on sources such as CANS where the flux is intrinsically limited may be worth the investment since (i) such sources could probably be designed to use VCN in an optimal way, (ii) the construction and handling of VCN sources would be much easier as radiative heating would be reduced by several orders of magnitude, in the range of hundreds of watts.