Opportunities in the use of Very Cold Neutrons in reflectometry techniques

Reflectometry techniques are especially suited for time-of-flight measurements. The possibility of using 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 due to the higher reflectivity. 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 improved optics performances. However, the possibility of performing such measurements would be bound to the fact that the pulse structure or the instrument lengths are still adequate. This would require to proportionally decrease the instrument length or to either 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 is already expected to be extremely high, it is questionable if it is worth investing in VCN 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 implementation 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, making the construction and handling of VCN sources a lot easier.