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Usually Ultra Cold Neutrons (UCN) and Very Cold Neutrons (VCN) are not efficiently scattered by individual atoms in matter due to their large wavelength. On the contrary, such neutrons are coherently scattered by nanoparticles, providing a powerful tool, in particular for VCN reflectors, for VCN storage in closed traps, probably for ‘quasi-specular’ reflectors for cold neutrons, and for cooling VCN to the UCN energy range. On the other hand, quasi-elastic scattering of UCN at nanoparticles and large molecules (as those of Fomblin oil) in all presently used UCN trap wall materials causes false effects in the neutron lifetime measurements because of poorly controlled spectrum evolution.

Ultra-cold neutrons (UCN) can be stored in a trap if their energy is lower than the trap wall potential. It is well known that the neutron density in a trap decreases due to neutron beta-decay, upscattering and absorption on surfaces but we have identified a complementary escape channel. This arises from a small increase in the energy of UCN during their interaction with a surface. Higher-energy neutrons can then escape into the bulk material or penetrate through the trap wall if it is thin enough.