Coherent scattering of slow neutrons at nanoparticles in particle physics experiments

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.