The reflection is caused by the coherent strong interaction of the neutron with atomic nuclei.
The kinetic energy of 300 neV corresponds to a maximum velocity of 7.6 m/s or a minimum wavelength of 52 nm.
As their density is usually very small, UCN can also be described as a very thin ideal gas with a temperature of 3.5 mK.
[1] Due to the small kinetic energy of an UCN, the influence of gravitation is significant.
[2] The consequence of such a potential would be the total reflection of neutrons slow enough and incident on a surface at a glancing angle.
[4] The storage of neutrons with very low kinetic energies was predicted by Yakov Borisovich Zel'dovich[5] and experimentally realized simultaneously by groups at Dubna[6] and Munich.
Such facilities have been built and are in operation: Any material with a positive neutron optical potential can reflect UCN.
The table on the right gives an (incomplete) list of UCN reflecting materials including the height of the neutron optical potential (VF) and the corresponding critical velocity (vC).
The production, transportation and storage of UCN is currently motivated by their usefulness as a tool to determine properties of the neutron and to study fundamental physical interactions.
Storage experiments have improved the accuracy or the upper limit of some neutron related physical values.
Using traps with different surface to volume ratios allowed them to separate storage decay time and neutron lifetime from each other.
The lowest value for the upper limit of the neutron electric dipole moment was measured with stored UCN (see main article).
Physicists have observed quantized states of matter under the influence of gravity for the first time.
The finding could be used to probe fundamental physics such as the equivalence principle, which states that different masses accelerate at the same rate in a gravitational field (V Nesvizhevsky et al. 2001 Nature 415 297).
UCN spectroscopy has been used to limit scenarios including dark energy, chameleon fields,[17] and new short range forces.
[18] see Mirror Matter The first reported measurement of the beta-asymmetry using UCN is from a Los Alamos group in 2009.
[19] The LANSCE group published precision measurements with polarized UCN the next year.