The condition for a neutron to be reflected from a surface is that the angle of approach is less than some critical angle. In other words, the component of its energy directed transverse to the surface must be below some critical energy (E_{crit}, typically ~ 150 neV).

If a neutron’s total energy is below E_{crit} it can reflect off the surface at any angle. It is trapped!

Neutrons with low enough energy to be trapped^{**} are called Ultracold Neutrons, or UCN.

A UCN has a velocity of ~ 5 m/s.

^{**Note: UCN can also be trapped in a magnetic bottle (150 neV corresponds to a magnetic field of 2.5 T) and a gravitational bottle (150 neV corresponds to a height of 1.5 m).}