Experiments with ultracold neutrons

Ultracold neutrons (UCN) form a tiny low-energy fraction of the Maxwellian spectrum of thermal neutrons in moderators of nuclear reactors and spallation sources. Their energy is extremely low (similar to 10(-7) eV), their velocity is a few meters per second, and their effective temperature is as low as similar to 1 mK. The specific feature of UCN is their nearly total elastic reflection by the nuclear-optical potential of many materials at any incidence angle, so that they could be stored in closed traps for many minutes, and used for extremely sensitive measurements. The fraction of UCN in a thermal neutron flux is as low as 10(-12)-10(-11), and serious efforts are under way all over the world to produce UCN in larger amounts. UCN are widely used in precision particle physics experiments. Applications of UCN are emerging in surface and nanoparticle physics. Here we focus on recent advances in the field. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3597610]