Ultracold neutrons

Abstract

Nearly twenty years ago Ya. B. Zeldovich pointed out that free neutrons, if sufficiently slow, could in principle be stored in a closed “bottle”—presumably for periods up to the beta decay lifetime, about ${10}^3\hspace{0.17em}\mathrm{seconds}.$ Apparently, according to Bruno Pontecorvo, Enrico Fermi had mentioned this possibility as early as the middle 1930's, but Zeldovich's paper was the first published discussion. The basis for the belief that neutrons could be stored in this way was the behavior of the effective potential for the interaction of neutrons with matter, which for many substances becomes repulsive when the neutrons are sufficiently slow. Thus there is a strong suppression of inelastic processes for these very slow neutrons (energies below about ${10}^{\text{−7}}\hspace{0.17em}\mathrm{eV}$ ), and they are totally reflected upon collisions with, for example, container walls.