ABSTRACT
This chapter considers the concept of ultra-cold neutron (UCN) gas and derive the necessary theory to describe the various features of neutron storage and guide transmission. UCN gas, although very similar to an ideal gas, has several distinct properties quite peculiar to itself. The low ucn densities obtainable outside of reactors and the small size of the neutron mean that neutron-neutron collisions have a negligible probability of occurrence. Neutron-neutron energy exchange therefore does not occur and individual neutron trajectories are completely independent of each other. The UCN mean free path between wall collisions is totally characterized by the geometry of the confinement system and the details of the UCN-wall collision mechanism. Bottled UCN gas densities decay during storage. This decay is due both to the effects of wall collisions and β-decay. Since both the rate of wall collisions and the loss probability per bounce for a UCN is energy-dependent this density decay rate will also be energy-dependent for a given system.
