The transport equation can be applied to a given part of phase space (time t, energy E, location r , and direction of travel Ω Ω ^ ^ ). The first term represents the time rate of change of neutrons in the system. The second terms describes the movement of neutrons into or out of the volume of space of interest. The third term accounts for all neutrons that have a collision in that phase space. The first term on the right hand side is the production of neutrons in this phase space due to fission, while the second term on the right hand side is the production of neutrons in this phase space due to delayed neutron precursors (i.e., unstable nuclei which undergo neutron decay). The third term on the right hand side is in-scattering, these are neutrons that enter this area of phase space as a result of scattering interactions in another. The fourth term on the right is a generic source. The equation is usually solved to find ϕ ϕ ( r , E ) , since that will allow for the calculation of reaction rates, which are of primary interest in shielding and dosimetry studies.