members of the series, which are solids, radon is a chemically-inert noble gas and can migrate in the environment.
A radon atom that is created deep within a grain of rock usually remains there until it decays. However, when a radon atom is created near the surface of a grain, it can recoil into the pore between grains; such radon atoms do not attach or bind to the matrix that contains the immediate precursor, radium. The amount of radon that reaches the pores is described by the emanation fraction. For typical soils or bedrock, the emanation fraction can range from 5% to 50% (see the review in Nazaroff 1992).
In most situations, the pore between grains of material contains a mixture of air and water. Often, a recoil radon atom will come to rest in the water and remain