When travelling through a medium they lose their energy by colliding with other atomic nuclei the most energy being transferred in collision with hydrogen nuclei. The hydrogen nucleus then becomes a densely ionising particle. Ionising radiation is emitted by radioactive materials , which more accurately are called radionuclides. Sources may either be closed sources or unsealed sources . 137 Cs in a cell irradiation facility.
Liquids are by far the most common.
Ionising radiation may also be produced by firing electrons at a metal target usually tungsten, copper or molybdenum. In this case a spectrum of x-ray energies is produced with a maximum energy determined by the accelerating voltage and a superimposed line spectrum determined by the target material. When x or gamma radiation interacts with living material it is absorbed by one of three processes- photoelectric absorption, Compton scatter or pair production. These all result in production of energetic electrons, i.e. charged particles which then lose energy by causing ionisation. In the case of alpha and beta particles there is no intermediate absorption process.