If the source of the ionizing radiation is a radioactive material or a nuclear process such as fission or fusion, there is particle radiation to consider. Particle radiation is subatomic particles accelerated to relativistic speeds by nuclear reactions.
radioactivity, property exhibited by certain types of matter of emitting energy and subatomic particles spontaneously. It is, in essence, an attribute of individual atomic nuclei.
Radioactivity is defined as the emission of particles and radiation from nuclear reactions. These nuclear reactions include radioactive decay by unstable atomic nuclei, fission, and fusion. It’s important to note that not all radiation comes from radioactivity.