Neutron

A neutron was a particle. It was one of the components of an atom, along with protons and electrons. (PROSE: The Infinity Doctors)

According to the First Doctor, the Time-Space Visualiser converted "neutrons of light energy into electrical impulses". (TV: The Chase)

Neutron eaters ate neutrons and existed in an alternate reality. When the Large Hadron Collider was first turned on, it opened a portal allowing a neutron eater to enter this universe. (AUDIO: Lost Souls)

Nuclear fission began when a large atom, such as Uranium-235, absorbed a neutron, resulting in it splitting in two. Free neutrons were also released as a result of this. (AUDIO: Power Play)