Neutrino

Neutrinos were subatomic particles that could pass through most matter. They could be detected with neutrino detectors, which were vats of cleaning fluid with sensors inside. In the rare instances when a neutrino interacted with an electron in the fluid, the sensors would detect it.

The Arkive communicated with neutrinos. In order to understand it, the Second Doctor had to create a neutrino detector in the ice of Mnemosyne. (PROSE: The Wheel of Ice)

When they activated the Hand of Omega in 1963, the Imperial Daleks observed the "primary neutrino release" as the reaction began. (TV: Remembrance of the Daleks)

Neutrinos made up 90% of the energy produced by a Zeiton-7-hymetusite cold fusion reaction. (PROSE: Lucifer Rising)

On Earth the muon neutrino was discovered by Melvin Schwartz, whom the Sixth Doctor met. (AUDIO: The Juggernauts)

Behind the scenes
The neutrino is a subatomic particle that has no electric charge and travels close to the speed of light. Over time it oscillates through three "flavors", the electron, muon, and tau neutrino.