Positron

A positron was a particle. It could be found in anti-matter. (TV: Planet of Evil)

Photons could be produced by the recombination of an electron and a positron. (TV: Time and the Rani)

An electron and a positron could also be combined to create orthopositronium. (AUDIO: The Time of the Daleks)

A fixed-atom force field created a shield of positrons which would painfully repel any intruders. (AUDIO: 1001 Nights)