Further notes on my Seetee researches here.

In his 1980 essay "The Word I Invented: Robotics," Isaac Asimov wrote:

When I first began writing science fiction stories, the positron had been discovered only six years before as a particle with all the properties of an electron except for an opposite charge. It was the first (and, at that time, still the only) bit of antimatter that had been discovered, and it carried a kind of science fictional flavor about it.

That meant that if I spoke of positronic robots rather than electronic robots, I would have something exotic and futuristic instead of something conventional.

"Reason," in the April 1941 Astounding, was the story in which Asimov introduced the word "positronic."