Thoron

A thoron is a radioactive isotope of radon, 220Rn, having a half-life of 54.5 seconds and produced by the decomposition of thorium.

Thoron fields can be employed in conjunction with duranium shadows to deceive sensors into registering fully functional weapons systems. (DS9: "Emissary", "The Way of the Warrior")

Thorons have some medical applications. (VOY: "Basics, Part II") When Captain Kathryn Janeway was injured by the crash of the Sacajawea in 2373, the Doctor tried a thoron pulse in attempting to repel a non-corporeal lifeform that had inhabited her cerebral cortex. (VOY: "Coda")

Crewman Lon Suder used a portable thoron generator from sickbay to fool the Starfleet tricorders being used by the Kazon. He said that it was an old Maquis trick. (VOY: "Basics, Part II")

Some directed energy weapons are based on thorons, though they can be unstable. Most of Entharan arms dealer Kovin's handheld disruptors were thoron-based; he claimed he was able to compensate for the instability by polarizing the emitter matrix. (VOY: "Retrospect")