One type of radioisotope are "delta-series radioisotopes", which can be produced in plasma conduits, and which can interact with temporal energy. Prolonged exposure to these isotopes can prove fatal for humanoids. The Star Trek Encyclopedia (4th ed., vol. 2, p. 201) defined "radioisotope" as a, "Form of chemical elements possessing unstable nuclei that spontaneously emit radiation in the form of alpha particles, beta particles, or gamma rays."