atoms or free radicals with energies greater than the energy of thermal motion.
Hot atoms, or energy recoil atoms, are formed in nuclear reactions; hot free radicals arise in photolysis, radiolysis. and electric discharge. The excess energy can be from several electron volts to hundreds of thousands. In collisions the energy of hot particles is either dissipated or expended in chemical conversions, in which reactions can take place that are thermodynamically impossible for ordinary particles.
Hot particles arise in the atmosphere through the testing of atomic weapons and other artificial or natural radioactive processes. The number of hot particles in the atmosphere is infinitesimal—one per hundreds of thousands of cubic meters of air—but their radioactivity is high—from 0.37 disintegrations per sec to 370 disintegrations per sec (from 10−11 to 10−8 curies per particle). In contact with body tissues (primarily of the gastrointestinal tract, respiratory passages, and skin), hot particles can cause the appearance of intense irradiation zones (where, within a radius of a few dozen microns, the total dose absorbed exceeds 10 joules per kg [1,000 rad]), and there is local skin destruction.