Hot Atoms

Hot Atoms 

fast atoms arising through nuclear conversions.

Every nuclear conversion is accompanied by a liberation of energy, which is distributed between the nucleus that has undergone conversion and the particle emitted, in accordance with the law of conservation of momentum. The fast atoms formed are called hot atoms because their kinetic energy corresponds to the energy of thermal motion of atoms “heated” to millions of degrees. They are also called energetic recoil atoms. In the emission of α-particles, the energy of a recoil atom attains a value of many dozens of kilo electron volts (keV), whereas for reactions of the (η,-y) type it reaches just a few keV. At the instant of formation a hot atom can lose a large number of electrons from the outer and inner electron shells, forming a highly charged ion. For example, in the radiative capture of neutrons by ⁷⁹Br nuclei, giving the ⁸⁰Br isomer, the hot atoms of the isomer carry a charge of up to +10. In addition to a high kinetic energy, an excited electron state is characteristic of hot atoms.

Because of their high kinetic energies, excited electron state, and high positive charge, hot atoms can participate in chemical reactions in which ordinary atoms do not. In most cases the momentum of the resultant hot atom is great enough to break one or more atomic bonds in chemical compounds; during this the hot atom can break away from the molecule containing it. This property of hot atoms forms the basis of the method of enrichment of radioisotopes in (η,γ) reactions. The energy of the resultant hot atom (or hot radical) is, in its turn, sufficient to cause the excitation and dissociation of several additional molecules. Several successive collisions reduce the kinetic energy of a hot atom, and it enters into various chemical reactions with molecules or radicals of the starting compound, or solvent, which is accompanied by the microsynthesis of new compounds or the return of the hot atom to the molecule of the starting compound. The ratio of the quantity of hot atoms stabilized as parent substance (or generally as other molecules) to the total quantity of hot atoms formed is called the retention. In assessing the behavior of hot atoms, it is necessary to take into account possible isotope exchange processes, resulting in a steady distribution of hot atoms between all chemical forms containing the particular atom. There is the possibility of future use of hot atom reactions in technology for polymerization, ammonia synthesis, and synthesis of tagged compounds.

REFERENCES

Radiokhimiia i khimiia iadernykh protsessov. Edited by A. N. Murin (et al.). Leningrad, 1960.
Nuclear Chemistry. New York-London, 1968. Page 185.

K. B. ZABORENKO