Atomic and Nuclear PhysicsDOE-HDBK-1019/1-93MODES OF RADIOACTIVE DECAYHeavy elements, such as uranium or thorium, and their unstable decay chain elements emitradiation in their naturally occurring state. Uranium and thorium, present since their creationat the beginning of geological time, have an extremely slow rate of decay. All naturallyoccurring nuclides with atomic numbers greater than 82 are radioactive.NuclearDecayWhenever a nucleus can attain a more stable (i.e., more tightly bound) configuration by emittingradiation, a spontaneous disintegration process known as radioactive decay or nuclear decay mayoccur. In practice, this "radiation" may be electromagnetic radiation, particles, or both.Detailed studies of radioactive decay and nuclear reaction processes have led to the formulationof useful conservation principles. The four principles of most interest in this module arediscussed below.Conservation of electric chargeimplies that charges are neither created nor destroyed. Singlepositive and negative charges may, however, neutralize each other. It is also possible for aneutral particle to produce one charge of each sign.Conservation of mass numberdoes not allow a net change in the number of nucleons. However,the conversion of a proton to a neutron and vice versa is allowed.Conservation of mass and energyimplies that the total of the kinetic energy and the energyequivalent of the mass in a system must be conserved in all decays and reactions. Mass can beconverted to energy and energy can be converted to mass, but the sum of mass and energy mustbe constant.Conservation of momentumis responsible for the distribution of the available kinetic energyamong product nuclei, particles, and/or radiation. The total amount is the same before and afterthe reaction even though it may be distributed differently among entirely different nuclidesand/or particles.Rev. 0Page 23NP-01
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