MODES OF RADIOACTIVE DECAYDOE-HDBK-1019/1-93Atomic and Nuclear PhysicsNP-01Page 26Rev. 0GammaEmission()Gamma radiation is a high-energy electromagnetic radiation that originates in the nucleus. It isemitted in the form of photons, discrete bundles of energy that have both wave and particleproperties. Often a daughter nuclide is left in an excited state after a radioactive parent nucleusundergoes a transformation by alpha decay, beta decay, or electron capture. The nucleus will dropto the ground state by the emission of gamma radiation. InternalConversionThe usual method for an excited nucleus to go from the excited state to the ground state is byemission of gamma radiation. However, in some cases the gamma ray (photon) emerges from thenucleus only to interact with one of the innermost orbital electrons and, as a result, the energy of thephoton is transferred to the electron. The gamma ray is then said to have undergone internalconversion. The conversion electron is ejected from the atom with kinetic energy equal to thegamma energy minus the binding energy of the orbital electron. An orbital electron then drops toa lower energy state to fill the vacancy, and this is accompanied by the emission of characteristicx-rays.IsomersandIsomericTransitionIsomeric transition commonly occurs immediately after particle emission; however, the nucleus mayremain in an excited state for a measurable period of time before dropping to the ground state at itsown characteristic rate. A nucleus that remains in such an excited state is known as a nuclear isomerbecause it differs in energy and behavior from other nuclei with the same atomic number and massnumber. The decay of an excited nuclear isomer to a lower energy level is called an isomerictransition. It is also possible for the excited isomer to decay by some alternate means, for example,by beta emission.An example of gamma emission accompanying particle emission is illustrated by the decay ofnitrogen-16 below.
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