MASS DEFECT AND BINDING ENERGYDOE-HDBK-1019/1-93Atomic and Nuclear PhysicsAn atom cannot stay in the excited state for an indefinite period of time. An excited atom willeventually transition to either a lower-energy excited state, or directly to its ground state, byemitting a discrete bundle of electromagnetic energy called an x-ray. The energy of the x-raywill be equal to the difference between the energy levels of the atom and will typically rangefrom several eV to 100,000 eV in magnitude.EnergyLevelsof the NucleusThe nucleons in the nucleus of an atom, like the electrons that circle the nucleus, exist in shellsthat correspond to energy states. The energy shells of the nucleus are less defined and lessunderstood than those of the electrons. There is a state of lowest energy (the ground state) anddiscrete possible excited states for a nucleus. Where the discrete energy states for the electronsof an atom are measured in eV or keV, the energy levels of the nucleus are considerably greaterand typically measured in MeV.A nucleus that is in the excited state will not remain at that energy level for an indefinite period.Like the electrons in an excited atom, the nucleons in an excited nucleus will transition towardstheir lowest energy configuration and in doing so emit a discrete bundle of electromagneticradiation called a gamma ray (g-ray). The only differences between x-rays and g-rays are theirenergy levels and whether they are emitted from the electron shell or from the nucleus. The ground state and the excited states ofFigure 7 Energy Level Diagram - Nickel-60a nucleus can be depicted in a nuclearenergy-level diagram. The nuclearenergy-level diagram consists of a stack ofhorizontal bars, one bar for each of theexcited states of the nucleus. The verticaldistance between the bar representing anexcited state and the bar representing theground state is proportional to the energylevel of the excited state with respect tothe ground state. This difference inenergy between the ground state and theexcited state is called the excitation energyof the excited state. The ground state ofa nuclide has zero excitation energy. Thebars for the excited states are labeled withtheir respective energy levels. Figure 7 isthe energy level diagram for nickel-60.NP-01Page 20Rev. 0
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business