DOE-HDBK-1017/2-93ATOMIC DISPLACEMENT DUE TO IRRADIATIONPlant Materialsof these displacements are temporary. At high temperatures, the number of permanentlydisplaced atoms is smaller than the initial displacement.During a lengthy irradiation (for large values of the neutron fluence), many of the displacedatoms will return to normal (stable) lattice sites (that is, partial annealing occurs spontaneously).The permanently displaced atoms may lose their energy and occupy positions other than normalcrystal lattice sites (or nonequilibrium sites), thus becoming interstitials. The presence ofinterstitials and vacancies makes it more difficult for dislocations to move through the lattice.This increases the strength and reduces the ductility of a material.At high energies, the primary knock-on (ion) will lose energy primarily by ionization andexcitation interactions as it passes through the lattice, as shown in Figure 3. As the knock-onloses energy, it tends to pick up free electrons which effectively reduces its charge. As a result,the principle mechanism for energy losses progressively changes from one of ionization andexcitation at high energies to one of elastic collisions that produce secondary knock-ons ordisplacements. Generally, most elastic collisions between a knock-on and a nucleus occur at lowkinetic energies below A keV, where A is the mass number of the knock-on. If the kineticenergy is greater than A keV, the probability is that the knock-on will lose much of its energyin causing ionization.SummaryThe important information in this chapter is summarized below.Atomic Displacement Due To Irradiation SummaryBeta and gamma radiation produce ionization and excitation of electrons, whichdoes very little damage.Heavier particles, such as protons, a-particles, fast neutrons, and fissionfragments, usually transfer energy through elastic or inelastic collisions to causeradiation damage. These particles in organic material break the chemical bonds,which will change the material's properties.Knock-on is a target nucleus (or recoiling atom) that is displaced.Vacancy is the vacated site when a metal atom is ejected from its crystal lattice.Interstitial is a permanently displaced atom that has lost its energy and isoccupying a position other than its normal crystal lattice site.MS-05Page 34Rev. 0
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