1.03.3.1 Irradiation Dose

As discussed in Chapter 1.01, Fundamental Proper­ties of Defects in Metals; Chapter 1.02, Fundamental Point Defect Properties in Ceramics; and Chapter

1.11, Primary Radiation Damage Formation, the

international standardized displacement per atom (dpa) unit for radiation damage34 is a useful parame­ter for comparing displacement damage levels in a variety of irradiation environments. The calculated damage level is directly proportional to the pro­duct of the fluence and the average kinetic energy transferred to the host lattice atoms (damage energy). The effective damage cross-sections for 1 MeV particles incident on copper range from ^30 barns (1 barn = 1 x 10-24 cm2) for electrons35 to ^600 barns for neutrons36 and ~2 x 109 barns for Cu ions.37

The dpa unit is remarkably effective in correlating the initial damage production levels over a wide range of materials and irradiating particles and is the singular most important parameter for quantifying radiation effects in materials. Numerous aspects of microstruc­tural evolution are qualitatively equivalent on a dpa basis for materials irradiated in widely different irradiation environments. However, the dpa unit does not accurately capture some of the complex differences in primary damage production for ener­getic displacement cascade conditions compared to isolated Frenkel pair production.38 For example, defect production at cryogenic temperatures (where long-range defect migration and annihilation does not occur) for neutron and heavy ion-irradiated materials is about 20-30% of the calculated dpa value due to athermal in-cascade recombination processes.38,39 In addition, the accumulated damage, as evident in the form of point defect clusters or other microstructural features, typically exhibits a complex nonlinear rela­tionship with irradiation dose that depends on irradi­ation temperature and several other factors. The impact of other experimental variables on the dose- dependent damage accumulation behavior is dis­cussed in Sections 1.03.3.2—1.03.3.9.