whereas in future fusion reactors employing deute­rium (D) and tritium (T) as fuel, the neutrons are the result of DT fusion. Spallation neutron sources, which are used for a variety of material research purposes, generate neutrons as a result of spallation reactions between a high-energy proton beam and a heavy metal target. Neutron exposure can lead to substantial changes in the microstructure of the materials, which are ultimately manifested as observ­able changes in component dimensions and changes in the material’s physical and mechanical properties as well. For example, radiation-induced void swelling can lead to density changes greater than 50% in some grades of austenitic stainless steels1 and changes in the ductile-to-brittle transition temperature greater than 200 °C have been observed in the low-alloy steels used in the fabrication of reactor pressure vessels.2,3 These phenomena, along with irradiation creep and radiation-induced solute segregation are discussed extensively in the literature4 and in more detail elsewhere in this comprehensive volume (e. g., see Chapter 1.03, Radiation-Induced Effects on Microstructure; Chapter 1.04, Effect of Radiation

on Strength and Ductility of Metals and Alloys; and Chapter 1.05, Radiation-Induced Effects on Mate­rial Properties of Ceramics (Mechanical and Dimensional)). The objective of this chapter is to describe the process of primary damage production that gives rise to macroscopic changes. This primary radiation damage event, which is referred to as an atomic displacement cascade, was first proposed by Brinkman in 1954.5,6 Many aspects of the cascade dam­age production discussed below were anticipated in Brinkman’s conceptual description.

In contrast to the time scale required for radiation — induced mechanical property changes, which is in the range of hours to years, the primary damage event that initiates these changes lasts only about 10~ns. Similarly, the size scale of displacement cascades, each one being on the order of a few cubic nan­ometers, is many orders of magnitude smaller than the large structural components that they affect. Although interest in displacement cascades was initi­ally limited to the nuclear industry, cascade damage production has become important in the solid state processing practices of the electronics industry also.7 The cascades of interest to the electronics industry arise from the use of ion beams to fabricate, modify, or analyze materials for electronic devices. Another related application is the modification of surface layers by ion beam implantation to improve wear or corrosion resistance of materials.8 The energy and mass of the particle that initiates the cascade provide the principal differences between the nuclear and ion beam applications. Neutrons from nuclear fission and DT fusion have energies up to about 20 MeV and 14.1 MeV, respectively, while the peak neutron energy in spallation neutron sources reaches as high as the energy of the incident proton beam, ~1 GeV in modern sources.9 The neutron mass of one atomic mass unit (1 a. m.u.^1.66 X 10-27 kg) is much less than that of the mid-atomic weight metals that comprise most structural alloys. In contrast, many ion beam applications involve relatively low-energy ions, a few tens of kiloelectronvolts, and the mass of both the incident particle and the target is typically a few tens of atomic mass unit. The use of somewhat higher energy ion beams as a tool for investigating neutron irradiation effects is discussed in Chapter 1.07, Radiation Damage Using Ion Beams.

This chapter will focus on the cascade energies of relevance to nuclear energy systems and on iron, which is the primary component in most of the alloys employed in these systems. However, the description of the basic physical mechanisms of displacement cascade formation and evolution given below is gen­erally valid for any crystalline metal and for all of the applications mentioned above. Although additional physical processes may come into play to alter the final defect state in ionic or covalent materials due to atomic charge states,10 the ballistic processes observed in metals due to displacement cascades are quite similar in these materials. This has been demon­strated in molecular dynamics (MD) simulations in a range of ceramic materials.11-15 Finally, synergistic effects due to nuclear transmutation reactions will not be addressed; the most notable of these, helium production by (n, a) reactions, is the topic of Chapter

1.6, The Effects of Helium in Irradiated Struc­tural Alloys.