The recoil energy enters the PBM through the cas­cade parameters er and eg (see eqn [138]). Direct experimental evaluation of the recoil energy effect on void swelling was made by Singh et a/.,133 who compared the microstructure of annealed copper irradiated with 2.5 MeV electrons, 3 MeV protons, and fission neutrons at ^520 K. For all irradiations, the damage rate was ^10—8dpas—1. The average recoil energies in those irradiations were estimated133
to be about 0.05, 1, and 60keV for electron, proton, and neutron irradiations, respectively, thus, produc­ing the primary damage in the form of FPs (elec­trons), small cascades (protons), and well-developed cascades (neutrons). The cascade efficiency, 1 — er, hence, the real damage rate, was highest for electron irradiation (no cascades, the efficiency is equal to unity) and minimal for neutron irradiation (~0.1, see Section 1.13.3). If dislocation bias is the mecha­nism responsible for swelling, the swelling rate is proportional to the damage rate and therefore must be highest after electron and lowest after neutron irradiation. However, just the opposite was found; the swelling level after neutron irradiation was ^50 times higher than after electron irradiation, with the value for proton irradiation falling in between (see Figure 5). These results represent direct experimen­tal confirmation that damage accumulation under cascade damage conditions is governed by mechan­isms that are entirely different from those under FP production.

The results obtained in this study can be under­stood as follows. Under electron irradiation, only the first term on the right-hand side of eqn [138] oper­ates, as eg = 0. The swelling rate is low in this case because of the low dislocation density, as discussed in Section 1.13.6.2.1. Under cascade damage condi­tions, the damage rate is smaller because of the low cascade efficiency. In this case eg = 0 and the second term on the right-hand side of eqn [138] plays the main role, which is evident from the theoretical treatment of the experiment carried out in the fol­lowing section.24