In copper which was irradiated by fission neutrons at 300 deg C in a temperature controlled rig to 0.0003 dpa with the damage production rate of 6 x10~-5 dpa/hr,the number of voids exceeds the number of gas atoms of hydrogen and helium which were generated by the transmutation reaction.The number density of voids was almost the same in as-received and gas-atomremoved copper which was neutron irradiated to 0.0003 dpa.THese results suggest that voids can be formed by clustering of only vacancies.In 5 MeV Ni-ion irradiated copper,voids could not be observed in specimens which were irradiated between 200 deg C and 450 deg C from 0.1 dpa to 30 dpa with the damage production rate of 6 dpa/hr.Voids were observed only in copper which was Ni-ion irradiated at 500 deg C.The difference of void formation on neutron-irradiation and ion-irradiation is discussed.It is concluded that the production rate of damage is an important factor on the void formationin irradiated metals.
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