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IMPERFECTIONS INDUCED IN SOLIDS BY 

 FAST-PARTICLE IRRADIATION 



P. G. Klemens 



Division of Physics, Commonwealth Scientific and Industrial Research Organization, 



University Grounds, Sydney 



INTRODUCTION 



When solids are irradiated with fast particles, stable and metastable changes 

 are induced in them. These changes vary widely with the nature of the solid 

 and the character of the incident radiation. We shall he mainly concerned 

 with radiation damage due to atomic displacements, caused either by the 

 direct action of the incident particle, or by the action of other displacements. 



This is not the only type of change, for there are also possible changes 

 due to electronic excitations, except in metals, where excited electronic 

 states are short-lived. However, the difference between a change due to 

 electronic excitation and one due to displacements is not always clear-cut; 

 for example, bond rupture may occur, followed by an atomic rearrangement, 

 the energy for the movement of atoms being supplied by thermal vibrations. 



Some instances of changes due to displacements in solids have been known 

 for some time, for example from the study of radio-active minerals^ The 

 introduction of nuclear reactors has given a great impetus to research in 

 this field. The reason is partly that radiation damage may, in some cases, 

 affect important physical properties of the solid components of reactors, and 

 partly that reactors have made available a strong source of fast and pene- 

 trating particles, which can be used to study, for more fundamental purposes, 

 the nature of radiation damage and consequent changes in physical properties. 

 It should be noted that neutrons are not the only source of radiation damage: 

 beams of high-energy electrons, protons, deuterons and alpha particles are 

 all being used ; however, where high dosage or uniform penetration of thick 

 specimens is required, the nuclear reactor has obvious advantages. 



THE NATURE OF RADIATION DAMAGE 



Let us first look, in a qualitative way, at the physical processes of radiation 

 damage. A charged particle, or even a neutral atom, travelling through a 

 solid, will lose energy by electron excitation. Competing with this process 

 will be energy loss by collision with the atoms. If sufficient energy is trans- 

 ferred to an atom, it will break away from its lattice site and move through 

 the crystal, itself dissipating its kinetic energy by the above processes. If 

 the energy imparted to an atom bound in the crystal lattice is below a 

 certain threshold E^, it will only excite lattice vibrations. Typical values of 

 Ej are about 25 eV, but the existence of a sharp threshold is of course an 

 oversimplification. 



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