IMPKRKHCniONS IXDUCKD I.\ SOLIDS \\\ I AS'l'-PARriC:i.r. IRRADI.VIION 



stifrcniiig of the network, with a profound circct on plastic properties. 

 H()\ve\er, a polymer contains some crystalline ret^ions, and the effect of the 

 bond changes of any actual displacement is to reduce the crystallinity". 



CHANGES IN PHYSICAL PROPERTIES 



The structural imperfections induced in solids by irradiation, as well as those 

 produced by other means, affect many of the physical properties. Some 

 properties will only suffer small changes, the relative change being com- 

 parable to the fractional volume (on an atomic scale) which is damaged: 

 such properties include density, lattice spacing, elastic constants and specific 

 heat. Other properties are very sensitive to small concentrations of imper- 

 fections. The various conduction properties, for example, depend upon the 

 mean free paths of the carriers of heat or electricity, and these are structure 

 sensitive, particularly at low temperatures. In the case of semi-conductors, 

 the number of carriers, too, is influenced very strongly. Various mechanical 

 properties depend upon the number of mobile imperfections, their sources, 

 sinks and mobilities, and are, therefore, also structure sensitive. 



There have been many investigations of the effects of irradiation of various 

 materials on their physical properties, including their crystal structure as 

 deduced from diffraction. They are too numerous even to catalogue here, 

 but have been reviewed, for example, by Dienes^, Glen^, Kinchin and 

 Pease^ and by Seitz and Koehler*. 



Imperfections in solids and their effect on physical properties constitute 

 a large and important part of solid-state physics. These imperfections may 

 be produced not only by irradiation, but also by the introduction of impui'- 

 ities, by plastic deformation and by quenching, and they may be removed 

 or rearranged by heat treatment. In each case it is necessary to identify 

 the imperfections, determine their concentrations, trace out possible genetic 

 relationships between them, and determine quantitatively the effect of each 

 imperfection on various physical properties. It is usually not possible to do 

 so unambiguously, but by measuring and correlating several physical pro- 

 perties and by making use of theory (which, unfortunately, can usually be 

 relied upon only to an order of magnitude), it is sometimes possible to arrive 

 at a self-consistent picture. It is obviously of advantage to use irradiation 

 studies in conjunction with other investigations, (a) because it is possible to 

 vary the concentration of radiation-induced imperfections in a controlled 

 manner, even if their absolute concentrations are uncertain, (b) because one 

 can estimate theoretically at least the order of magnitude of these concen- 

 trations more reliably than in the case of quenching or deformation, and 

 (c) because the principal imperfections differ from those produced by other 

 methods, being mainly vacancies and interstitials in equal concentration. 



The difficulties in the interpretation of the imperfections and of their 

 effect on physical properties may be illustrated by the case of copper, a 

 material to which much attention has been paid. 



In a pure metal the most convenient indicator of crystal imperfections is 

 the increase of electrical resistivity caused by them, though this increase 

 occurs to varying degree for all imperfections, and therefore does not identify 

 them. However, the resistivity is reduced to its original value by annealing, 

 since this removes the imperfections. The minimum temperature required 



276 



