IMPl'RFKCriOXS IM)l(;i,l) l.\ SOLIDS U\ IAS 1 -1' \K I ICI.I, IRRADIATION 



to impurities of neutron-irradiated copper that the removal of intcrstitials 

 during stage I is not a simple process of point defect diffusion. 



These considerations illustiate the complexity of the ijroblem. It is almost 

 certain that our ideas on this subject will undergo some change before 

 attaining finality. 



THERMAL CONDUCTIVITY: EFFFX.T OF IRRADIATION 



Another physical property which is very sensitive to radiation damage is 

 the thermal conductivity. This is particularly so for non-metals, where the 

 thermal conduction is due to the transport of energy by lattice waves. The 

 thermal conductivity is then given l)y: 



K = -^ j S{<jo) V I (co) doj (4) 



where S{(x)) dot is the contribution of waves of frequency cu, dcu to the specific 

 heat per unit volume, and /(cu) is the mean free path of lattice waves. 

 Lattice waves are scattered by the anharmonicities of the lattice forces, by 

 lattice imperfections and by the crystal boundaries. The temperature 

 dependence of the conductivity is governed by the temperature and fre- 

 quency variation of l(co), and since different imperfections scatter with 

 different frequency variations, it is possible to identify the dominant imper- 

 fections from their thermal resistance-^- ^^^ ^^. It is, however, necessary to 

 study the thermal conductivity over a wide range of temperatures, usually 

 from liquid helium to licjuid air temperatures. 



The sensitivity of the conductivity to lattice imperfections and the 

 possibility of identifying them lends interest to thermal conductivity studies 

 of irradiated solids. This applies, of course, only to lattice thermal con- 

 ductivities: the electronic thermal conductivities of metals only yield the 

 same information as the electrical resistivities-^. In the case of alloys, 

 however, it is possible to separate out the lattice component of thermal 

 conductivity, and this can yield information about lattice imperfections-^- ^^. 

 While this technique could, no doubt, be applied to irradiation damage in 

 alloys, no such investigation has yet been published. 



The first measurements of the effect of irradiation on the thermal 

 conductivity were made on neutron-irradiated quartz^- -^' -'. Measurements 

 were made of the thermal conductivity of the original crystal, and of the 

 same crystal after three successive doses of neutron irradiation. From the 

 temperature variation of the additional thermal resistance it was deduced 

 that at least two types of imperfections were present : isolated point defects 

 and more extensive regions of damage. The latter would, of course, be 

 expected from the theory of radiation damage, as displacements should be 

 clustered near the end of the range of a knocked-on atom. 



Berman also studied the annealing behaviour of this thermal resistance. 

 No change occurred at annealing temperatures lower than 300"C; this, 

 together with the proportionality of the resistance to exposure, indicates that 

 no recovery took place during irradiation. 



It is very noticeable that the thermal conductivity curves of the heavily 

 irradiated specimen approach that of fused silica, and Klemens suggested 

 that the regions of extensive damage are vitreous inclusions. In a localized 



278 



