IMPERFECTIONS INDUOED IN SOLIDS BY FAST-PARTICLE IRRADIATION 



density, I)ut c\xmi exceeded it for heavier exposures. It thus appears that the 

 lattice is expanded mainly by the isolated point defects, and becomes less 

 rigid as the defect concentration is increased. 



Cohen (reference 36 and private communication) measured the thermal 

 conducti\ity of fused quartz before and after neutron irradiation. As 

 predicted by Klemens,^- the thermal conductivity increased on neutron 

 irradiation, due to the destruction of short-range order. After prolonged 

 neutron irradiation, the thermal conductivity curve reached a terminal 

 form, showing that the effects of irradiation saturate. On annealing, the 

 original (vitreous silica) curve was recovered. When crystalline quartz was 

 irradiated and subsequently annealed, the conductivity curve approached 

 that of the unirradiated crystal, as previously found by Berman'-". However, 

 on prolonged irradiation, a saturation curve was reached, identical with 

 the saturation curve of irradiated quartz. When the material was now 

 annealed it reverted, not to the form of the original crystal, l)ut to the form 

 of unirradiated glass; in other words, the heavily irradiated crystal had 

 become completely amorphous, losing all memory of its original crystalline 

 form. 



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280 



