CHARACTERISTICS OF THE DISLOCATION STRUCTURE OF LiF CRYSTALS DEFORMED IN CONTACT WITH LIQUID HELIUM.

An investigation was made of the dislocation structure of LiF crystals (doped with similar 5 multiplied by 10** minus **4 wt. % Mg**2** plus ) deformed in contact with liquid **4He and **3He. A local reduction (by a factor of 2) in the density of screw dislocations N//s was observed in a surface layer whose thickness (5-10 mu ) was correlated with the depth of penetration of helium into crystals when they were deformed in liquid helium. The values of N//s were constant over the cross sections of crystals deformed at 4. 2 and 300 degree K in vacuum. The results led to the conclusion that the local reduction in N//s near the outer surfaces of LiF crystals was due to the ''dynamic diffusion'' of helium atoms along newly created and moving dislocations, which hindered the dislocation multiplication process and was probably of athermal origin.