Radiation hardening of a surface under ion bombardment

Nominally pure LiF crystals were irradiated by Kr ions (210 MeV) at fluences of 10¹⁰-10¹⁴ cm^-2 and changes in the optical absorption and microhardness investigated. These results were compared with those obtained from high-energy (5 MeV) electron irradiation. Under electron irradiation, microhardness increased only after F-centre saturation, but Kr ion bombardment was able to increase the microhardness with only a small dose. Under electron bombardment at a fluence of 10¹⁷ cm^-2 the MH increased by a factor of 2, whereas under ion bombardment the MH increased by a factor of 3. The increase in MH is assumed to be connected to a slowdown of dislocations at radiation defects. These defects cannot be point defects, such as F-centres, or simple aggregate centres, such as F₂ and F₃ or the hole centres complementary to them, because dislocation does not detect these centres. The slowdown is apparently caused by a defect containing no fewer than six particles.