Inert gas bubbles in neutron-irradiated magnesium oxide

In magnesium oxide, irradiated with neutrons at 150°c, 600°C and 1000°c to doses greater than 10 20 nvt> 1 Mev, rectilinear bubbles have been observed by transmission electron microscopy after annealing at temperatures above 1500°c. In specimens irradiated at 150°c the bubbles had {100} surfaces and sides ranging from 40 to 400 Å and measurements of the residual growth of the crystals after annealing have been correlated with the volume occupied by the bubbles. Electron microscope and gas release studies indicate that the bubbles form primarily by vacancy condensation and subsequently act as sinks for the diffusion of neon and helium produced by the transmutation of magnesium and oxygen. It is suggested that mechanical release of these gases, by crushing the crystals at room temperature, could take place by two processes: the intersection of bubbles by cracks and by transport of gas via irradiation-induced dislocations. In a crystal irradiated at 1000°c, large precipitates were formed that, on annealing, dissolved, leaving behind bubbles up to 2000 Å in size, having a variety of crystallographic shapes.