Fission fragment damage to crystal lattices: heat-sensitive crystals

Abstract

An earlier paper (Bowden & Chadderton 1962) has described the damage produced by uranium fission fragments in the lattices of stable crystals. There is evidence that the lattice disarray is caused by both displacement and ionization processes. This paper deals with damage in thermally unstable crystals. Thin single crystals of silver cyanamide, the thiocyanates of silver and mercury (II), the picrates of ammonium, potassium and thallium, ammonium iodate, and ammonium borofluoride have been irradiated and directly examined by transmission electron microscopy. A considerable variation was found in the type and amount of damage. According to the irradiated material, each fragment produces a track of up to 140Å in width, which can represent either missing material (white track) or an alteration to the crystal structure (black track), and which can be either continuous or discontinuous. With some materials the damage does not take the form of a visible track. The observed amount of damage is found to be consistent with that calculated on the basis of a model of damage by heat alone, involving either decomposition or evaporation. An important parameter is the radius of the cylindrical volume within which the energy lost by the fission fragment appears as heat. This radius is determined primarily by the range of the electrons which are released by the passage of the fragment, and which transfer its energy to the crystal lattice. It is likely that only in an intrinsic semiconductor is the range of these electrons low enough for the production of visible tracks in single crystals. The possibility of further damage or explosion, subsequent to irradiation, is briefly considered.