Absorption of optical radiation by nanoclusters of sputtered metallic uranium and its oxides

Abstract

Thin films of uranium-containing materials are used as energy-releasing elements in nuclear-pumped lasers. They serve as a source of fission products which ionize and excite the gaseous medium. In this pumping method, a uranium layer is sputtered by its own fission products. This is one of the factors that limit the service life of energy-releasing elements. The products of sputtering can enter the gas and affect the optical and kinetic properties of laser media. In the present work, calculations are performed of the electrodynamic cross-sections for photoabsorption of optical radiation by nanoparticles of metallic uranium and its oxides. The main space-time relaxation parameters are determined. The effect of sputtered uranium-containing nanoparticles on the absorption of optical radiation in a laser element is evaluated.Thin layers of uranium-containing materials are used as energy-releasing elements in the nuclear-pumped lasers. They serve as a source of fission products which ionize and excite the gaseous medium. As a rule, thin-film uranium-containing coatings are characterized by a finely dispersed structure and high porosity, which are due to the method used to obtain the films (magnetron spraying, deposition from the vapor phase, and others). Such coatings have anomalously high sputtering coefficients, reaching 10 3 -10 5 atoms per fission fragment [1]. If it is assumed that 10 4 uranium atoms enter the gas together with a fission fragment, which at the exit from the uranium layer possesses characteristic energy 40 MeV, then the typical energy input 1 J/cm 3 during nuclear pumping will correspond to the average density of sputtered atoms 1.5·10 15 cm −3 . This quantity of contaminants, for example, water vapor, is sufficient not only to weaken but also make it completely impossible to obtain lasing. In reality, the picture is more complicated. Atomization of the coating material into individual atoms does not occur. Most of the sputtered material is contained in clusters ranging in size from several to tens of nanometers [2].The objective of the present work is to study the optical properties of sputtered nanoparticles containing uranium and to develop recommendations for eliminating the harmful effect of the products of sputtering and for extending the service life of laser elements.The coefficient of sputtering of thin-film coatings by fission fragments depends on many factors, the most important ones being the microstructure of the film and the structure of the boundary between crystallites. The physical reason for the anomalous sputtering of coatings is evaporation of surface grains as result of their being heated by decelerating fission fragments [1]. The probability of sputtering of a grain as a whole is determined by the ratio of the energy absorbed inside a grain, the flux of energy on neighboring crystallites through the complicated intergrain boundary, and the energy required to detach a grain from the substrate. The model of a completely isolated grain gives a qualitative description of the sput...