The preparation of thin film nuclear targets by vacuum deposition techniques

A survey of other techniques used in the preparation of nuclear targets is first briefly discussed. These can be roughly classified as chemical, mechanical and physical proparations. Vacuum evaporation is the most generally used technique and will be discussed in detail. A typical system for high vacuum evaporation is described and the three main methods of heating the target material to evaporating temperatures are discussed (i.e. conduction heating, radio frequency heating and heating by means of electron-bombardment). Various electron-bombardment guns are compared in greater detail. Stages in the fabrication and mounting of self-supporting foils as thin as 5 μg/cm2 are described in detail, with emphasis given to the preparation of thin self-supporting carbon foils used as target backings and stripper foils. A technique for the preparation of relatively thick (1-10 mg/cm2), isotopically pure, rare-earth targets (Yb, Sm, Dy, Er, Nd, Gd, Lu) is given. The simultaneous reduction of the oxide and deposition of the metal is described. Final fabrication of the target by mechanically rolling the small head of condensed metal between strips of molybdenum, is described in detail. The need for, and achievement of, target purity and conservation of expensive materials, is emphasised and discussed. (author)