Deposition techniques for the preparation of thin film nuclear targets

Title Deposition techniques for the preparation of thin film nuclear targets
Author: Muggleton, A H F
Vacuum, 1987
More details: Of all the diverse physical, chemical and mechanical techniques used to make thin film nuclear targets, material deposition by vacuum evaporation is the most widely employed. This review commences with a brief description of the basic principles that regulate vacuum evaporation and the physical processes involved in thin film formation, followed by a description of the experimental methods used. The principal methods of heating the evaporant are detailed and the means of measuring and controlling the film thickness are elucidated. Types of thin film nuclear targets are considered and various film release agents are listed. Thin film nuclear target behaviour under ion bombardment is described and the dependence of nuclear experimental results upon target thickness and uniformity is outlined. Parameters such as thermal effects, radiation damage and sputtering of target material all influence the useful lifetime of thin nuclear targets. Target impurities can also have a serious effect upon experimental results; these effects are briefly considered. Special problems associated with preparing suitable targets for lifetime measurements are discussed. Carbon is used extensively as a target material and also for the manufacture of stripper-foils to change the polarity of the accelerated ions. The causes of stripper-foil thickening and breaking under heavy-ion bombardment are considered. A comparison is made between foils manufactured by a glow discharge process and those produced by vacuum sublimation. Consideration is given to the methods of carbon stripper-foil manufacture and to the characteristics of stripper-foils made by different techniques. Various methods to increase stripper-foil lifetimes are considered. Finally, techniques are described that have been developed for the fabrication of special targets, both from natural and isotopically enriched material, and also of elements that are either chemically unstable, or thermally unstable under irradiation. The reduction of metal oxides by the use of hydrogen or by utilising a metallothermic technique and the simultaneous evaporation of reduced rare earth elements is described. A comprehensive list of the common targets is presented.

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