Experimental studies of vacuum evaporated thin film nuclear targets
Creators: name: Muggleton, Albert Harold Frank
Publisher: name: The Australian National University
Year: 1984
Subjects: subject: Thin films
subject: Thin film devices
subject: Vacuum technology

description: 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 thesis commences with a description of the basic principles that regulate vacuum evaporation and the physical processes involved in thin film formation. Next follows a description of the experimental methods used. The principal methods of heating the evaporant are detailed and means of measuring and controlling film thickness are elucidated. Various types of thin film nuclear targets are considered and the results of an experiment, undertaken to verify that the film release agent has an influence on film surface topography, are reported. 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 the 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 considered in detail. Special problems associated with preparing suitable targets for lifetime measurements, utilising Doppler-Shift phenomena, are discussed. Carbon is used extensively as a target backing material and also for the manufacture of stripper foils to change the polarity of the accelerator beam. The effects of heavy-ion irradiation of carbon foils are discussed and the causes of stripper foil thickening and breaking under heavy-ion bombardment are considered. The crystal structural changes caused by radiation damage are described and a comparison 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. The development of techniques for increasing stripper foil lifetimes is discussed at length. 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 series of techniques for the preparation of isotopic oxygen targets are investigated and comparison made between the isotopic enrichment of the different targets. A detailed description is given of the technique developed to produce thin, single-crystal gold films, utilising a self-latching electromechanical shutter of novel design for accurate control of film thickness. Transmission electron microscope micrographs and selective area diffraction patterns were used to evaluate the mono-crystal gold films produced.