Experimental studies of vacuum evaporated thin film nuclear targets
Date
1984
Authors
Muggleton, Albert Harold Frank
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Abstract
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.
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