Preparation of Nuclear Accelerator Targets by Vacuum Evaporation

In many cases an accelerator target for physical or chemical research work has to be prepared in form of a uniform, as far as possible self supporting thin film of thickness 0.1-10 μm. Typical target materials are stable or radioactive isotopes for nuclear physics and nuclear chemistry experiments, mixtures of different elements for material analysis with ion beams as well as single crystal samples for special solid state physics applications. A powerful method to prepare thin film nuclear accelerator targets is the procedure of vacuum evaporation-condensation. In the present paper the physical and technological background of this method is discussed and the construction of suitable evaporation systems and related equipment is displayed. Different sublimation techniques including resistance-, induction- and electron beam heating as well as ion beam sputtering are described considering their specific benefits and disadvantages. Particular attention is paid to the collection efficiency of these various techniques in view of the low abundance and high cost of isotopically enriched target material. Since in many cases a target backing severely disturbs the actual measurement, emphasis is also given to procedures yielding unsupported high purity targets. In addition, special techniques for preparation, handling and storage of chemical reactive targets and of radioactive targets are discussed. Finally the preparation of single crystal thin film targets is briefly mentioned as well as target preparation by chemical vapour deposition, which in a physical sense is closely related to the vacuum evaporation-condensation method.