On the deposition mechanism and irradiation behaviour of molecular plating thin films
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Abstract
Superheavy elements are produced by heavy ion fusion reactions using intense beams that irradiate suitable targets. For the heaviest elements, lead/bismuth and actinide targets are primarily used for this purpose. The production of actinide targets has been mainly based on the molecular plating (MP) method for decades.
The MP method is also widely used in the preparation of actinide and lanthanide samples for other research areas. The mechanism of the MP process has not yet been conclusively elucidated. The majority of work on the subject is limited to morphological studies. Spectroscopic studies have been limited to X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX). The reason for this limited spectroscopic data situation is to be found in the special challenges arising from the radioactivity of the actinide samples. Technical restrictions and legal requirements thus prevent access to modern methods, as the relevant laboratories lack the clearance to handle radioactive material. Besides radioactivity, high costs in acquisition and disposal prevent mass experiments, e.g. to optimise electrochemical parameters. Therefore, it is common to limit oneself to experiments on lanthanide samples. The present work was also limited to lanthanide samples, but radioactive tracers could be generated using the Mainz research reactor TRIGA Mark II. In this way, short-lived lanthanide isotopes were used to test working methods and analytical procedures that can also be transferred to future procedures with actinides. The electrochemical production of actinide targets is virtually limited worldwide to the MP process and some closely related methods. Developments in the technically more important field of lanthanide electrochemistry have not been transferred to target production. Thus, the established MP method is still limited by significant deposition of unwanted by-products and thus low maximum deposition rates of desired actinides.
When MP films are irradiated at the heavy ion accelerator, changes in the alpha spectra of the irradiated actinide thin films are a clear spectroscopic indication of chemical changes due to irradiation. Photographs taken before and after the accelerator experiments provide evidence of drastic morphological changes, which has been confirmed by some initial publications on lanthanide substitutes, using scanning electron microscopy (SEM). By understanding the chemical processes involved, it is hoped to improve the quality and stability of the MP targets used.
This dissertation presents two publications on mechanistic studies of lead and lathanide targets, presenting new insights into the MP process and the irradiation behaviour of the thin films. The third publication attempts to adapt established new methods for the electrochemical deposition of lanthanides to the needs of target production and to present the first irradiation tests with these thin films.