Radioactive Molecular Beams at CERN-ISOLDE

Abstract

The present thesis addresses aspects of molecular beam developments for thick-target radioactive ion beam facilities such as CERN-ISOLDE. At these facilities, an intense and energetic driver beam impinges on a target. Radioisotopes are produced in a target matrix from which they require to diffuse into an ion source. On their way, numerous encounters with structural materials and the target material occur, which enable desired and undesired chemical interactions. The extracted ions are separated by their mass- to-charge ratio and supplied to various experiments for investigations e.g. in nuclear structure, reactions and applications. The motivation to extract radioisotopes as molecular beams is multifold. Firstly, the elements with the highest melting and boiling points (refractory elements) require embedding in a volatile carrier molecule to enable their transport to the ion source. Secondly, a common issue are isobaric contaminations which come along with the isotope of interest after mass separation. They can render impossible the intended user experiments. A powerful tool to produce and purify these beams is the extraction as molecular sideband which synthesizes volatile compounds in-situ and shifts the mass as seen by the mass separator away from the contamination. Finally, a field of investigation on radioactive molecules themselves was recently arising. This thesis is based on four publications which cover different facets of molecular beam developments. The first publication concerns the redesign and implementation of the ISOLDE yield database which provides valuable information of available beams and demonstrates the need for molecular beam developments. This implementation also contains new data and models to predict ion beam yields based on measured data. The first extraction of radioactive boron beams at a thick-target ISOL-facility is discussed in the second publication. Boron is a reactive and refractory element which cannot be extracted in elemental form. Its release properties from multi-walled carbon nanotubes are described. The measured yield of 8B is presented along with predictions for yields of 12B and 13B, based on the derived release model. As an example for beam purification by molecule formation, the third publication provides investigations by online and offline measurements towards a reliable extraction of carbonyl selenide sidebands.The investigations towards extraction of transition metals as carbonyl complexes are a major contribution to this thesis. Most of the refractory transition metals are not avail able as ISOL beam since no suitable carrier molecules to enable their transport have been identified yet. This is often due to the harsh conditions in the target an ion source unit that typically operates at high temperatures. It was studied by simulation, calculation and exploratory experiments, if carbonyl compounds could be used within a cold target concept. To avoid slow diffusion processes, the recoil momentum in fission and spallation reactions is exploited. Transition metal carbonyl compounds (e.g. Mo(CO)6) form in-situ upon their thermalization in a carbon monoxide containing atmosphere already at ambient pressure and temperature. However, these are delicate compounds which easily decompose when exposed to heat, electron bombardment or plasma. The included publication presents first exploratory experiments with an electron beam induced arc discharge ion source operated at ambient pressure. In contrast to classical thermionic electron production, electrons are liberated by a laser from a cold ion source.

The addendum of the thesis documents experimental results towards ionization of the fragile carrier molecule Mo(CO)6 with available ISOLDE ion sources. Their ionization efficiency turned out insufficient and underlines the need for development of a cold electron impact ion source, as proposed in the publication. Moreover, an experimental setup has been built and tested to study the formation of neutral transition metal carbonyl complexes at ISOLDE. The included documents describe its purpose, mode of operation and management of involved risks.