Negative Ion Source Development and Photodetachment Studies at ISOLDE

Astatine is one of the rarest elements on earth. The small amount of existing astatine is either created in decay chains of heavier elements or artificially. One of its longer lived isotopes, 211At, is of interest for targeted alpha therapy, a method of treating cancer by using the alpha decay of radioactive elements directly at the location of a tumor. However, its chemical properties are yet to be determined due to the short life time of astatine. A milestone towards the determination of the electronegativity of astatine was the measurement of its ionization potential (IP) at CERN-ISOLDE. However, its electron affinity (EA, the binding energy of the additional electron in a negative ion), is still to be measured. In order to determine the EA of radioisotopes by laser photodetachment spectroscopy, the Gothenburg ANion Detector for Affinity measurements by Laser Photodetachment (GANDALPH) has been built in recent years. As a proof-of-principle, the EA of the 128I negative ion, produced at the CERN-ISOLDE radioactive ion beam facility, was mea- sured. This was the first ever photodetachment measurement of a radioactive isotope. During those first campaigns, it was evident that the astatine experiment would not be successful under the current conditions. In this thesis, steps are taken to address the issues from two sides: Increase the ionization efficiency of elements with low electron affinity by finding new low work function materials and improving the GANDALPH beam-line. For the negative ion source development, a dedicated ion source test stand was con- ceived and constructed. Candidates for new ion source materials were determined via x-ray diffraction. The electron emission of the samples were investigated and compared to the standard source material, LaB6, as a benchmark. Second, several upgrades to im- prove the beam tuning capabilities of the GANDALPH beam-line were implemented. A dedicated negative ion source was conceived and constructed which was used for off-line testing and fine-tuning of the neutral atom detector, as well as the electrostatic elements in the beam-line. These results will be presented and the feasibility to conduct an EA measurement of astatine will be discussed.