A new vacuum-ultraviolet spectroscopy setup (Resonance Ltd, VM180) was characterized and connected to the beamline at ISOLDE, CERN. Unfortunately, pure actinium-229 beam was not available at ISOLDE, but a beam of mass A=229, mostly comprising of francium-229 and radium-229, both of which decay into actinium-229 was available. During the experimental campaign IS658, beam of mass A=229 was implanted onto calfium fluoride, (5 mm thick crystal from Thorlabs) as well as 50 nm thick foil on silicon substrate produced at IMEC and magnecium fluoride crystals (5mm crystal from Thorlabs). The VUV photons from the de-excitaion of the isomer was observed for the first time enabling the measurement of the isomer's excitation energy to a precision of 0,4 nm, dominated by systematic uncertainties. The measured excitaion energy value is a seven fold improvement in the precision compared to the previous results. The lifetime of this VUV-signal of the thorium-229 isomer implanted on a magnesium fluoride was also measured and the measured values are in good agreement with the theoretical predictions.
Improvements in the VUV-spectrometer as well as the data acquisition system was performed after the IS658 measurements at ISOLDE. To improve the precision of the isomer's excitation energy, new optimized calibration method and experimental procedures were developed to minimize the systematic uncertainty. Another experiment campaign IS715 was performed in July, 2023, end of the fellowship period. Several different crystals were tested during this experiment campaign and the VUV-efficiency and the isomer' lifetime have been measured in different crystals.
The second part of this project was the measurement of the hyperfine structure of the isomer and the ground state of a singly charged thorium-229 ion. An important first step for this part was a develpment of an efficient ionization scheme. Due to the lack of such efficient ionization scheme, this part of the project was delayed in order to search for an efficient ionization shcme and to measure the second ionization potential of thorium-229. A more efficient ionization scheme has been developed and the measurements for the second ionization potential is still ongoing.
