Characterization of the spectral resolution of the in-gas-jet laser ionization spectroscopy method

Laser spectroscopy studies, which are complementary to nuclear spectroscopy in the actinides and superheavy elements (SHE) region, give access to nuclear ground- and isomeric-state properties, such as mean-square charge radii, magnetic dipole and electric quadrupole moments and spins. The information on these observables is essential to validate nuclear models and to increase their predictive power. However, the efficient separation and study of the actinides and SHE is a challenging task which requires a high-resolution and high-efficiency experimental technique. High spectral resolution ensures that the uncertainties on the extracted nuclear properties will be dominated by the uncertainties from atomic theory rather than by the experimental errors, while the high efficiency enables the studies of the elements produced at a small rate of about 1 atom in 10 s. With the in-gas-jet method, which is an extension of the In Gas Laser Ionization and Spectroscopy (IGLIS) technique, the spectral resolution can be improved by more than one order of magnitude in comparison to in-gas-cell ionization spectroscopy, while maintaining a high efficiency. In this method, the ionization of the isotopes of interest takes place in supersonic gas jets with flow velocities exceeding the local speed of sound. An exhaustive characterization of the in-gas-jet method was performed at the offline IGLIS laboratory with a goal to further investigate the limits of the attainable spectral resolution. The flow properties of supersonic jets to be used for high-efficiency and high-resolution IGLIS studies of short-lived nuclei have been characterized in offline conditions using Planar Laser-Induced Fluorescence (PLIF)-spectroscopy on copper atoms. PLIF-spectroscopy was validated prior to this by applying it on well-known flow parameters of supersonic free jets. It was shown that long (> 80 mm) and quasi-uniform jets were formed when the nozzle's inner contour was optimized on the µm scale, manufactured with high accuracy and minimal surface roughness and the pressure was matched between the nozzle exit and a background medium. The projected temperature associated (Doppler) broadening, that can be attained with the upgraded in-gas-jet method, was estimated to be about 140 MHz for the nobelium isotopes. As an outlook, the future steps of the characterization and enhancement of the in-gas-jet method are discussed. Some technical details, which enables the prominent performance of the IGLIS setup, are given as well.status: publishe