Optical Parametric Oscillators for Spectroscopy of Actinides

Abstract

The interactions between a nucleus, composed of neutrons and protons, and its surrounding electronic cloud, are governed by fundamental electromagnetic forces. As the nucleus it is not a point-like charge but possesses a finite volume and shape, these interactions can be observable through small shifts in the energy of an atomic transition known as the hyperfine structure. By probing and measuring these shifts, information about the nucleus' size and shape can be obtained. These nuclear characteristics are ultimately determined by the interactions of protons and neutrons within the nucleus, governed by the fundamental strong nuclear force. Understanding these interactions at the finest level requires high-resolution laser spectroscopy, which reveals fundamental properties of a nucleus, such as spins, magnetic and electric moments, and charge radii. Precise measurements of these observables are essential for testing state-of-the-art theoretical models and exploring the limits of nuclear existence.

Experimental investigations of the hyperfine structure face significant challenges due to low production yields, short half-lives, and limited laser technology, in particular for heavy and superheavy elements with transitions in the ultraviolet range (330–370 nm). To address these limitations, this work implemented and validated a solid-state laser system based on a continuous-wave optical parametric oscillator (cw-OPO) combined with pulsed amplification. This approach enables narrow-linewidth, high-energy pulses with optical linewidths on the order of 100 MHz, covering spectral gaps often inaccessible to conventional Titanium:Sapphire (Ti:Sa) and dye lasers. The system was successfully used for high-resolution spectroscopy of exotic silver isotopes via different Doppler-free techniques at the radioactive ion beam facilities ISOLDE-CERN and IGISOL-JYFL. Furthermore, high-resolution spectroscopy at the RISIKO mass separator was performed on fermium and neptunium isotopes. Using different two-step excitation schemes and advanced laser ion-source techniques, the nuclear ground-state moments of these heavy nuclei were determined.

These results demonstrate that OPO-based laser systems offer a versatile and efficient solution for extending high-resolution spectroscopy to new regions of the nuclear chart. Future developments include a fully solid-state cw-OPO seeded optical parametric amplifier to further enhance wavelength coverage and stability. By bridging technological gaps and expanding the experimental toolkit, this thesis contributes to the groundwork for future laser spectroscopy studies at upcoming facilities like at GSI-FAIR and the S3 LEB at GANIL, where access to short-lived nuclei will push the limits of nuclear knowledge and technology.