Resonance ionization spectroscopy of thorium isotopes - towards a laser spectroscopic identification of the low-lying 7.6 eV isomer of Th-229

TL;DR

This study developed a resonance ionization spectroscopy method for thorium isotopes, identifying new energy states and ionization schemes, to aid in laser spectroscopic detection of the Th-229 isomer near 7.6 eV.

Contribution

It presents a new efficient three-step ionization scheme for thorium, with detailed level energy measurements and isotope shift data, advancing the spectroscopic identification of Th-229m.

Findings

Identified over 20 new intermediate states of thorium.

Measured isotope shifts for Th-228, Th-229, Th-230 relative to Th-232.

Achieved an overall detection efficiency of 0.6.

Abstract

In-source resonance ionization spectroscopy was used to identify an efficient and selective three step excitation/ionization scheme of thorium, suitable for titanium:sapphire (Ti:sa) lasers. The measurements were carried out in preparation of laser spectroscopic investigations for an identification of the low-lying Th-229m isomer predicted at 7.6 +- 0.5 eV above the nuclear ground state. Using a sample of Th-232, a multitude of optical transitions leading to over 20 previously unknown intermediate states of even parity as well as numerous high-lying odd parity auto-ionizing states were identified. Level energies were determined with an accuracy of 0.06 cm-1 for intermediate and 0.15 cm-1 for auto-ionizing states. Using different excitation pathways an assignment of total angular momenta for several energy levels was possible. One particularly efficient ionization scheme of thorium, exhibiting saturation in all three optical transitions, was studied in detail. For all three levels in this scheme, the isotope shifts of the isotopes Th-228, Th-229, and Th-230 relative to Th-232 were measured. An overall efficiency including ionization, transport and detection of 0.6 was determined, which was predominantly limited by the transmission of the mass spectrometer ion optics.