Jens Lassen

Resonance ionization spectroscopy of fermium (Z=100)☆☆☆

Laser spectroscopy has been applied for the first time to measure resonant transition frequencies of fermium (Zs 100). A number of 2.7=10 atoms was electrodeposited on a Ta filament and covered with a 1 mm Ti layer. Fm 10

An ion guide laser ion source for isobar-suppressed rare isotope beams

Modern experiments at isotope separator on-line (ISOL) facilities like ISAC at TRIUMF often depend critically on the purity of the delivered rare isotope beams. Therefore, highly selective ion sources are essential. This article presents the development and successful on-line operation of an ion guide laser ion source (IG-LIS) for the production of ion beams free of isobaric contamination. Thermionic ions from the hot ISOL target are suppressed by an electrostatic potential barrier, while neutral radio nuclides effusing out are resonantly ionized by laser radiation within a quadrupole ion guide behind this barrier. The IG-LIS was developed through detailed thermal and ion optics simulation studies and off-line tests with stable isotopes. In a first on-line run with a SiC target a suppression of surface-ionized Na contaminants in the ion beam of up to six orders of magnitude was demonstrated.

RIMS measurements for the determination of the first ionization potential of the actinides actinium up to einsteinium

The ionization potential is a fundamental property of an element. In order to determine the first ionization potential, a method based on the measurement of photoionization thresholds in the presence of a well-defined electrical field is used. By one or two step resonant laser excitation, the investigated atoms are promoted to a highly excited state. Ionization is obtained by scanning the wavelength of an additional tunable laser across the threshold Wth, which is detected by a sudden increase of the ion signal. Extrapolation of Wth to zero field strength yields directly the first ionization potential. Using this method the first ionization potentials of Ac, Am, Cm, Bk, Cf, and Es have been determined for the first time. Furthermore, the ionization potentials of Th, U, Np, and Pu were remeasured.

Nuclear and in-source laser spectroscopy with the ISAC yield station

A new decay station has been built for the ISAC facility at TRIUMF for the rapid and reliable characterization of radioactive ion beam (RIB) compositions and intensities with the capability of simultaneously collecting α, β, and γ decay data from RIB with intensities between a few and ≈10(11) ions per second. It features user-friendly control, data acquisition, and analysis software. The analysis of individual decay time structures allows the unambiguous assignment of α and γ lines even with substantial isobaric contamination present. The capability for accurate half-life measurements is demonstrated with the example of (46)K. The coupling of the yield station to the laser ion source, TRILIS, allows the correlation of radiometric data with automated laser frequency scans. First results of in-source laser spectroscopy measurements on astatine are discussed.

First Determination of the Ionization Potential of Actinium and First Observation of Optical Transitions in Ferminm

For the determination of the first ionization potential of actinium, 227Ac was electrodeposited on a Ta backing and covered with ~1 μm Zr. From this filament, Ac atoms were evaporated at ≥ 1250 °C. By resonant excitation with UV light of 388.67 nm and subsequent excitation with light of ca. 568 nm, Ac was ionized in an external electrical field. By determining the ionization thresholds as a function of the electrical field strength and by extrapolation to zero field strength, the first ionization potential of 43398(3) cm−1 = 5.3807(3) eV was measured. About 1 ng of 255Fm, half life 20.1 h, was prepared at ORNL by milking from 255Es produced in the High Flux Isotope Reactor and shipped to Mainz. Here, a fraction of 2.7 · 1010 atoms was electrodeposited on a Ta backing and covered with ~ 1 μm Ti. From this filament, Fm atoms were evaporated at ~ 1000 °C in the buffer gas of an optical cell and resonantly excited and ionized with two beams of an excimer-dye-laser combination. The ions were identified by mass-selective detection employing the Ion-Guide-Quadrupole-Mass Separation (IGQMS) technique. Two resonant transitions were found at energies of 25099.8 cm−1 and 25111.8 cm−1.