Penning-trap mass spectrometry: commissioning of PIPERADE and measurements of neutron-rich A = 100-120 nuclei at JYFLTRAP

Abstract

Penning-trap mass spectrometry allows to measure masses of atomic nuclei with a very high precision. Nuclear binding energies, determined from the atomic masses, are used to study the structure of exotic nuclei, and are also key inputs for nucleosynthesis models. In this thesis, two Penning traps have been utilized. The PIPERADE (PIèges de PEnning pour les RAdionucléides à DESIR) double Penning trap was commissioned at the LP2i Bordeaux. The results from the first trapping of ions, the implementation of the buffer gas cooling technique, a method to select the ions of interest, all the way to the results of the first time-of-flight ion-cyclotron-resonance (TOF-ICR) mass measurement, are reported. PIPERADE will be dedicated to precision mass measurements and ion of interest selection in the new research facility DESIR (Désintégration, Excitation et Stockage d’Ions Radioactifs) at the GANIL (Grand Accélérateur National d’Ions Lourds) Accelerator laboratory. The JYFLTRAP double Penning trap was utilized at the Ion Guide Isotope Separator On-Line (IGISOL) facility of the Accelerator Laboratory of University of Jyväskylä to perform precision mass measurements of neutron-rich rhodium, ruthenium, molybdenum, niobium, zirconium and yttrium isotopes in the mass region <i>A</i> ≈ 100 − 120. In total 15 ground- and eight isomeric-state masses were measured. Altogether eight masses were determined for the first time, while for the rest the precision of the atomic mass was improved significantly. The phase-imaging ion-cyclotron-resonance technique (PI-ICR) allowed for the first time the separation of the isomeric states from the ground states, and precision mass measurement of each state independently. The most exotic rhodium, ruthenium, molybdenum and niobium isotopes were measured using the TOFICR technique. In addition, the half-lives of the ground and isomeric states of ¹¹²Rh were measured. From the measured masses, several observables, such as the two-neutron separation energies, were extracted and compared to the values predicted by the Hartree-Fock-Bogoliubov based global mass models BSkG1 and BSkG2. The impact of the new mass values on the neutron-capture rates needed for the astrophysical rapid neutron capture process was also studied. Keywords: Penning trap, PIPERADE, JYFLTRAP, isomers, nuclear structure.