Towards Gas Phase Ion Chemistry of Transactinide Elements

The study of fundamental physical and chemical properties of $Z\geq104$ elements, also known as transactinide elements (TAN), are difficult because TANs can only be produced from nuclear reactions where the production rates are as low as 1 particle/week. The low production rates of TANs require prolonged experiments (in excess of 8 weeks) at accelerator facilities such as the 88-Inch Cyclotron Facility necessitating the need for stockpiles of projectile material, such as isotopically enriched $^{48}$Ca and $^{50}$Ti (isotopes that are critical for TAN studies), and targets that can withstand irradiation for up to weeks at a time. Efforts were made to establish in-house reduction capabilities for isotopically enriched $^{48}$Ca and $^{50}$Ti since the material was purchased and received in a chemical form not suitable for ion beam generation. Additionally, the circular target employed in TAN studies was re-designed. The re-design was prompted by observed target damage from a recent TAN spectroscopy experiment as a result of an over-focused beam. Simulations of target heating were undertaken to guide the target re-design before being fabricated and characterized in the $^{209}$Bi($^{48}$Ca, 2n)$^{255}$Lr reaction./nThe identification of fundamental physical and chemical properties such as reaction rate constants, ionization potentials, and bond dissociation energies of elements is critical for their correct placement on the periodic table. Theory predicts that TANs will have chemical behavior that differs from their lighter homologs due to relativistic effects that arise from the strong Coulombic interaction between the orbiting electrons and the high-$Z$ nuclei. Due to the difficulty of studying TANs, few direct measurements of these properties have been made. In order to improve upon current theoretical understanding, experimental measurements of these properties are required. Recent efforts and results are presented and discussed for the development of an on-line gas-phase ion chemistry technique at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron Facility to investigate the chemical behavior of TANs./nThe gas-phase ion chemistry technique was tested and performed with the recently commissioned apparatus For the Identification of Nuclide A (FIONA) coupled downstream to the Berkeley Gas-Filled Separator (BGS). Separated nuclear reaction products from the BGS enter FIONA where they are first stopped and thermalized in the 1+ or 2+ charge states in a radiofrequency (RF) gas catcher (GC). The ions are then extracted from the GC where they are bunched and trapped in a linear radiofrequency quadrupole (RFQ) trap. While trapped, the ions are exposed to a reactive gas for a period of time (10 ms to 1000 ms). After the set reaction time, the RFQ-trap is dumped and the ions are separated according to their mass-to-charge ratio (\textit{A/q})./nThe first reactions utilizing this technique were performed with O$_2$ as the reactive gas. The reaction between Ho$^+$ and O$_2$ gas to form HoO$^+$, a previously studied reaction, was first performed to calibrate the system for O$_2$ gas. The calibration was required because the pressure of O$_2$ at the RFQ-trap during measurements could not be directly measured. In order to determine the oxygen concentration in the RFQ-trap the measured reaction rate constant and the known reaction could be used with an estimation of the O$_2$ pressure from a residual gas analyzer (RGA) located approximately two meters away from the reaction site. Upon completion of the calibration of O$_2$ gas with the Ho$^+$ reaction, O$_2$ was used as the reactive gas to investigate gas-phase ion chemistry of No ($Z=102$) and Lr($Z=103$), elements at the end of the actinides where few chemical studies have been performed to date. These investigations focused on measuring reaction rate constants of the reduction of 2+ ions of No and Lr with O$_2$ gas to the 1+ charge state as well as investigating the formation of NoO$^+$ and LrO$^+$ from the interaction between No$^+$/Lr$^+$ with O$_2$ gas. The results from the calibration reaction and of No and Lr in the gas-phase are presented and discussed as well as prospects for future gas-phase ion chemistry experiments of TANs.

Advisor: Cerny, Joseph