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Open Access Publications from the University of California

The development and characterization of the novel heavy element mass separator FIONA and the investigation of short-lived isomeric states in ^{255} Lr

  • Author(s): Esker, Nicholas
  • Advisor(s): Cerny, Joseph
  • et al.

A \ce{^{209}Bi}(\ce{^{48} Ca}, 2n)\ce{^{255} Lr} reaction was performed using the Berkeley Gas-filled separator (BGS) at the 88$''$ cyclotron in Lawrence Berkeley National Laboratory. This class of heavier actinide readily populates high angular momentum $K$-states with $>1$ms lifetimes, allowing for their separation and detection. The $K$-isomers of \Lr were identified by their conversion electron decays and their nuclear levels measured using coincident $\gamma$-ray spectroscopy. With a beam energy of 232 MeV, almost ten times the number of previous reported converted transitions were observed. From these higher statistics, the preliminary conclusions are presented, including the eight new low-lying rotational states observed in the de-excitation of \ce{^{255}Lr}. These isomeric studies provide a valuable view to the nuclear structure of the heaviest elements, testing the current explanations and models.

The BGS is currently being upgraded with a secondary, mass sensitive secondary separator to improve detection sensitivity and expand research scope. For this separator, known as FIONA, simulations were used to investigate the effective separation and transport efficiency. Based on crossed electric and magnetic fields, the ion trajectories in a variety of configurations were modeled. The degree of mass separation was explored to determine this approach's viability with respect to heavy and superheavy element mass identification. A simulated mass resolution of $R = 784 ~ A/ \Delta A$ is reported, indicating the required mass sensitivity for superheavy elements is achievable.

A testing set-up of the FIONA mass separation scheme was built and characterized to verify the simulated behavior. Initially designed from an available 0.7 T dipole magnet and a custom electrode array, the independent beam-line and separator was built to record the ion trajectories and compare them to simulation. Using an off-line ${}^{nat}$Xe${}^{+}$ ion source, a 0.5~keV ion beam was delivered to the crossed field region with $\vec{B} = 0.3667$ T and $\vec{E} = 1240$ V. For the ${}^{nat}$Xe${}^{+}$ ions, which have a similar $m/q$ as a superheavy element, a mass resolution of $\bar{R} = ~ 628.6 A/ \Delta A$ is reported, well within the necessary predictions for superheavy element mass separation. Agreement with simulation and further prospect with the BGS is discussed.

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