Precision measurements with trapped radioactive ions using the Canadian Penning Trap Mass spectrometer and associated ion traps at the Argonne National Laboratory

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This work centres on the precise measurement of the atomic masses and other properties of nuclei far from stability. The results provide information on the energy released in nuclear reactions involving nuclei in these exotic regions. Such information is important in predicting the rates and pathways of nuclear reactions that power explosive astrophysical events that are candidates for the nucleosynthesis of the heavier elements (through the rp-, nu-p- and r-processes). They also allow the study of fundamental symmetries in particle physics theories and the evolution of nuclear structure in regions where the nuclei have extreme neutron-to-proton ratios. The measurements are conducted with the Canadian Penning Trap Mass Spectrometer (CPT) which operates on-line to the ATLAS accelerator system at the Argonne National Laboratory. The energetic products of nuclear reactions are slowed in a gas cell and collected, cooled and purified in a series of ion traps. This ion-cloud of exotic nuclides is then transferred to a precision Penning trap where the mass of the ions can be measured to precisions of a few parts per billion. Alternately, the ions can also be transferred to a radio-frequency quadrupole ion trap (RFQ) where they are suspended in helium gas. Such a cloud of exotic nuclei, nearly at rest, provides a unique opportunity to study the correlations between the beta particles and the neutrinos and other particles emitted in their decay. The following initiatives are either planned or in progress: • The CPT has been moved from its original location and installed on-line to the low-energy beam line of the new CARIBU facility at ATLAS. CARIBU utilizes a large scale version of the same gas stopping technique described above to provide a source of very neutron-rich nuclides, produced in the fission of 252-Cf. With this new source the measurements can push even closer to the neutron drip line and provide data that are very useful for improving our models for r-process nucleosynthesis. This campaign is expected to conclude in 2014. • Subsequently, we plan to move the CPT mass spectrometer to a location in experimental area III at ATLAS to make use of the unique availability of high-intensity energetic beams of heavy nuclei, to produce very neutron-rich nuclei in regions heavier than those available through fission. These very exotic nuclei will once again be stopped in a gas catcher and presented for measurement into a newly constructed multiple-reflection time-of-flight mass spectrometer and the CPT mass spectrometer systems. This new arrangement should provide us with unique opportunities to push the boundaries of our knowledge of exotic nuclei further towards the N=126 neutron-rich region critical for the formation of the final abundance peak in the r-process. • Our original target and gas catcher system are now used to load an open geometry RFQ trap that we call the BPT. The trap is surrounded by various detectors that allow us to study beta-neutrino correlations in the decay of the trapped particles. We are examining the beta decay of the mass 8 system (8-Li and 8-B) which are sensitive individually to tensor currents, and when combined, to second class currents. • A second RFQ trap of is under construction at the CARIBU facility where it will be used to study beta-delayed neutron emission from the trapped ions. The results will delineate the decay modes of neutron-rich nuclei which play a role in nucleosynthesis. They are also relevant to understanding nuclear reactor kinetics and inform safety considerations with nuclear power plants. • Other mass measurements with trapped ions of particular interest will be undertaken as opportunities arise.