Mass measurements of neutron-rich nuclides for the astrophysical r process using the Canadian Penning trap mass spectrometer

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Ray, Dwaipayan
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About half of the elements heavier than iron (Z = 26) are believed to be formed via the r process (rapid neutron capture process), of which our understanding is limited. Running theoretical simulations to gain insight into this r process rely on the availability of nuclear data (like nuclear masses, beta decay lifetimes, neutron capture cross sections and others) with reduced uncertainties for nuclides near the expected r-process path. Such data is available in limited capacity due to the challenges in producing those exotic nuclides. The astrophysical conditions required for the r process, and the events that generate them are still open questions. The CAlifornium Rare Isotope Breeder Upgrade (CARIBU) at Argonne National Laboratory (ANL) uses the spontaneous fission from a Californium-252 (252Cf) source and is one the few facilities world-wide to produce beams of such neutron-rich isotopes around the mass numbers A = 105 and A = 140. The extracted ions are subject to multiple stages of cooling and purification, before they are sent to the Canadian Penning trap (CPT) mass spectrometer, where their masses are measured using the Phase-Imaging Ion-Cyclotron- Resonance (PI-ICR) technique. The main focus of this work was to provide data on atomic masses that would allow simulations to replicate the rare-earth peak in the observed r-process abundance pattern. Masses of a total of 34 nuclides are presented in this thesis, of which seven were measured for the first time, including the first-time measurements isotopes of Ce (Z = 58): A = 152−155. The results of these Ce isotopes, with their uncertainties reduced from the extrapolated values of ≥ 200 keV/c2 in the latest Atomic Mass Evaluation (AME2020), to ≤ 6 keV/c2, agree with mass predictions from a reverse engineered Markov Chain Monte Carlo technique for a hot r-process environment, hence validating this “hot” r-process model for this mass region.
Phase-imaging ion-cyclotron-resonance, Penning trap mass spectrometry, astrophysical r process, Nuclear masses, Penning trap