11/09/2023
By Sanjanee Waniganeththi

The Kennedy College of Sciences, Department of Physics and Applied Physics, invites you to attend a doctoral dissertation defense by Sanjanee Waniganeththi on "β-Decay Spectroscopy Of Light Rare-Earth Nuclei."

Candidate Name: Sanjanee Waniganeththi
Degree: Doctoral
Defense Date: Monday, Nov. 20, 2023
Time: 11 a.m. - 1 p.m.
Location: 202 Pinanski North Campus

Dissertation Title: β-Decay Spectroscopy Of Light Rare-Earth Nuclei

Committee: 

  • Advisor Andrew M. Rogers, Department of Physics and Applied Physics, UMass Lowell
  • Partha Chowdhury, Department of Physics and Applied Physics, UMass Lowell
  • Archana Kamal, Department of Physics and Applied Physics, UMass Lowell

Abstract: Probing the intrinsic and collective structure of atomic nuclei provides insight into the evolution of nuclear deformation and the properties of nuclei far from stability. The rapid neutron capture process (r-process) is responsible for synthesizing approximately half of those heavy unstable nuclei in the solar system. The formation of the rare-earth peak in the r-process solar abundance pattern has yet to be fully understood. So, rare-earth nuclei are also currently the subject of many experimental investigations to understand their low-laying isomers, which strongly influence nucleosynthesis pathways. The region contains a significant expanse of heavy nuclei with varying nuclear shapes. Production of these neutron-rich unstable nuclei is challenging; as a result, the basic properties of many of them must be calculated with nuclear models. β-decay half-lives, nuclear masses, and neutron-emission probabilities are among the most important properties that affect r-process abundances. Consequently, this understanding is critical for exploring the formation of the rare-earth peak in the r-process abundance pattern, where new data can influence constraints on the possible astrophysical sites and conditions.

The odd-odd nucleus 158Pm is a particularly interesting case as a predicted isomeric state has yet to be clearly established. This thesis provides experimental evidence addressing the missing 158Pm isomer and new β-decay data on the relevant A=158 and 160 isobars. Investigating such isomers is critical to determining their structure as well as to remove ambiguities that can arise in direct mass measurements. To probe this region, a β-decay experiment was performed at Argonne National Laboratory (ANL) using the HPGe clover detectors of X-Array (XA) combined with the Scintillator And Tape Using Radioactive Nuclei (SATURN) decay station. Radioactive ions produced by the Californium Rare Isotope Breeder Upgrade (CARIBU) facility were isobarically separated and delivered to the XA and SATURN. Various tape cycles were optimized for the decay of a given species, with a focus on 158Pm, 158Sm, and 160Sm. This sensitive β-decay experiment was utilized to constrain the spins assigned for low-lying energy levels of 158Pm and the ground state of 158Pm. A correlation method was employed to deduce the half-life, decay branches, and other properties of the β-decay. It was discovered that the previously assigned 121 keV isomeric γ-ray transition in 158Pm is related to 158Nd (preceding isotope in the isobar) β-decay. The first experimental observation of β-decay feeding to the 121 and 930 keV state of 158Pm. The intensity of observed 809 and 930 keV γ-ray transitions provides evidence that the ground state spin of 158Pm must be 0+, supporting the favored energy orientation of Gallagher-Moszkowski rules. These observations provide the first high-resolution spectroscopic data to extend into more unknown neutron-rich isotopes.