Combined γ-ray and electron spectroscopy : study of shape coexistence in 186Pb

Abstract

Structures in ¹⁸⁶Pb have been under extensive investigation for several decades. ¹⁸⁶Pb is the system having a low-lying triplet of zero-spin states, which are assigned with different shapes (spherical, prolate and oblate). This nucleus has been previously studied using α-decay and in-beam γ-ray spectroscopic methods but the transitions near the ground state remained a mystery due to the large conversion coefficient. In this thesis, the low-lying structure of ¹⁸⁶Pb was experimentally investigated by combined γ-ray and electron spectrometer, SAGE, and utilising the recoil-decay tagging method. As a result, the E0 2⁺₂ → 2⁺₁ and 4⁺ ₂ → 4⁺₁ interband transitions could be assessed. Also, the feeding of the first excited 0⁺ ₂ state was observed for the first time. Results allowed for reassigning the shapes of 0⁺ states in ¹⁸⁶Pb, which provide input for new theoretical calculations. Recent developments for in-beam γ-ray spectroscopy at the Accelerator Laboratory of Jyväskylä forms part of this thesis work. The properties of the JUROGAM 3 germanium array, such as energy resolution, add-back factor and photopeak detection efficiency, can be found in the fifth chapter. Also, the sum-peak method to determine the absolute photopeak detection efficiency of a germanium array is explained in this chapter. The issue with pile-up marking in the DAQ system is also raised to notify other potential users.