PhD defence: From ideal vibrations to possible anomalies Insights from tellurium lifetime measurements and modelling

by Ebba Ahlgren Cederlöf (KTH Physics)

Friday 31 Oct 2025, 10:00 → 13:00 Europe/Stockholm

Pärlan plan 6 (Albano Building 1)

Opponent: Professor Joakim Cederkäll, Lund University

Supervisor: Professor Ayse Nyberg, Kärnvetenskap och kärnteknik; Universitetslektor Torbjörn Bäck, Kärnvetenskap och kärnteknik; Professor Johan Nyberg, Uppsala University; Universitetslektor Magnus Wolke, Uppsala Universitet

Abstract

The work presented in this thesis explores the structure of tellurium (Te) isotopes throughprecise measurements of nuclear lifetimes. Lifetime measurements offer a way of probing theinner structure of the atomic nucleus through its close connection to transition probabilities andcollectivity. The Te isotopic chain acts as an ideal laboratory for studying collectivity in generaland quadrupole vibrations in particular. However, previous measurements have left key gapswhich hindered further understanding of nuclear collectivity and the evolution of structure bothnear the midshell region and towards the most neutron deficient Te nuclei. This work aims to fill those gaps by providing new lifetime data for both even-A (¹⁰⁸Te, ¹¹⁸Te) and odd-A (¹¹⁷Te,¹¹⁹Te) isotopes and interpret them using theoretical models.Two experiments were conducted at the JYFL accelerator laboratory in Jyväskylä, Finland. Excited states in Te isotopes were populated using fusion-evaporation reactions. The RecoilDistance Doppler Shift (RDDS) method was the primary tool used for lifetime measurements. In the case of the very neutron deficient ¹⁰⁸Te, Recoil Decay Tagging (RDT) helped in isolating the gamma rays associated with its decay. Lifetimes of the 2⁺ → 0⁺ and 4⁺ → 2⁺ transitions were measured in ¹¹⁸Te with improved precision. With this reduced uncertainty in the B(E2) values of ¹¹⁸Te, together with new lifetime measurements in the odd-mass nuclei ¹¹⁷Te and ¹¹⁹Te presented in this work, the evolution of the structure in midshell region Te is starting to emerge. In ¹⁰⁸Te, a new measurement of the 4⁺→ 2⁺ lifetime allowed for the calculation of the B_4/2 ratio, an important indicator of collectivity. These results contribute towards resolving the unexplained B_4/2 anomaly seen in several isotopes, among them ¹¹⁴Te. Finally, a newly developed Python package for IBM/IBFM calculations ispresented, together with results from applications within the Te chain.