In this licentiate thesis, lifetimes of low lying excited states in the midshell region of tellurium isotopes are discussed. The nuclear lifetimes are interesting properties to study, since they are inversely proportional to the transition probabilities, which contain information on the underlying structure of the nucleus. By measuring the lifetime of a transition, information can be gained on e.g. the collectivity of the state.

The main results presented in this thesis are the lifetime measurements of the 2⁺ → 0⁺ and 4⁺ → 2⁺ transitions in ¹¹⁸Te described in Paper I. The principal aim of this paper was to reduce the relative uncertainty in the 2⁺ lifetime, to better see the trend of the 2⁺ → 0⁺ transition probabilities over the tellurium isotopic chain.

The work presented in Paper I is based on data from an experiment conducted in 2017 at the JYFL accelerator facility in Jyväskylä, Finland. In this experiment, excited states of were populated in the fusion-evaporation reaction ¹⁰⁰Mo(²²Ne,4n)¹¹⁸Te at a beam energy of 75 MeV. The lifetimes were extracted using the Recoil Distance Doppler Shift (RDDS) technique, with the Jurogam II γ-ray spectrometer coupled to the Differential Plunger for Unbound Nuclear States (DPUNS). Data analysis was performed using the Differential Decay Curve Method (DDCM) in coincidence mode. The lifetimes were determined to be τ₂₊ = 7.46(19) ps and τ₄₊ = 4.25(23) ps, in agreement with previous measurements, but with reduced uncertainty. In the case of the 2⁺ lifetime, the new result reduces the relative uncertainty of the lifetime from 16% to 2.5%, allowing for more precise tests of theoretical predictions.

In addition to ¹¹⁸Te, an analysis of lifetimes of low-lying excited states in the odd-A nuclei ¹¹⁷Te and ¹¹⁹Te is ongoing and an experiment aiming to measure lifetimes in ¹¹⁶Te has been accepted. These nuclides will also be discussed in this thesis.

I denna licentiatavhandling kommer livstider för lågt liggande exciterade tillstånd i telluriumisotoper att diskuteras. Livstid för ett exciterat tillstånd är intressant att studera eftersom den är omvänt proportionell mot övergångssannolikheten, som innehåller information om den underliggande strukturen i kärnan. Genom att mäta livstiden för en övergång kan man få information om t.ex. tillståndets kollektivitet.

Huvudresultatet som presenteras i denna avhandling är livstidsmätningen av övergångarna 2⁺ → 0⁺ och 4⁺ → 2⁺ i ¹¹⁸Te som beskrivs i Paper I. Det huvudsakliga syftet med detta arbete var att minska den relativa osäkerheten i 2⁺-livstiden, för att bättre se trenden för övergångssannolikheterna över isotopkedjan för tellurium. 

Arbetet som presenteras i Paper I är baserat på data från ett experiment utfört 2017 vid acceleratoranläggningen JYFL i Jyväskylä, Finland. I detta experiment populerades exciterade tillstånd i 118Te i kompoundkärne-reaktionen ¹⁰⁰Mo(²²Ne,4n)¹¹⁸Te, vid en strålenergi på 75 MeV. Livstiderna extraherades med Recoil Distance Doppler Shift (RDDS) tekniken, med Jurogam II-gammaspektrometern kopplad till DPUNS (Differential Plunger for Unbound Nuclear States). Dataanalysen utfördes med Differential Decay curve Method (DDCM) i koincidensläge. Livstiderna bestämdes till  τ₂₊ = 7.46(19) ps och τ₄₊ = 4.25(23) ps, i överensstämmelse med tidigare mätning, men med reducerad osäkerhet. När det gäller 2⁺ livstiden innebär det nya resultatet en minskning av den relativa osäkerheten från 16% till 2,5%, vilket möjliggör mer exakta test av teoretiska förutsägelser.

Utöver ¹¹⁸Te pågår även analysis av livstider hos lågt liggande exciterade tillstånd i de udda-A kärnorna ¹¹⁷Te och ¹¹⁹Te, och ett experiment som syftar till att mäta livstider i ¹¹⁶Te har blivit accepterat. Dessa kärnor kommmer också att diskuteras i denna avhandling.

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.