Several instances of anomalously low values of the B(E2;41+→21+)B(E2;21+→0gs+) ratio (B4/2)<1 have previously been observed in several nuclei among the highly neutron deficient W-Os-Pt isotopes with even numbers of neutrons and protons. The relatively large number of cases suggests a striking new phenomenon that currently has no theoretical explanation. A clue to its origin might be achieved by comparing the B4/2 ratios in even-even nuclei with the corresponding ratios of transition rates in neighboring odd-mass nuclei. However, until now only one case (Os169) has been observed with an unusually low corresponding B4/2 ratio, approximately 1σ below 1. Electromagnetic transition rates between excited states in the extremely neutron deficient nuclide Os167 were measured using 15 single-crystal germanium detectors from the jurogam 3 array coupled to the RITU gas-filled separator. The excited states of interest were populated via the Mo92(Kr78,2pn) fusion-evaporation reaction and studied by selecting the rare reaction channel via recoil-α-decay tagging. The lifetimes of low-lying states were measured using the recoil distance Doppler shift method with the APPA plunger device. From the measured lifetimes in the rotational-like band structure assigned to the νi13/2 yrast intruder configuration, the B(E2;17/2+→13/2+) and B(E2;21/2+→17/2+) values were deduced, resulting in the ratio B(E2;21/2+→17/2+)B(E2;17/2+→13/2+)=0.49(10). The results were compared with previous measurements in the Os isotopic chain and theoretical mean-field calculations, implying that the odd neutron valence particle has limited, if any, influence on the B4/2 values for this particular single-particle configuration.

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.

Two new low-lying isomeric states and two new rotational bands have been identified in the odd-odd nucleus 116Cs from a high-statistics experiment using the 58Ni(64Zn, αpn) 116Cs fusion-evaporation reaction and the JUROGAM 3 + MARA setup. The spin-parity of the known T1/2 = 3.85 s isomer is firmly fixed as 6+, and its configuration is established as πg−1 9/2 ⊗ νg7/2 from the properties of the strongly coupled band built on it. The 8+ and 7− bandheads of the two rotational bands to which we assign πh11/2 ⊗ νh11/2 and πh11/2 ⊗ νd5/2 configurations, respectively, have lifetimes of the order of a few nanoseconds and can therefore be considered as isomeric states. Several weak transitions depopulate the 8+ isomer, feeding states that can be based on oblate shapes. The analysis of the alignment properties, combined with the systematics of the doubly-odd cesium nuclei, and the results of extensive particle number conserving cranked shell-model calculations, led to a very good understanding of the bands structures. The spins and parities of two bands of 118Cs with properties similar to the bands built on the 6+ and 7− isomers of 116Cs have been changed to 7+ and 7− and reassigned to the πg−1 9/2 ⊗ νg7/2 and πh11/2 ⊗ νd5/2 configurations, respectively. The band of 118Cs built on the πh11/2 ⊗ νd5/2 configuration has been extended to higher spin. The new results enrich the band structure in very light cesium nuclei and unveil the existence of multiple high-K and possible shape isomers built on two-quasiparticle configurations close to the proton drip line.

With the recent lifetime measurements of the 2 + states in Te 116 and Te 118 , the evolution of the collectivity in the midshell region of the Te isotopic chain is starting to reveal itself. However, more information on the structure of the ground-state bands and the nature of the collectivity can be gathered from studying the odd-mass systems, in which lifetimes have not been extensively studied.

The purpose of the present work is to investigate the structure of low-lying excited states of the ν h 11 / 2 band in the odd-mass systems Te 117 and Te 119 , that correspond to the ground-state band in even-even Te.

Lifetimes of low-lying states in the ν h 11 / 2 band of Te 117 are measured for the first time and remeasured in Te 119 , using the recoil distance Doppler shift technique, and analyzed with the differential decay curve method in coincidence mode.

The lifetimes and B (E2) values of the first two transitions, 15 / 2 − → 11 / 2 − and 19 / 2 − → 15 / 2 − , in the ν h 11 / 2 band, are determined in both Te 117 and Te 119 . From the lifetimes, the B (E2) values and corresponding B 4 / 2 ratios are determined. The results are compared to systematics in both even- A and odd- A Te isotopes, as well as with theoretical results from the large-scale shell model and interacting boson model calculations.

While the energy levels of Te 117 and Te 119 , similarly to Te 118 , exhibit textbook vibrational behavior, the unexpectedly small B 4 / 2 ratios of these odd-mass nuclei point to a puzzling discrepancy and question the vibrational nature of these states.

In recent years, several cases of nuclei presenting the so-called "B-4/2 anomaly" have been observed in the neutron-deficient region close to Z = 50 and Z = 82. In the last region, the osmium isotopic chain is of particular interest, as three consecutive isotopes, 168,169,170Os, have shown the presence of this peculiar phenomenon. An experiment aimed at extending the study to Os-167 was performed at the Accelerator Laboratory of the University of Jyvaskyla, using a beam of 78Kr at 360 MeV impinging on a Mo-92 target. Lifetimes of several low-lying states were measured using the Recoil-Distance Doppler Shift method. The preliminary analysis and the study of the influence of unobserved feeders are discussed.

The very neutron-deficient strongly deformed 117Cs nucleus has been studied using the 58Ni(64Zn, 1 alpha 1p) reaction and JUROGAM 3 gamma -ray detector array coupled to the MARA recoil-mass separator. Three previously known and three newly identified rotational bands were observed up to very high spin and excitation energy. All bands are firmly assigned to 117Cs based on measured mass spectra and interconnecting transitions. The ground-state spin and parity are suggested based on the systematics of low-lying states in odd-even cesium nuclei. The systematics of bandheads in cesium nuclei reveal maximum collectivity and deformation is reached for neutron numbers 64-66, corresponding to the middle of the N = 50-82 magic shell. The rotational frequencies of the first and second crossings in the different bands are similar to those observed in the corresponding bands in the neighboring 119Cs nucleus, suggesting similar deformations of the two nuclei, but enhanced softness in 117Cs. Particle number conserving cranked shell model calculations describe the observed band structures well.

Excited states have been observed for the first time in the very neutron-deficient odd-odd nucleus 57120La63. The observed γ rays have been assigned based on coincidences with lanthanum X rays measured with the JUROGAM 3 array and with A=120 fusion-evaporation residues measured with the MARA separator. The observed γ rays form a rotational band which decays to the ground state via a cascade of four low-energy transitions. Based on the systematic comparisons with the heavier odd-odd La isotopes we assign spin-parity 4+ to the ground state and a πh11/2⊗νh11/2 configuration to the rotational band. The nuclear shape has been investigated by the cranked Nilsson-Strutinsky model. Two quasiparticle plus triaxial rotor model calculations including the np interaction nicely reproduce the spin of the inversion between the even- and odd-spin cascades of E2 transitions, giving credit to the np interaction as an important parameter responsible for the mechanism inducing the inversion. The position of the Fermi levels, in particular for neutrons, also has a strong impact on the observed inversion in the chain of lanthanum nuclei.

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 electromagnetic transition probabilities of the yrast 2 + states in the midshell Te isotopes, two protons above the closed shell at Sn, are of great importance for the understanding of nuclear collectivity in these isotopes and the role played by the neutron-proton interactions and cross-shell excitations. However, the large uncertainty of the experimental data for the midshell nucleus 118Te and the missing data for 116Te make it difficult to pin down the general trend of the evolution of transition probabilities as a function of the neutron number. In this work, the lifetime of the yrast 2 + state in 118Te was measured, with the aim of reducing the uncertainty of the previous measurement. The result is τ2 += 7.46 (19) ps. In addition, the lifetime of the 4 + state was measured to be τ4 += 4.25 (23) ps. The experimental transition rates are extracted from the measured lifetimes and compared with systematic large-scale shell-model calculations. The trend of the B(E2 ; 0 +→ 2 +) values in the midshell area is in good agreement with the calculations and the calculated B4 / 2 ratio provide evidence for 118Te as a near perfect harmonic vibrator.