Philosophiae Doctor - PhD / Lying in a rapidly-changing transitional region between nuclei that may behave as spherical vibrators (N ≤ 88) or good rotors (N ≥ 92), the N = 90 isotones present a unique testing ground where new nuclear collective phenomena may be uncovered. As part of a general investigation of collective structures and the coupling of single-particle states to these structures, the 150Sm(12C,4nγγ) and 150Sm(13C,4nγγ) reactions at a beam energy of Elab = 65 MeV were used to study the low and medium spin structure of 158,159Er nuclei below spin 20~. The γ-γ coincidence events were detected in the nine escape-suppressed HPGe Clover detectors using the AFRODITE γ-ray spectrometer at the iThemba Laboratory for Accelerator Based Sciences. DCO ratios and γ-ray polarization measurements were used to establish the spins and parities of newly observed and confirm previously established rotational bands. The observed bands will be discussed in terms of both traditional quadrupole rotational-vibration models and recent predictions of octupole correlations in the N = 90 isotones and neighbouring nuclei. The data obtained will be compared with the spectroscopic systematics of neighbouring nuclei.

<p>Познавање параметара нуклеарне структуре, попут шеме нивоа, функциjе густине стања и функциjе jачине прелаза су неопходне за разумевање<br />нуклеарних особина атомског jезгра. Ове величине су директно повезане са ефикасним пресеком, коjи jе битна физичка величина у нуклеарноj физици, као и теориjски са моделима jезгра. Прикупљање тачних и прецизних података о овим физичким величинама jе jедан од наjважниjих задатака у&nbsp; нискоенергетскоj нуклеарноj физици.У овом раду за одређивање ових величина коришћен jе метод двоструких гама каскада коjи се заснива на&nbsp; коинциденционоj детекциjи гама кваната коjи настаjу након захвата термалних неутрона на jезгрима мете. Циљ експеримента jе детекциjа два узастопна гама кванта коjи се емитуjу када се jезг-ро након захвата неутрона деексцитуjе са енергиjе захвата на основно или неко од нископобуђених стања.Jезгра коjа су испитивана у овом раду су⁹⁴Nb и⁵⁶Mn, а експерименти су извршени на BRR истра-живачком реактору у Будимпешти и FRM II истраживачком реактору на Техничком универзитету у Минхену, у Хаjнц Меjер-Лаjбниц центру, тим редом. Прикупљени експериментални подаци су анализирани и добиjене су информациjе о шеми нивоа и емитованих гама кваната приликом чега jе пронађен велики броj нових гама прелаза унутар ових jезгара (210 за ⁹⁴Nb, 50 за⁵⁶ Mn) , као и нових енергетских нивоа у овим jезгрима (29 за ⁹⁴ Nb, 24 за<br />⁵⁶ Mn). За детектоване каскаде израчунати су и интензитети. Након одређивања интензитета кас-када, за одређивање функциjе густине нивоа и jа-чине прелаза за<br />⁹⁴ Nb и ⁵⁶ Mn коришћен jе практични модел за гама распад неутронских резонанци, развиjен на Обjедињеном институту за нуклеарна истраживања у Дубни, Руска федерациjа. Такође, у овом раду jе одрађена и систематизациjа података за 43 jезгра коjа су раниjе испитивана овом методом и представљени су закључци добиjени овом систематизациjом.</p> / <p>Poznavanje parametara nuklearne strukture, poput šeme nivoa, funkcije gustine stanja i funkcije jačine prelaza su neophodne za razumevanje<br />nuklearnih osobina atomskog jezgra. Ove veličine su direktno povezane sa efikasnim presekom, koji je bitna fizička veličina u nuklearnoj fizici, kao i teorijski sa modelima jezgra. Prikupljanje tačnih i preciznih podataka o ovim fizičkim veličinama je jedan od najvažnijih zadataka u&nbsp; niskoenergetskoj nuklearnoj fizici.U ovom radu za određivanje ovih veličina korišćen je metod dvostrukih gama kaskada koji se zasniva na&nbsp; koincidencionoj detekciji gama kvanata koji nastaju nakon zahvata termalnih neutrona na jezgrima mete. Cilj eksperimenta je detekcija dva uzastopna gama kvanta koji se emituju kada se jezg-ro nakon zahvata neutrona deekscituje sa energije zahvata na osnovno ili neko od niskopobuđenih stanja.Jezgra koja su ispitivana u ovom radu su⁹⁴Nb i⁵⁶Mn, a eksperimenti su izvršeni na BRR istra-živačkom reaktoru u Budimpešti i FRM II istraživačkom reaktoru na Tehničkom univerzitetu u Minhenu, u Hajnc Mejer-Lajbnic centru, tim redom. Prikupljeni eksperimentalni podaci su analizirani i dobijene su informacije o šemi nivoa i emitovanih gama kvanata prilikom čega je pronađen veliki broj novih gama prelaza unutar ovih jezgara (210 za ⁹⁴Nb, 50 za⁵⁶ Mn) , kao i novih energetskih nivoa u ovim jezgrima (29 za ⁹⁴ Nb, 24 za<br />⁵⁶ Mn). Za detektovane kaskade izračunati su i intenziteti. Nakon određivanja intenziteta kas-kada, za određivanje funkcije gustine nivoa i ja-čine prelaza za<br />⁹⁴ Nb i ⁵⁶ Mn korišćen je praktični model za gama raspad neutronskih rezonanci, razvijen na Objedinjenom institutu za nuklearna istraživanja u Dubni, Ruska federacija. Takođe, u ovom radu je odrađena i sistematizacija podataka za 43 jezgra koja su ranije ispitivana ovom metodom i predstavljeni su zaključci dobijeni ovom sistematizacijom.</p> / <p>Knowledge of nuclear parameters, such as level scheme, level density function and radiative strength function is important in order to understand nuclear properties. These parameters are directly related to the value of the cross section, an important quantity in nuclear physics, and also, theoretically to nuclear models. Acquisition of precise and high quality data on these parameters is one of the most important<br />tasks in low-energy nuclear physics. To determine these parameters, in this thesis the method of the two step gamma cascade was used. This method is based on coincidence detection of gamma rays that are emitted after thermal neutron capture&nbsp; on the nucleus. The goal of the experiment is to record two consecutive gamma rays &nbsp; that are emitted as nucleus decays from the capture energy to the ground state<br />or some of the low-excited states.In this work, 94 Nb and 56 Mn nuclei were investigated and the experiments were carried out at the BRR research reactor at the&nbsp; PGAA facility of the Centre for Energy Research (MTA EK), Budapest, Hungary and FRM II research reactor at the Technische Universit&uml;at M&uml;unchen, Heinz Maier- Leibnitz&nbsp; Zentrum (MLZ), Garching, Germany, respectively. Collected data were analyzed and information about level scheme and emitted gamma rays were obtained. The analysis showed that a large number of new gamma transitions, as well as new energy levels (29 for 94 Nb, 24 for 56 Mn) was detected for these nuclei (210 for 94 Nb, 50 for 56 Mn). The intensities were also calculated for all recorded cascades. After calculating the intensities of cascades, to determine the level density function and radiative strength function the practical model of neutron resonances gamma decay developed at JINR, Dubna, Russia, was used. Also, in this thesis, the systematization of results obtained for 43 nuclei previously measured by this method,&nbsp; are presented.</p>

Philosophiae Doctor - PhD / In the first experiment high spin states in 194Tl, excited through the 181Ta (18O, 5n) heavyion fusion evaporation reaction were studied using the AFRODITE array at iThemba LABS. The γ-γ coincidences, RAD ratios and linear polarization measurements were carried out and the previously known level scheme of 194Tl was significantly extended. A total of five rotational bands four of which are new were observed. A pair of rotational bands associated with the πh9/2 ⊗ νi−1 13/2 configuration at lower spins and with the πh9/2 ⊗ νi−3 13/2 configuration at higher spins was found and interpreted as the first possible chiral bands followed above the band crossing. The two 4-quasiparticle bands show exceptionally close near-degeneracy in the excitation energies. Furthermore close similarity is also found in their alignments and B(M1)/B(E2) reduced transition probability ratios. In the second experiment lifetimes in 194Tl were measured using the DSAM technique with the excited states in this nucleus populated through the 181Ta (18O, 5n) reaction. A total of 25 lifetimes and 30 reduced transition probabilities of magnetic dipole B(M1) and electric quadrupole B(E2) have been evaluated. Furthermore B(M1) and B(E2) reduced transition probabilities in Bands 1 and 4, which have been regarded as chiral candidates, were found to be close to each other and reveals strong splitting along spin values. This further supports the proposed chiral nature of these two bands.