Abstract
This thesis reports on the utilisation of γ-ray coincidences between germanium and cerium doped lanthanum tri-bromide LaBr3(Ce) scintillation detectors for the determination of electromagnetic transition rates in the pico-to-nanosecond regime. The technique uses the high quality full energy peak resolution of the LaBr3(Ce) detectors coupled with their excellent timing responses in order to study discrete energy γ-ray cascades from nuclei populated using the 18O(18O,pn)34P, 130Te(12C,4n)138Ce and 128Te(16O,4n)140Nd fusion-evaporation reactions. These studies were carried out at the Tandem van de Graaff accelerator laboratory in Bucharest, Romania. Coincidence measurements were performed using a mixed array of eight HPGe and eight LaBr3(Ce) detectors. The HPGe detectors were used to define the decay path of nucleus of interest, while the LaBr3(Ce) detectors were used to measure the decay times spectra of selected levels. In the first experiment, excited states in 34P were populated using the 18O(18O,pn) fusion-evaporation reaction at beam energy of 36 MeV. In this experiment, a fast timing method was used to measure the half-life of the Iπ = 4− intruder state to be 2.0(1) ns. This structure and decay rate of this level are compared to predictions of restricted basis nuclear shell model calculations using on spdf basis which incorporates excitations into the f7/2 negative parity orbital. In the second experiment, N=80 isotones 138Ce and 140Nd were populated using the 130Te(12C,4n) and 128Te(16O,4n) fusion-evaporation reactions at incident beam energy of 56 MeV and 66 MeV, respectively. The half-life of the Iπ=6+ yrast state at 2294 keV in 138Ce was measured to be T1/2=880(19) ps. The B(E2; 6+ -> 4+) is compared with the predictions of the restricted basis shell model and the systematics of the region and shows an anomalous behaviour compared to the neighbouring isotonic and isotopic chains. Half-lives of the yrast Iπ = 5−, 11+ and 14+ states in 138Ce have also been determined for the first time. Reduced transition probabilities have been calculated for the electromagnetic decays from these states. For 140Nd, the half-life of the Iπ= 8− yrast state at 3240 keV was measured to be T1/2=100(12) ps. The centroid shift method was used to determine the half-life of this state because the half-life of the nuclear state was significantly shorter than the time resolution (full width at half-maximum) for the LaBr3(Ce) setup. Reduced transition probabilities B(E2) and B(M1) associated with the 178- and 1017 keV transitions decaying from the Iπ = 8− state in 140Nd have been calculated for the values of the E2/M1 mixing ratios.