Study of lifetimes of nuclear excited states near the N=20 island of inversion using the Doppler Shift Attenuation Method

In the region along the neutron-rich N = 20 shell closure, the ground-state structure of certain isotopes is known to be dominated by intruder configurations. In these configurations, neutrons occupy states above the N = 20 shell gap, while leaving vacancies in the shell below. This area on the nuclear chart is known as the N = 20 island of inversion. The trend extends to nearby nuclei, where sets of intruder states have been found among their excited states. Lifetime measurements are commonly used to unravel the nature and properties of nuclear states, as they are closely related to transition probabilities. Comparing those to theoretical predictions, indirect information on the nuclear wave-functions can be extracted. This thesis provides a first-step analysis of an experiment which aims to investigate the interplay between spherical and intruder configurations in the low-lying states of isotopes on the boundary of the N = 20 island of inversion, namely 34Si and 35P. The AGATA High-Purity Germanium γ-tracking array was used in coincidence with the PRISMA high-acceptance magnetic spectrometer to detect the γ rays emitted in the decay of the states of interest. From the γ-ray energy spectrum measured, the Doppler Shift Attenuation Method was used to extract the associated lifetimes, expected to lie in the range from 30 to 100 fs. The analysis involved processing data from both detectors. In order to extract the lifetimes, the shapes of the peaks in the γ-ray energy spectrum were compared to those produced by a Monte Carlo GEANT4 simulation, adapted to mimic the experimental conditions. Doing so, it was possible to estimate the lifetime of the first 2+ state in 36S and provide a first preliminary lifetime estimation for the 3/2+ state of 35P.