Speaker
Description
The region "south-west" of $^{132}$Sn is of interest for both theoretical and experimental efforts to comprehend nuclear shell structure in the vicinity of proton (Z=50) and neutron (N=82) shell closures. The half-lives of excited nuclear states are a crucial source of information on nuclear shell structure, and advanced experimental methods had to be developed to obtain the necessary data. Since possible half-lives, even taking into account only fast-timing spectroscopy, span over a few orders of magnitude from sub-picosecond up to nanosecond scale, the challenge for experimental nuclear physics becomes even more demanding. Due to the difficulties in studying such exotic nuclei, experimental data in the aforementioned region remain limited. This abstract will present the benefits and practical problems. In-flight $gamma$-ray spectroscopy will be discussed based on experimental data on $^{129}$In and $^{128}$Cd obtained during the HiCARI campaign in November 2020 at RIKEN (Japan). Nuclei of interest were produced via nucleon knock-out reactions from $^{130}$In projectile impinging with velocity of approximately 0.55c on 6mm $^{9}$Be target following induced, in-flight fission of $^{238}$U primary beam. HiCARI, an array comprised of 3 different types of segmented HPGe detectors aimed at detecting prompt gamma rays emitted after the knock-out reaction, was located in close proximity to the target. Based on the reconstructed velocity of ions and position of $\gamma$-ray emission during their de-excitation event-by-event, $\gamma$-ray spectra were obtained for each reaction channel. The line shape of identified transitions carries information about their energy ($E$) and half-life ($T_{1/2}$). Taking into account precisely measured HiCARI geometry, the response function of every HPGe crystal to the $\gamma$-ray of given $E$ and $T_{1/2}$ was simulated using the Geant4 package. However, line shape is in general influenced by the contaminant $\gamma$-rays emitted in the rest frame. The origin of these $gamma$-rays is connected with the projectile energy loss, and the response function to this kind of radiation was also taken into account. Finally, $E$ and $T_{1/2}$ of the excited states were extracted by minimizing $\chi^{2}$ of response functions fitted to the experimental data. The method allows access to short half-lives ranging from single ps to a few hundred ps, making it a perfect tool to study low-lying excited states of the nuclei in the vicinity of $^{132}$Sn. Moreover, knock-out reactions populate short-lived states directly, in contrast to methods that require a nuclear isomer. Thanks to this, previously unobserved states can be discovered. Results will be interpreted employing state-of-the-art shell model calculations.