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Experimental Studies of Neutron Deficient Nuclei in the A ≈ 110 and A ≈ 170 Mass Regions

Time: Thu 2022-04-28 15.00

Location: Kollegiesalen, Brinellvägen 8, Stockholm

Language: English

Subject area: Physics

Doctoral student: Wei Zhang , Fysik

Opponent: Professor Piotr Bednarczyk, The Henryk Niewodniczański Institute of Nuclear Physics, Polish Academy of Sciences

Supervisor: Bo Cederwall, Kärnfysik

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This doctoral thesis describes several experimental studies on the structures of neutron-deficient nuclei with mass numbers A≈110 and A≈170. The experiments were carried out at the Accelerator Laboratory of the University of Jyväskylä, Finland, using heavy-ion fusion-evaporation reactions induced by beams of energies around 5 megaelectronvolts/nucleon. The experimental setups used for these studies are based on the high-resolution germanium-detector array Jurogam in combination with the recoil ion separators RITU or MARA and their respective focal plane detector systems, as well as with other ancillary devices. 

Using the recoil distance Doppler shift technique, lifetimes of excited states in the neutron-deficient osmium isotopes 169,171,173Os were measured for the first time by coupling the Jurogam germanium detector array with the ancillary device DPUNS (differential plunger for unbound nuclear states), the gas-filled recoil separator RITU, and GREAT the decay spectrometer situated at the RITU focal plane. The differential decay curve analysis method was used to extract lifetimes of the excited states of interest. An anomalously low value was observed for the ratio of the reduced electric quadrupole transition probabilities between consecutive gamma-ray transitions in the yrast band of 169Os. This observation adds to the handful of nuclides known to exhibit such behavior, which cannot be explained by state-of-the-art theory. Based on data from the same experiment, new alpha decay fine structures were identified for the osmium isotopes 171,172Os and iridium isotopes 171,172,174Ir, relying on the correlations of the alpha decays with both prompt and delayed gamma rays. It is suggested that the relatively enhanced hindrance factors observed for some of the newly observed decays might be related to changes in the superposition degree of different geometrical shapes (shape coexistence) in the final-state wave functions of the respective daughter nuclei.

Another experimental setup used in the studies was based on the vacuum-mode mass separator MARA and its focal plane detector system in conjunction with the germanium array Jurogam and the charged-particle detector JYTube which was placed around the target position. In this work, the extremely neutron deficient proton emitting lanthanum isotope 116La was discovered. Enhanced proton emission probability and smaller proton-decay Q value were observed for 116La compared with its nearest less neutron-deficient isotope 117La. This unusual circumstance was proposed to be a possible effect of rarely observed neutron-proton pair correlations, as a result of their effect on the spatial distribution of the valence proton wave function. 

 Using the highly selective recoil-decay tagging technique, excited states in the extremely neutron-deficient tellurium isotope 107Te were identified from two independent experiments based on the two setups described above. The deduced level scheme was compared with shell model calculations and TRS (Total Routhian Surface) calculations, indicating that the valence neutron occupying the h11/2 orbit has an important effect on the collectivity. The thesis includes two published articles and two manuscripts in advanced preparation. The experimental techniques and data analysis used in this work are summarized and the theoretical background to the interpretations of the experimental results is briefly outlined.