In May 2022 at GSI Helmholtzzentrum f¨ur Schwerionenforschung, a high-intensity, 208Pb primary

beam of 1 GeV/u was fired at a 9Be target to populate neutron-rich fragmentation products at

neutron number N ≈126. The study of such nuclei forms an important part of our understanding

of nuclear structure and the testing of nuclear shell model predictions. In addition, investigating

increasingly neutron-rich N =126 nuclei brings us closer to important astrophysical nuclei, aiding in

the refinement of current r-process path predictions. Fragmentation products from GSI’s fragment

separator were implanted in two 1 mm thick 24×8 cm2 AIDA active Si stoppers and two scintillating

βplastic detectors. The implanted ions were surrounded by eight newly developed DEGAS and

two well-established EUROBALL, high-purity Ge detectors.

The primary aim of this experiment was to study the structure of N =126 nuclei through observing

isomeric transitions. To this aim, GSI’s fragment separator was centred on the N = 126 nuclei

203Ir and 202Os for ∼ 1 day and ∼ 3 days, respectively. A total of 62 different neutron-rich

nuclei were populated in this experiment, where γ-ray spectroscopy could be carried out for 40

different nuclei. Where possible, experimental level schemes and available transition strengths for

N = 126, 125 nuclei have been determined and compared with predictions from three different

shell model parametrisations.

Key findings in this thesis include a revised level scheme for 203Ir based on newly observed transitions

and a much improved isomeric lifetime measurement. This nucleus, possessing five proton

holes with respect to 208Pb, is the most neutron rich N =126 nucleus with previous spectroscopic

information. Furthermore, a tentative partial level scheme is given for 202Os, the most neutron-rich

N =126 nucleus ever observed. In addition, the first spectroscopic information on 197Re and 200Re

is presented, where the nucleus 200Re has been observed for the first time in the present experiment.

In 197Re, a short isomeric half-life, lack of strong X-rays, and a relatively large γ-ray energy are

in contrast to those observed in 192,193,194,196Re. This suggests that the predicted prolate-oblate

shape transition in rhenium isotopes occurs between A=196 and 197.