Speaker
Description
The interplay between single-particle and collective degrees of freedom in atomic nuclei constitutes a fundamental aspect in quantum many-body physics, particularly manifest in odd-nucleon systems where unpaired nucleons couple to the even-even core. The region southwest of 208Pb offers a rich landscape for such studies. For example, across the Au isotopic chain, isomeric bands based on πh11/2 and πh11/2 ⊗ νi13/2 configurations in odd-even and odd-odd Au, respectively, provide a systematic probe of how high-j unique-parity nucleons couple to the core. However, neutron-rich Au isotopes approaching N = 126 remain largely unexplored, where decoupling or weak-coupling limits are expected. Investigating the high-spin structure of these nuclei is therefore of considerable interest. Furthermore, detailed spectroscopic knowledge in this region is valuable for astrophysical r-process calculations, particularly for constraining first-forbidden β-decay rates.
Producing neutron-rich nuclei in this region and achieving unambiguous particle identification pose significant experimental challenges. To address the difficulties, MNT reactions between a 136Xe beam (7 MeV/u) and a 198Pt target were employed at GANIL, combined with a suite of complementary spectrometers. Projectile-like fragments (PLFs) were fully identified using the VAMOS++ spectrometer, and the corresponding target-like fragments (TLFs) near N ≈ 126 were selected based on isotopically identified PLFs and reconstructed excitation energies. Prompt γ rays were detected with AGATA, a state-of-the-art HPGe tracking array, enabling high-spin spectroscopy of the TLFs. Additionally, CATLIFE—a time-of-flight spectrometer coupled with the EXOGAM HPGe array—was employed to measure delayed γ rays and determine TLF mass numbers prior to neutron evaporation, providing crucial independent fragment characterization. A novel kinetic-energy calibration method based on supervised machine-learning techniques was implemented for the VAMOS++ data, improving ion charge-state identification at energies near the Bragg peak.
In this contribution, we will report new results on the high-spin structure of 195-202Au isotopes. New level schemes have been constructed and known structures extended above the long-lived isomers. Notably, new (25/2⁺) isomers in 199Au and 201Au have been identified, with half-lives of T1/2 = 140(20) μs and 15.2(29) μs, respectively. The excitation energies in the Au isotopes reflect structures inherited from corresponding states of the neighboring Hg cores and evolve consistently across the extended isotopic chain. A local dip in the level-energy systematics, deviating from the monotonic trend toward the N = 126 shell closure, is observed at N = 119. Furthermore, our measurements reveal the disappearance of the odd-J mirror bands in the level schemes of odd-odd Au isotopes for N ≥ 117. These experimental findings, interpreted within the framework of large-scale shell-model calculations, advance our understanding of the interplay between high-j orbitals and the collective core near the N = 126 shell closure.