Speaker
Description
The interplay between single-particle and collective degrees of freedom in atomic nuclei constitutes a fundamental aspect in quantum many-body physics. High-spin excited states of odd-even or odd-odd nuclei typically arise from the interplay between the odd nucleons in high-j orbitals and the collective behaviour of the underlying core. In this context, Au (Z = 79) isotopes provide an excellent opportunity to explore how collective nuclear modes evolve along with high-j single-particle excitations over a long isotopic chain serving as a good playground to probe such an interplay. However, high-spin states in neutron-rich Au isotopes approaching N = 126 remain largely unexplored due to limited methods for their production. The detailed spectroscopic knowledge in this region is also relevant for astrophysical r-process calculations, particularly for constraining first-forbidden β-decay rates.
Producing and achieving unambiguous particle identification of neutron-rich nuclei in this region pose significant experimental challenges. Multi-Nucleon Transfer reactions between a 136Xe beam (7 MeV/u) with 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.
Among the large number of isotopic chains identified, this contribution focuses on new results on the high-spin structure of 195-202Au isotopes [1,2]. New level schemes have been constructed and known structures extended well above the long-lived high-spin isomers. New (25/2⁺) isomers in 199Au and 201Au have been identified. The measurements reveal the disappearance of the odd-J mirror bands in the level schemes of odd-odd Au isotopes for N ≥ 117. The minima in the excitation energies of the bands as a function of N built on the 11/2− and 12− isomers occur at N = 119, show a dip against an otherwise smooth evolution.
The measured excitation energies in the Au exhibit remarkable regularity as a function of neutron number and are seen to be inherited from those of the yrast-band members in the corresponding Hg (Z = 80) isotope. Large-scale shell-model calculations reproduce the observed regularity and show that these states arise from the unique-parity orbitals π0h11/2 and ν0i13/2 coupled to the Hg core. This regularity is attributed to the dominant proton configurations of the Hg and Au isotopes, where the level energies are almost independent of different neutron-orbital occupancies. The calculated Au wave functions show significant higher-spin components of the corresponding Hg core, unlike what is expected in the conventional interpretation of inheritance in terms of the weak-coupling/decoupling limits of the particle-core coupling model.
[1]. Y. Cho et al. (Submitted to Phys. Letts. B)
[2]. Y. Cho et al. (Submitted to Phys. Rev . C) )
| Contribution category | Experiment |
|---|---|
| Presenter status | Postdoc |