2023-08-16 オークリッジ国立研究所(ORNL)
◆ナトリウム-32核の興奮状態を使用して行った実験で、核の形状進化に関する予想外の結果が得られました。この結果は、原子核の形状とエネルギーが時間とともに変化することを探求する重要な研究です。新たに観測された長寿命の興奮状態は、従来の理論とは異なる可能性を示唆しており、原子核の性質に関する理解を深める上で重要です。
＜関連情報＞
- https://www.ornl.gov/news/long-lived-quantum-state-points-way-solving-mystery-radioactive-nuclei
- https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.242501
FRIBで観測されたN=20形状反転島のマイクロ秒異性体 Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB
T. J. Gray et al.
Physical Review Letters Published 13 June 2023
DOI:https://doi.org/10.1103/PhysRevLett.130.242501
ABSTRACT
Excited-state spectroscopy from the first experiment at the Facility for Rare Isotope Beams (FRIB) is reported. A 24(2)−μs isomer was observed with the FRIB Decay Station initiator (FDSi) through a cascade of 224- and 401-keV γ rays in coincidence with 32Na nuclei. This is the only known microsecond isomer (1 μs≤T1/2<1 ms) in the region. This nucleus is at the heart of the N=20 island of shape inversion and is at the crossroads of the spherical shell-model, deformed shell-model, and ab initio theories. It can be represented as the coupling of a proton hole and neutron particle to 32Mg, 32Mg+π−1+ν+1. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of 32Mg, where the onset of spherical-to-deformed shape inversion begins with a low-lying deformed 2+ state at 885 keV and a low-lying shape-coexisting 0+2 state at 1058 keV. We suggest two possible explanations for the 625-keV isomer in 32Na: a 6− spherical shape isomer that decays by E2 or a 0+ deformed spin isomer that decays by M2. The present results and calculations are most consistent with the latter, indicating that the low-lying states are dominated by deformation.
