Collapse of the N=28 shell closure in (42)Si

Area of Science:

• Nuclear Physics
• Atomic Physics
• Quantum Mechanics

Background:

• The traditional nuclear shell model predicts stable spherical nuclei at specific "magic numbers" of protons (Z) and neutrons (N), such as Z=14 and N=28.
• Investigating neutron-rich isotopes is crucial for understanding how nuclear structure evolves away from stability.
• Deviations from expected shell closures provide insights into the fundamental forces governing atomic nuclei.

Purpose of the Study:

• To investigate the excited state energies of very neutron-rich silicon-42 (⁴²Si) and phosphorus-41, 43 (⁴¹P, ⁴³P).
• To explore the impact of proton-neutron tensor forces on nuclear shell structure in exotic isotopes.
• To determine the structural properties of ⁴²Si, specifically its deviation from a spherical configuration.

Main Methods:

• In-beam gamma-ray spectroscopy was employed to measure excited state energies.
• Experiments utilized the fragmentation of secondary beams of sulfur-42 and sulfur-44 (⁴²S, ⁴⁴S) at an energy of 39 MeV per nucleon.
• Nuclear shell model calculations were performed to interpret the experimental results.

Main Results:

• The low-lying 2+ excited state energy in ⁴²Si was measured to be 770(19) keV.
• The measured level schemes of ⁴¹P and ⁴³P were determined.
• These results provide evidence for the weakening or disappearance of the predicted Z=14 and N=28 spherical shell closures.

Conclusions:

• The observed low 2+ energy in ⁴²Si and the level schemes of the phosphorus isotopes indicate a breakdown of traditional shell closures.
• The proton-neutron tensor force is identified as the primary mechanism responsible for this shell evolution.
• Shell model calculations suggest that ⁴²Si exhibits characteristics of a well-deformed oblate rotor, rather than a spherical nucleus.