Large low-energy M1 strength for ^{56,57}Fe within the nuclear shell model

Area of Science:

• Nuclear Physics
• Astrophysical Nucleosynthesis

Background:

• A recent discovery revealed a significant enhancement in the gamma-ray strength function at low energies for iron-56 and iron-57.
• Understanding this phenomenon is crucial for nuclear structure and astrophysical reaction rates.

Purpose of the Study:

• To theoretically investigate the origin of the low-energy enhancement in the gamma-ray strength function of iron-56 and iron-57.
• To analyze the contributions of M1 and mixed E2 transitions to this enhancement.
• To explore the broader implications for nuclear physics and astrophysics.

Main Methods:

• Theoretical calculation of gamma decay spectra for states up to approximately 8 MeV in excitation for iron-56 and iron-57.
• Analysis of B(M1) and B(E2) transition strengths.
• Identification of key nuclear configurations, specifically high-ℓ diagonal terms.

Main Results:

• Theoretical gamma decay spectra successfully explain the experimentally observed low-energy enhancement.
• Large B(M1) values at low gamma-ray energies are identified as the primary cause.
• Mixed E2 transitions contribute only a minor fraction to the observed enhancement.
• High-ℓ(=f) diagonal terms are found to be most significant for the strong low-energy M1 transitions.

Conclusions:

• The study provides a theoretical explanation for the low-energy gamma-ray strength enhancement in iron isotopes.
• The findings suggest that such low-energy M1 enhancements are likely prevalent across the nuclear chart.
• This has significant implications for understanding the M1 strength function and astrophysical reaction rates.