Speaker
Description
The low-energy enhancement (LEE) of the dipole γ-ray strength function has been observed in many nuclei, yet its microscopic origin remains debated. We investigate the LEE in $^{50}$V using large-scale shell-model calculations that treat electric and magnetic dipole transitions consistently within a single framework. Calculations are performed in a sd–pf–sdg valence space with a $1\hbar\omega$ truncation, employing the SDPFSDG-MU interaction and the KSHELL code. The model space yields several thousand eigenstates and nearly two million individual E1 and M1 transitions.
Benchmark comparisons demonstrate excellent agreement with experimental data: low-lying levels are reproduced within 300 keV, the calculated level density matches Oslo-method data up to $E_x\!\approx\!7.5$ MeV, and the dipole γ-strength function follows the measured shape over the full experimental γ-energy range, including the LEE. By separating electric and magnetic contributions, we show that the enhancement in $^{50}$V is entirely of magnetic dipole origin. Both spin and orbital components of the M1 operator are essential, with constructive interference between them providing a significant additional enhancement at low γ energies.
Analysis of one-body transition densities identifies $0f_{7/2}\!\rightarrow\!0f_{7/2}$ proton transitions as the dominant microscopic mechanism driving the LEE, while transitions between different orbitals govern the higher-energy M1 strength. These results establish a direct link between specific shell-model configurations and emergent statistical properties of γ decay, providing a quantitative microscopic explanation of the LEE in this mass region.