Microscopic study of the low-energy enhancement in the gamma-decay strength of \(^{50}\)V

Abstract

We address the microscopic origin of the low-energy enhancement (LEE) in \(^{50}\)V with large-scale shell-model calculations to obtain $E1$ and $M1$ transitions within the same theoretical framework. The valence space spans the three major shells $sd$, $pf$ and $sdg$ and is treated with the SDPFSDG-MU interaction using the KSHELL code. With a \(1 \hbar ω\) truncation, 3600 energy eigenstates and a basis of $7.02\times10^{6}$ positive and $5.94\times10^{8}$ negative parity states, the calculations yield nearly two million individual dipole transitions. The fourteen lowest experimental levels are reproduced within $0.30$~MeV, the calculated total level density excellently reproduces Oslo-method data up to $E \approx 7.5$~MeV, and the calculated dipole gamma strength function follows the experimental shape -- including the LEE -- for the full gamma-energy range covered by the Oslo experiment. The LEE is shown to be entirely magnetic dipole in origin. Both spin and orbital parts of the \(\hat{M}1\) operator are required to reproduce the LEE, with constructive interference between the spin and orbital parts giving an extra enhancement to the LEE. Reduced one-body transition densities identify $0f_{7/2} \rightarrow 0f_{7/2}$ proton transitions as the principal driver of the LEE.