Magnetic dipole excitations of $^{50}$Cr

TL;DR

This study investigates low-lying magnetic dipole excitations in $^{50}$Cr using nuclear resonance fluorescence, revealing details about the nature of $1^{+}$ states and comparing experimental results with theoretical models.

Contribution

First detailed experimental analysis of $^{50}$Cr's $M1$-strength up to 9.7 MeV with theoretical interpretation using Skyrme QRPA and Large-Scale Shell Model.

Findings

Fifteen $1^{+}$ states observed between 3.6 and 9.7 MeV.

Lowest $1^{+}$ state at 3.6 MeV is an isovector orbital mode.

Results support Lipparini and Stringari's schematic model over the scissors-like picture.

Abstract

The low-lying $M1$-strength of the open-shell nucleus $^{50}$Cr has been studied with the method of nuclear resonance fluorescence up to 9.7 MeV, using bremsstrahlung at the superconducting Darmstadt linear electron accelerator S-DALINAC and Compton backscattered photons at the High Intensity $\gamma$-ray Source (HI$\gamma$S) facility between 6 and 9.7 MeV of the initial photon energy. Fifteen $1^{+}$ states have been observed between 3.6 and 9.7 MeV. Following our analysis, the lowest $1^{+}$ state at 3.6 MeV can be considered as an isovector orbital mode with some spin admixture. The obtained results generally match the estimations and trends typical for the scissors-like mode. Detailed calculations within the Skyrme Quasiparticle Random-Phase-Approximation method and the Large-Scale Shell Model justify our conclusions. The calculated distributions of the orbital current for the lowest $1^{+}$-state suggest the schematic view of Lipparini and Stringari (isovector rotation-like oscillations inside the rigid surface) rather than the scissors-like picture of Lo Iudice and Palumbo. The spin M1 resonance is shown to be mainly generated by spin-flip transitions between the orbitals of the $fp$-shell.