Physical Review Letters ( IF 8.385 ) Pub Date :
T. Beck; V. Werner; N. Pietralla; M. Bhike; N. Cooper; U. Friman-Gayer; J. Isaak; R. V. Jolos; J. Kleemann; Krishichayan; O. Papst; W. Tornow; C. Bernards; B. P. Crider; R. S. Ilieva; B. Löher; C. Mihai; F. Naqvi; S. Pascu; E. E. Peters; F. M. Prados-Estevez; T. J. Ross; D. Savran; J. R. Vanhoy; A. Zilges

The size of a $\Delta K=0$ $M1$ excitation strength has been determined for the first time in a predominantly axially deformed even-even nucleus. It has been obtained from the observation of a rare $K$-mixing situation between two close-lying ${J}^{\pi }={1}^{+}$ states of the nucleus ${}^{164}$Dy with components characterized by intrinsic projection quantum numbers $K=0$ and $K=1$. Nuclear resonance fluorescence induced by quasimonochromatic linearly polarized $\gamma$-ray beams provided evidence for $K$ mixing of the ${1}^{+}$ states at $3159.1\left(3\right)$ and $3173.6\left(3\right)$,keV in excitation energy from their $\gamma$-decay branching ratios into the ground-state band. The $\Delta K=0$ transition strength of $B\left(M1;{0}^{+}\to {1}_{K=0}^{+}\right)=0.008\left(1\right)\phantom{\rule{0.167em}{0ex}}{\mu }_{\mathrm{\text{N}}}^{2}$ was inferred from a mixing analysis of their $M1$ transition rates into the ground-state band. It is in agreement with predictions from the quasiparticle phonon nuclear model. This determination represents first experimental information on the $M1$ excitation strength of a nuclear quantum state with a negative $ℛ$-symmetry quantum number.

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