Collective excitations built on the 2⁺ _γ state in ¹⁶⁸Er

Abstract:

To examine the collectivity of the previously proposed K ^π=4⁺γγ-two-phonon state in ¹⁶⁸Er at an excitation energy of 2056 keV and of the first excited K ^π=0⁺ band at 1217 keV (b-band), a Coulomb excitation experiment has been carried out at the Heidelberg-Darmstadt Crystal Ball spectrometer. The B(E2) value connecting the 4⁺ _γγ state to the γ-band was remeasured to be B(E)2,4⁺ _γγ→ 2⁺ _γ)=(600± 130) e²fm⁴, which almost exhausts the harmonic expectation. In addition, the B(E2) values connecting the band head of the lowest lying excited 0⁺ _b band to the 2⁺ _g and 2⁺ _γ states were measured to be B(E2,2⁺ _g→ 0⁺ _b)=(4.4±0.6) e²fm⁴ and B(E2, 2⁺ _γ→ 0⁺ _b)=(30.4±4.6) e²fm⁴; the latter is almost a factor of 10 smaller than the B(E2,2⁺ _γ→ 0⁺ _g) value, which shows that the 0⁺ _b band head has no significant contribution of the K ^π= 0⁺γγ-two-phonon state in its wave function.