High-spin states in the semimagic nucleus ${}^{89}\mathrm{Y}$ and neutron-core excitations in the $N=50$ isotones

The semimagic nucleus ${}^{89}\mathrm{Y}$ has been investigated using the ${}^{82}\mathrm{Se}({}^{11}\mathrm{B},4n)$ reaction at beam energies of 48 and 52 MeV. More than 24 new transitions have been identified, leading to a considerable extension of the level structures of ${}^{89}\mathrm{Y}$. The experimental results are compared with the large-basis shell model calculations. They show that cross-shell neutron excitations play a pivotal role in high-spin level structures of ${}^{89}\mathrm{Y}$. The systematic features of neutron-core excitations in the $N=50$ isotones are also discussed.

Figure 1

Level scheme of ${}^{89}\mathrm{Y}$ established in the present work. New transitions and levels observed in the current work are denoted in red.

Figure 2

Spectra of $\gamma$-rays gated on the 456.1, 470.5+470.7, 403.0, and 880.2+328.9 keV transitions, respectively. The newly observed transitions are indicated by asterisks. Peaks labeled C1, C2, and C3 in the spectra originate from the contaminations of ${}^{89}\mathrm{Sr}$ [23], ${}^{87}\mathrm{Rb}$ [24], and ${}^{84}\mathrm{Kr}$ [25], respectively.

Figure 3

Comparison of the positive parity states of ${}^{89}\mathrm{Y}$ with the SM-I and SM-II results performed using the code oxbash and GWBXG interaction.

Figure 4

Similar to Fig. 3 but for the negative parity states.

Figure 5

Excitation energies as a function of angular momentum for the states in the $N=50$ even-even isotones (left) and odd-$A$ isotones (right) coming from neutron-core excitations across the $N=50$ gap. The data are from Refs. [8, 9, 10, 11, 12, 13, 27, 28] and the present work. Open and filled symbols denote the positive and negative parity states, respectively.