100 Years of Nuclear Isomers

Around closed shells, intruder orbitals with large angular momentum difference and opposite parity compared to the ground state orbital lead to an accumulation of isomeric states. Below the $N=82$ shell, low-lying states in the even-$Z$, $N=81$ isotones are the $J^{\pi }=1/2^{+},3/2^{+}$ and $11/2^{-}$ neutron-hole states, associated with the $s_{1/2}$, $d_{3/2}$, and $h_{11/2}$ orbitals, respectively. From ${}_{\phantom{1}50}^{131}$Sn to ${}_{\phantom{1}68}^{149}$Er, the $J^{\pi }=11/2^{-}$ states are isomeric, in ${}_{\phantom{1}48}^{129}$Cd this becomes the ground state [1]. In this chain of isomers, the excitation energy remains constant at $750$keV from ${}_{\phantom{1}58}^{139}$Ce to ${}_{\phantom{1}68}^{149}$Er, which is a unique feature on the nuclear chart for long-lived isomeric states.

Recently [2], this chain was extended towards the proton drip line using TITAN's multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) at TRIUMF. MR-TOF-MS are a powerful tool to discover long-lived isomers [4, 5] and study their properties [6]. Masses of neutron-deficient Yb isotopes including the ground and isomeric state in ${}_{\phantom{1}70}^{151}$Yb were measured, and the excitation energy of the $J^{\pi }=11/2^{-}$ isomer in ${}_{\phantom{1}70}^{151}$Yb was thus derived. The new value falls in line with the observed systematics. State-of-the-art mean field calculations including shape degrees of freedom were performed to unravel the constancy of the excitation energies. The measurements and the theoretical results will be presented.

[1] V. Manea et al., Phys. Rev. Lett. 124, 092502 (2020)
[2] S. Beck et al., Phys. Rev. Lett. 127, 112501 (2021)
[3] C. Hornung et al., Phys. Lett. B 802, 135200 (2020)
[4] C. Izzo et al., Phys. Rev. C 103, 025811 (2021)
[5] I. Miskun et al., Eur. Phys. J. A 55, 148 (2019)