Impact of Coriolis mixing on a two-quasi-neutron isomer in ${}^{164}\mathbf{Gd}_{100}$ and other $N=100$ isotones

We report on two complementary delayed gamma-ray spectroscopic studies on ${}_{64}{}^{164}\mathrm{Gd}_{100}$. The nucleus is produced via spontaneous fission of ${}^{252}\mathrm{Cf}$ sources used in selective experimental setups. An isomeric state at 1095 keV with a half-life of 605 (30) ns is confirmed as well as its decay path toward the ground state band. From comparison to other $N=100$ isotones and with calculations based on the axially symmetric-deformed quasiparticle random phase approximation (QRPA) using the up-to-date Gogny interaction a $4^{-}$ spin and parity is proposed for the isomer. The state is interpreted as a two-quasi-neutron excitation in line with available data for others $N=100$ isotones. The variation of the lifetime of the isomeric state along the $N=100$ isotones is interpreted in terms of Coriolis mixing implemented for the first time within axially symmetric-deformed QRPA microscopic calculations.

Figure 1

Scaled schematic view of the experimental setup used in the first experiment. The lead shielding around and inside the vacuum chamber is reported in dark gray. The vacuum chamber is colored in green and the fission chamber is represented in light blue.

Figure 2

(a) Normalized anode pulse height measured for fission fragments entering the fission chamber used in Setup 1 (black curve). Arbitrarily scaled distributions gated by the detection of a gamma-ray deexecting isomeric states in ${}^{146}\mathrm{La}$, ${}^{132}\mathrm{Sb}$, and ${}^{99}\mathrm{Zr}$ are also reported. (b) Normalized anode pulse height measured with the twin ionization chamber used in Setup 2. (c) Two-dimensional spectrum of anode pulse height measured for fission fragments detected in coincidence in the ionization chamber. (d) Portion of kinetic energies plot showing structures associated with neutron-less fission events.

Figure 3

(a) Gamma-ray spectrum measured in coincidence with a heavy fragment in the fission chamber in Setup 1 and in coincidence with a delayed $\gamma$-ray at 854 keV. (b) Single delayed $\gamma$-ray spectrum obtained for fission events with a heavy fragment postneutron mass of 164 and $\mathrm{TKE}>175\mathrm{MeV}$. (c) Time pattern associated with the 168 keV $\gamma$ ray. (d) Deduced partial level scheme for ${}^{164}\mathrm{Gd}$.

Figure 4

Low-energy level structure for $N=100$ isotones discussed in the text. Diamond (circle) symbols show experimental (theoretical) data. The number reported on top of the experimental $4^{-}$ point corresponds to the measured half-live (in ns) of the isomer. The theoretical ${(2–8)}^{+}$ excitation energies are calculated within the 5DCH approach, while the theoretical $4^{-}$ energy, shown as filled pink circle, is the result of QRPA calculations.

Figure 5

Excitation energy of the $J^{\pi }=4^{-}$ states built on $K=0$, 1, 2, 3, and 4 QRPA band heads for $N=100$ isotones of interest. States are labeled by their $K$ values.