Lifetimes and shape-coexisting states of ${}^{99}\mathrm{Zr}$

Lifetimes of intermediate-spin states in two rotational bands of ${}^{99}\mathrm{Zr}$ have been measured. These states were populated following the neutron-induced fission of ${}^{235}\mathrm{U}$ at the PF1B beamline of the Institut Laue-Langevin, Grenoble, during the EXILL-FATIMA campaign. The nucleus ${{}^{99}\mathrm{Zr}}_{59}$ exhibits shape coexistence and lies precisely on the border of an abrupt change in ground-state deformation when going from $N=58$ to $N=60$, making its study interesting for understanding the mechanisms involved in the rapid onset of deformation here. The $B\left(E2\right)$ values extracted for decays in the $\nu 3/2\left[541\right]$ band allow quadrupole deformations of ${\beta }_2=0.34\left(1\right)$ and 0.26(3) to be determined for the 821.6- and 1236.6-keV members, whereas ${\beta }_2=0.32\left(3\right)$ was found for the 850.5-keV member of the $\nu 3/2\left[411\right]$ band. Some of the excited states known in ${}^{99}\mathrm{Zr}$ have been reasonably described with interacting boson-fermion model (IBFM) calculations. Type-II shell evolution is proposed to play a major role in modifying single-particle energies in ${}^{99}\mathrm{Zr}$.

Figure 1

Partial decay scheme of ${}^{99}\mathrm{Zr}$. Transition and level energies are taken from Ref. [18]. Some known weak decay branches are not shown.

Figure 2

Energy spectra obtained when gating on the 536.0-keV transition with the ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ detectors and the 192.6-keV transition (blue) and a background energy of 208.0 keV with the Ge detectors, respectively.

Figure 3

Delayed time spectra obtained when gating on the 536.0- and 121.7-keV transitions with the ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ detectors and additional gates on the 192.6-keV transition (blue) and a background energy of 208.0 keV (red).

Figure 4

Energy spectra obtained when gating on the 121.7- and 192.6-keV $\gamma$ rays in the Ge and ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ detectors, respectively.

Figure 5

Time spectra of the 657.9-keV level obtained with $\gamma$-ray gates on the 121.7-keV transition (Ge) and on the 536.0- and 192.6-keV $\gamma$ rays (${\mathrm{LaBr}}_3$).

Figure 6

Centroid distribution as a function of ${\mathrm{LaBr}}_3$ energy with gates set on the 121.7-keV transition (Ge) and on the 536.0-keV $\gamma$ ray (${\mathrm{LaBr}}_3$).

Figure 7

Centroid distribution as a function of ${\mathrm{LaBr}}_3$ energy with gates set on the 121.7-keV transition (Ge) and on the 192.6-keV $\gamma$ ray (${\mathrm{LaBr}}_3$).

Figure 8

Energy spectra obtained when gating on the 121.7-keV $\left[{\mathrm{Ge}/\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)\right]$, 536.0-keV (Ge), and 218.0-keV $\left[{\mathrm{Ge}/\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)\right]$ $\gamma$ rays.

Figure 9

Kinematic moments of inertia of $\nu 3/2\left[411\right]$ bands in ${}^{99,101,103}\mathrm{Zr}$ and ${}^{97}\mathrm{Sr}$.

Figure 10

Kinematic moments of inertia of $\nu 3/2\left[451\right]$ bands in ${}^{99}\mathrm{Zr}$.

Figure 11

Calculated and experimental level schemes of ${}^{99}\mathrm{Zr}$. Experimental states are labeled with a number corresponding to the order number of their theoretical counterpart for a given spin. For tentative assignments this number is placed within parentheses.