Experimental investigation of high-spin states in ${}^{90}\mathrm{Zr}$

High-spin states of ${}^{90}\mathrm{Zr}$ have been investigated using the heavy-ion fusion-evaporation reaction ${}^{82}\mathrm{Se}({}^{13}\mathrm{C},5n)$ at a beam energy of 60 MeV. Excited levels of ${}^{90}\mathrm{Zr}$ have been observed up to an excitation energy of $\approx 13$ MeV and a spin of $\approx 20\hslash$ with the addition of thirty-two new $\gamma$-ray transitions to the proposed level scheme. Structures of both the positive- and negative-parity states up to the highest observed spin have been interpreted with shell-model calculations using the GWBXG interaction and a ${}^{68}\mathrm{Ni}$ core. Calculations suggest the role of neutron excitations across the $N=50$ shell gap for states with greater than 7 MeV excitation energy. High-spin states in these bands are interpreted to be generated by the recoupling of stretched proton and neutron configurations.

Figure 1

Partial level scheme of ${}^{90}\mathrm{Zr}$ developed in the present work. The energies of the excited states and $\gamma$-ray transitions are given in keV units. The thickness of the arrows above the 3588 keV state are proportional to the $\gamma$-ray intensities mentioned in Table 1. Newly observed $\gamma$-ray transitions are marked by red labels. The spin-parity of the levels marked by blue labels have been measured. The purple labels represent the level energies in keV units. The experimentally observed values are given for $\gamma$-ray transition energies whereas the level energies are the output of the gamma-to-level (gtol) least-squares fitting code developed at NNDC [44]. The transition energies and level energies shown in the level scheme have been rounded to the nearest whole numbers. The values of $t_{1/2}$ of the 2319 and 3588 keV levels are adopted from Ref. [45].

Figure 2

Representative spectrum showing the sum of double-gated spectra with one gate on 213- and other on 620-, 1310-, 1658- and 2055-keV transitions. The newly observed transitions in ${}^{90}\mathrm{Zr}$ are labeled in red. The insets (a) and (b) depict the expanded energy ranges from 950 to 1650 keV and from 1825 to 2775 keV, respectively.

Figure 3

Representative spectrum showing the sum of double-gated spectra with one gate on 1427- and other on 141-, 213-, 1310- and 2055-keV transitions. The newly observed transitions in ${}^{90}\mathrm{Zr}$ are labeled in red. The inset shows the expanded energy range from 300 to 1000 keV for a better view of the weaker transitions.

Figure 4

Angular distribution of (a) $L=2$, 2055-keV (${10}^{+}\to 8^{+}$); (b) $L=1$, 1310-keV (${11}^{-}\to {10}^{+}$); and (c) $L=1$, 818-keV (${11}^{+}\to {10}^{-}$) $\gamma$-ray transitions in ${}^{90}\mathrm{Zr}$. The experimental data are represented by filled red circles and the fitted curve shown as a black solid line. The errors in relative yield have been multiplied by a factor of two for a better view.

Figure 5

A comparison of the experimental excitation energies of the positive-parity states (left) of ${}^{90}\mathrm{Zr}$ with those from shell-model calculations using the GWBXG effective interaction (right). For a given spin whenever more than one state is present, they have been marked with red (second-excited state).

Figure 6

A comparison of the experimental excitation energies of the negative-parity states (left) of ${}^{90}\mathrm{Zr}$ with those from shell-model calculations using the GWBXG effective interaction (right). For a given spin, whenever more than one state is present, they have been marked with red (second-excited state) and blue (third-excited state).