Experimental investigation of high-spin states in ${}^{91}\mathrm{Nb}$

High-spin states of ${}^{91}\mathrm{Nb}$ have been investigated using the heavy-ion fusion-evaporation reaction ${}^{65}\mathrm{Cu}({}^{30}\mathrm{Si}$, $2p2n)$ at a beam energy of 120 MeV. States of ${}^{91}\mathrm{Nb}$ up to excitation energy $\approx 12$ MeV and spin $41/2\hslash$ have been observed with the addition of thirteen new $\gamma$-ray transitions to the proposed level scheme. Structures of 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. High-spin states in this nucleus are dominated by the configurations involving both proton- and neutron-core excited structures. Owing to the presence of one valence proton in the $1g_{9/2}$ orbital, an important role of neutron excitation in the $1g_{7/2}$ orbital in generating the high-spin states in ${}^{91}\mathrm{Nb}$ has been identified.

Figure 1

Partial level scheme of ${}^{91}\mathrm{Nb}$ developed in the present work. The energies of the excited states and $\gamma$-ray transitions are given in keV units. Newly observed $\gamma$-ray transitions are marked by red labels. The spin-parity of the levels marked by blue labels has been measured. The green 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 National Nuclear Data Center (NNDC) [36]. The values of $T_{1/2}$ of the 105- and 2035-keV levels are adopted from Ref. [37].

Figure 2

Representative spectrum showing the sum of double-gated spectra with one gate on 2291.1- and other on 819.5- and 356.4-keV transitions. The newly observed transitions in ${}^{91}\mathrm{Nb}$ are labeled in red.

Figure 3

Representative spectrum showing the sum of double-gated spectra with one gate on 421.6- and other on 185.2-, 497.0-, 661.0-, and 747.8-keV transitions. The newly observed transitions in ${}^{91}\mathrm{Nb}$ are labeled in red.

Figure 4

Representative spectrum showing the sum of double-gated spectra with (a) one gate on 2291.1-, 819.5-, and 356.4-keV, and the other on 2076.7-, 730.2-, 645.0-, and 795.4-keV transitions; (b) one gate on 919.2- and other on 254.2- and 884.5-keV transitions. The newly observed transitions in ${}^{91}\mathrm{Nb}$ are labeled in red.

Figure 5

Angular distribution of – (a) $L=2$, 2291.1-keV ($13/2^{+}\to 9/2^{+}$), (b) $L=2$, 356.4-keV ($21/2^{+}\to 17/2^{+}$), and (c) $L=1$, 747.8-keV ($35/2^{+}\to 33/2^{+}$) $\gamma$ transitions in ${}^{91}\mathrm{Nb}$. The experimental data are represented by open red circles and the fitted curve by the blue solid line. The errors in relative yield are multiplied by a factor of 2 for a better view.

Figure 6

A comparison of the experimental excitation energies of the positive-parity states (left) of ${}^{91}\mathrm{Nb}$ with those from shell-model calculations using the GWBXG effective interaction (right). For a given spin, whenever more than one state is present, they are marked with red (second-excited state), blue (third-excited state), and green (fourth-excited state).

Figure 7

A comparison of the experimental excitation energies of the negative-parity states (left) of ${}^{91}\mathrm{Nb}$ with those from shell-model calculations using the GWBXG effective interaction (right). For a given spin, whenever more than one state is present, they are marked with red (second-excited state) and blue (third-excited state).