High-spin level structure of the semi-magic nucleus ${}^{91}\mathrm{Nb}$

The high-spin level structure of the semi-magic nucleus ${}^{91}\mathrm{Nb}$ has been investigated via the ${}^{82}$Se(${}^{14}$N,$5n$)${}^{91}\mathrm{Nb}$ reaction at a beam energy of 60 MeV. Based on these experimental results, a new level scheme is established that modifies and extends earlier schemes. Shell-model calculations have been carried out to interpret the level structure of ${}^{91}\mathrm{Nb}$. The calculated results show that the inclusion of neutron particle-hole excitation across the $N=50$ shell gap is essential to adequately describe states above $I=(21/2–23/2)\hslash$, with excitation energies above $\sim$6.0 MeV.

Figure 1

The level scheme of ${}^{91}\mathrm{Nb}$ established in the present work. The energies of the $\gamma$ transitions and of the levels are given in keV. Spins and parities in parentheses are tentative assignments. New $\gamma$ transitions observed in this work are denoted with asterisks.

Figure 2

Background-corrected coincidence spectrum obtained by gating on the 356.3 keV $\gamma$ ray in group A. The energies with single or double asterisks are contaminations from the reaction channel ${}^{82}$Se(${}^{14}$N,$1p3n$)${}^{92}$Zr or the single-escape peak of the 2290.4 keV $\gamma$ ray in ${}^{91}\mathrm{Nb}$, respectively.

Figure 3

Background-corrected coincidence spectrum obtained by double gating on the 185.3 and 497.5 keV $\gamma$ rays in group A. The energies with single or double asterisks are contaminations from the reaction channels ${}^{82}$Se(${}^{14}$N,4$n$)${}^{92}$Nb or ${}^{82}$Se(${}^{14}$N,3$n$)${}^{93}$Nb, respectively.

Figure 4

Background-corrected coincidence spectrum obtained by gating on the 1484.1 keV $\gamma$ ray in group A. The energies with single asterisk are contaminations from the reaction channel ${}^{82}$Se(${}^{14}$N,4$n$)${}^{92}$Nb.

Figure 5

Background-corrected coincidence spectrum obtained by gating on the 2062.0 keV $\gamma$ ray in group B.

Figure 6

Comparison of experimental and calculated energy levels of positive-parity yrast states in ${}^{91}\mathrm{Nb}$ within SM-I and SM-II configuration spaces.

Figure 7

Comparison of experimental and calculated energy levels of negative-parity non-yrast states in groups B and D of ${}^{91}\mathrm{Nb}$ within SM-I and SM-II configuration spaces.