Experimental study of the level structure in ${}^{90}\mathrm{Nb}$ and systematics of level structure characteristics near $A=90$

In beam $\gamma$-ray spectroscopic investigations of the ${}^{90}\mathrm{Nb}$ nucleus have been explored by employing the ${}^{76}\mathrm{Ge}({}^{19}\mathrm{F},5n){}^{90}\mathrm{Nb}$ reaction. Twenty-eight new transitions belonging to ${}^{90}\mathrm{Nb}$ have been identified and placed in the proposed level scheme through the analysis of $\gamma \text{−}\gamma$ coincidence spectra. Shell model calculations have been performed in the restricted $\pi (p_{3/2},f_{5/2},p_{1/2},g_{9/2})\otimes \nu (p_{1/2},g_{9/2},d_{5/2},g_{7/2})$ configuration space, and compared to the new level scheme. According to the calculations, the newly observed 2614.6 keV $\gamma$ ray depopulating the $\left({15}_3^{+}\right)$ level at 4421.2 keV is explicated as the breakup of the $N=50$ neutron closed shell, while the $\gamma$ ray with 2774.0 keV feeding the $\left({13}_1^{-}\right)$ level at 2812.9 keV is interpreted as protons excitation across the $Z=38$ subshell.

Figure 1

Proposed level scheme for ${}^{90}\mathrm{Nb}$ originated from the present and previous work [34, 35]. New $\gamma$ rays are marked with red asterisks. The level and $\gamma$ transition energies are given in keV. Arrow width stands for the relative $\gamma$ ray intensity. Spin and parity assignments for levels are tentative.

Figure 2

$\gamma \text{−}\gamma$ coincidence spectrum with the sum gated on the 755.0 and 495.8 keV transitions. The numbers displayed in the spectra are the peak energies in keV. The peaks marked with circles, a square, a pentagon, and a star originated from the reactions ${}^{76}\mathrm{Ge}({}^{19}\mathrm{F},4n){}^{91}\mathrm{Nb}$, ${}^{27}\mathrm{Al}(n,n^{\prime }$), Coulomb excitation of ${}^{56}\mathrm{Fe}$, and electron positron annihilation, respectively.

Figure 3

$\gamma \text{−}\gamma$ coincidence spectra gated on the 1695.0 keV $\gamma$ ray. The numbers displayed in the spectra are the peak energies in keV. The peaks with black circles are contaminated by the 1692.8 keV $\gamma$ ray.

Figure 4

$\gamma \text{−}\gamma$ coincidence spectrum with the sum gated on the 537.2 and 576.0 keV transitions. The numbers displayed in the spectra are the peak energies in keV. The peaks with a pentagon and a star originated from the Coulomb excitation of ${}^{56}\mathrm{Fe}$ and electron positron annihilation, respectively.

Figure 5

$\gamma \text{−}\gamma$ coincidence spectra from gates on the 384.0 and 678.0 keV $\gamma$ rays. The peaks with red and black lines originated from the gates on 384.0 and 678.0 keV, respectively.

Figure 6

Comparison between calculated levels and experimental ones in ${}^{90}\mathrm{Nb}$.

Figure 7

(a) The level energies in Nb odd-$A$ isotopes; (b) the same as (a), but for Nb odd-odd isotopes; (c) the ratios of the two lowest level energies observed relative to ground states. $R=[E_{I+4}-E_I]/[E_{I+2}-E_I]$, with $I=9/2$ for odd-$A$ nuclei and $I=8$ for odd-odd nuclei.

Figure 8

(a) Comparison between the energy levels up to the ${16}^{+}\hslash$ in ${}^{90}\mathrm{Nb}$ and its odd-odd neighbors nuclei ${}^{88}\mathrm{Nb}$, ${}^{90}\mathrm{Tc}$ and ${}^{92}\mathrm{Tc}$; (b) Comparison between the energy levels up to the $25/2^{+}\hslash$ in ${}^{89}\mathrm{Nb}$ and its odd-$A$ neighbors nuclei ${}^{91}\mathrm{Nb}$, ${}^{91}\mathrm{Tc}$, and ${}^{93}\mathrm{Tc}$.

Figure 9

(a) Energy levels and isomer half-lives in the $N=50$ isotones; (b) the same as (a), but for $N=49$ isotones.