Evidence for enhanced neutron-proton correlations from the level structure of the $N=Z+1$ nucleus ${}_{43}{}^{87}\mathrm{Tc}_{44}$

The low-lying excited states in the neutron-deficient $N=Z+1$ nucleus ${}_{43}{}^{87}\mathrm{Tc}_{44}$ have been studied via the fusion-evaporation reaction ${}^{54}\mathrm{Fe}({}^{36}\mathrm{Ar}, 2n1p){}^{87}\mathrm{Tc}$ at the Grand Accélérateur National d'Ions Lourds (GANIL), France. The AGATA spectrometer was used in conjunction with the auxiliary NEDA, Neutron Wall, and DIAMANT detector arrays to measure coincident prompt $\gamma$ rays, neutrons, and charged particles emitted in the reaction. A level scheme of ${}^{87}\mathrm{Tc}$ from the ($9/2_{\mathrm{g}.\mathrm{s}.}^{+}$) state to the ($33/2_1^{+}$) state was established based on six mutually coincident $\gamma$-ray transitions. The constructed level structure exhibits a rotational behavior with a sharp backbending at $\hslash \omega \approx 0.50$ MeV. A decrease in alignment frequency and increase in alignment sharpness in the odd-mass isotonic chains around $N=44$ is proposed as an effect of the enhanced isoscalar neutron-proton interactions in odd-mass nuclei when approaching the $N=Z$ line.

Figure 1

(a) Sum of the spectra gated on the 712.0-, 886.5-, 1007.4-, 845.9-, and 956.3-keV lines (red), and the summed background spectrum (black) gated 4 keV above these lines. The background spectrum was normalized to the gated spectrum according to the statistics below 2 MeV. $\gamma$-ray peaks due to contaminant reactions are indicated. The coincidence spectrum in (b) is the difference between the two spectra shown in (a). (c) Level scheme of ${}^{87}\mathrm{Tc}$ deduced from the present work. The tentative spin-parity assignments were made based on the systematics as well as shell-model calculations, and the width of the arrows are proportional to the relative intensities of the $\gamma$ rays. The relative intensities: 100(4), 80(3), 65(5), 37(4), 26(3), and 13(3).

Figure 2

Systematics of positive-parity yrast states below 6 MeV in ${}^{87}\mathrm{Tc}$ together with the odd-$A N=44$ isotones [31, 32] in comparison with neighboring technetium isotopes ${}^{89,91,93}\mathrm{Tc}$ [33, 34, 35]. The results for ${}^{87}\mathrm{Tc}$ are from the present work.

Figure 3

Kinematic moments of inertia [$J^{\left(1\right)}$] as a function of rotational frequency for the positive parity and positive signature bands in the $N=44$ even-even [13, 37, 38] isotones (a), $N=44$ odd-mass isotones (b), and $N=43$ odd-mass [39, 40, 41] isotones (c).

Figure 4

Left: The experimental level scheme of ${}^{87}\mathrm{Tc}$ in comparison with the shell-model calculations in the $pg$ model space based on the slgmt0 [43] interaction (slgmt0* denotes the calculation with the spin-aligned neutron-proton interaction reduced by 200 keV). The cartoon illustrates the coupling between the unpaired neutron with the spin-aligned np pair which is the dominant component of the wave function for the higher-spin states. Right: Kinematic moments of inertia as a function of rotational frequency for the positive parity and positive signature bands in ${}^{87}\mathrm{Tc}$ and ${}^{89}\mathrm{Tc}$ from the experimental data and theoretical calculations (slgmt0 and slgmt0*).