Evidence for spherical-oblate shape coexistence in ${}^{87}\mathrm{Tc}$

Excited states in the neutron-deficient nucleus ${}^{87}\mathrm{Tc}$ have been studied via the fusion-evaporation reaction ${}^{54}\mathrm{Fe}({}^{36}\mathrm{Ar},2n1p){}^{87}\mathrm{Tc}$ at 115 MeV beam energy. The AGATA $\gamma$-ray spectrometer coupled to the DIAMANT, NEDA, and Neutron Wall detector arrays for light-particle detection was used to measure the prompt coincidence of $\gamma$ rays and light particles. Six transitions from the deexcitation of excited states belonging to a new band in ${}^{87}\mathrm{Tc}$ were identified by comparing $\gamma$-ray intensities in the spectra gated under different reaction channel selection conditions. The constructed level structure was compared with the shell model and total Routhian surface calculations. The results indicate that the new band structure in ${}^{87}\mathrm{Tc}$ is built on a spherical configuration, which is different from that assigned to the previously identified oblate yrast rotational band.

Figure 1

Comparison of the total $2n$-gated spectrum (a) and the $2n$-gated spectrum after the purification (b). The enhanced and new peaks in the spectrum (b) come from $2n$ related channels. The four red dotted lines indicate the positions of the 712, 887, 719, and 863 keV peaks from ${}^{87}\mathrm{Tc}$.

Figure 2

For the newly identified $\gamma$ rays, (a) sum of the gated spectra (red) on 718.6, 731.6, 791.7, 862.8, 905, and 1042.4 keV lines with the width of 2 keV, and summed normalized background spectra (black) gated 4.75 keV above these lines with the width of 2.5 keV. The coincidence spectrum in (b) is the difference between the two spectra in (a). For the previously reported yrast band, (c) sum of the background-subtracted coincidence spectra for the yrast band in ${}^{87}\mathrm{Tc}$ [10] gated on the 712.0, 886.5, 1007.4, 845.9, and 956.3 keV lines.

Figure 3

Intensity ratios for different $\gamma$-ray transitions deduced from the $1p$-gated spectrum and the $0p$-gated spectrum. All ratios were measured using other coincident transitions as gates. The transition energies of ${}^{49}\mathrm{Cr}$ and ${}^{87}\mathrm{Mo}$ are from Ref. [19, 20].

Figure 4

Intensity ratios for different $\gamma$-ray transitions deduced from the $0n$-gated spectrum, the $1n$-gated spectrum, and the $2n$-gated spectrum. In the measurement, triple $\gamma \text{−}\gamma \text{−}\gamma$ coincidences are used for a better peak-to-background ratio.

Figure 5

The background-subtracted spectra gated at 791.7 keV (a), 731.6 keV (b), 718.6 and 862.8 keV (c). The width of all gates was 3 keV. The background spectra were gated in the range from 3 to 18 keV above these lines, excluding all areas covered by clear peaks in these energy intervals.

Figure 6

Level scheme of ${}^{87}\mathrm{Tc}$. The left part (black) is for the yrast band from Ref. [10]. The right part (red) is deduced in the present work. The width of the arrows are proportional to the relative intensities of the $\gamma$ rays listed in Table 1. Based on TRS calculations, we propose the newly discovered ($7/2_1^{+}$) state as the ground state, and the ($9/2_1^{+}$) state reported in Ref. [10] as the excited state. “$+\mathrm{X}$” in the left part is used to indicate an overall shift due to the unknown excitation energy of the ($9/2_1^{+}$) state.

Figure 7

Total Routhian surfaces in the (${\beta }_2,\gamma$) plane for the ($\pi ,\alpha$) = ($+$, $+1/2$) and ($+$, $-1/2$) configurations in ${}^{87}\mathrm{Tc}$. The distance between contour lines is 0.2 MeV. The positions of spherical and oblate minima on the surfaces are marked with red and black dots, respectively.

Figure 8

Comparison of the experimental level scheme (left) and the shell model prediction (right).