Structure of ${}^{83}\mathrm{As},$ ${}^{85}\mathrm{As},$ and ${}^{87}\mathrm{As}$: From semimagicity to $\gamma$ softness

The structure of ${}^{83}\mathrm{As}$, ${}^{85}\mathrm{As}$, and ${}^{87}\mathrm{As}$ have been studied in fusion-fission reaction ${}^{238}\mathrm{U}+{}^9\mathrm{Be}$. Fission fragments were identified in mass and atomic number using the VAMOS$++$ spectrometer and the coincident $\gamma$ rays were detected in the $\gamma$-ray tracking array AGATA. New transitions in ${}^{83}\mathrm{As}$ and ${}^{85}\mathrm{As}$ are reported and placed in the level schemes. A level scheme of the excited states in ${}^{87}\mathrm{As}$ is proposed for the first time. The data are interpreted in frame of large-scale shell-model calculations, SU3 symmetries, and beyond mean-field frameworks. A spherical regime at magic number $N=50$ is predicted and the location of the proton $g_{9/2}$ orbital is proposed for the first time. Development of collectivity in a prolate deformed, $\gamma$-soft regime in the open shell cases ${}^{85}\mathrm{As}$ and ${}^{87}\mathrm{As}$, most neutron-rich isotopes beyond $N=50$, is concluded. Data and theoretical calculations give confidence to a relatively high extrapolated excitation energy about 4 MeV of the $9/2^{+}$ state in ${}^{79}\mathrm{Cu}$, one proton above ${}^{78}\mathrm{Ni}$.

Figure 1

Tracked $\gamma$-ray spectrum in coincidence with ${}^{83}\mathrm{As}$ ions identified in VAMOS$++$. (♥) ${}^{83}\mathrm{As}$ transitions reported for the first time in this work; ($♦$) ${}^{83}\mathrm{As}$ transitions previously observed in Refs. [12, 13, 14, 15]; ($♠$) ${}^{82}\mathrm{As}$ contaminant transitions; and ($♣$) ${}^{84}\mathrm{As}$ contaminant transitions.

Figure 2

Left: Partial level scheme of ${}^{83}\mathrm{As}$, containing the transitions observed in this work. The transitions indicated in color are new. The transitions marked in green eventually decay to the 1542.6- and 1865.6-keV levels. The transitions marked in blue follow other paths. Right: LSSM predictions with the DF2882 interaction.

Figure 3

(a) Tracked $\gamma$-ray spectrum in coincidence with ${}^{83}\mathrm{As}$ ions identified in VAMOS$++$. Enlargement of the region near 320 keV. The 317.2-keV transition is highlighted in green. The weak 313.2(1)-keV line is a contaminant from ${}^{84}\mathrm{As}$ (see text for details); (b) background-subtracted gate on 317.2-keV transition in ${}^{83}\mathrm{As}$ [as highlighted in panel (a)].

Figure 4

Background-subtracted tracked $\gamma$-ray spectrum in coincidence with the ${}^{83}\mathrm{As}$ ions identified in VAMOS, gated on the 2466-keV $\gamma$-ray line.

Figure 5

Tracked $\gamma$-ray spectrum in coincidence with ${}^{85}\mathrm{As}$ ions identified in VAMOS$++$. (♥) ${}^{85}\mathrm{As}$ transitions reported for the first time in this work; ($♦$) ${}^{85}\mathrm{As}$ transitions previously observed in [28]; and ($♣$) ${}^{84}\mathrm{As}$ contaminant transitions.

Figure 6

Left: Partial level scheme of ${}^{85}\mathrm{As}$, established in this work. Right: Level scheme predicted by the LSSM calculations.

Figure 7

(a) Tracked $\gamma$-ray spectrum region seen in coincidence with ${}^{85}\mathrm{As}$ identified in VAMOS$++$, fitted with three Gaussian components, (b) background-subtracted gate on the 793.6-keV transition, and (c) background-subtracted gate on the 796.6-keV transition.

Figure 8

Tracked $\gamma$-ray spectrum in coincidence with ${}^{87}\mathrm{As}$ ions identified in VAMOS$++$. (♥) ${}^{87}\mathrm{As}$ transitions reported for the first time in this work; ($♠$) ${}^{87}\mathrm{Se}$ contaminant transitions; and ($♣$) ${}^{86}\mathrm{As}$ contaminant transitions.

Figure 9

Tracked $\gamma$-ray spectrum observed in coincidence with the isotopically identified ${}^{87}\mathrm{As}$ and the 997.3-keV $\gamma$-ray.

Figure 10

Left: Level scheme of ${}^{87}\mathrm{As}$, established in this work. Right: Predictions from the LSSM calculations.

Figure 11

Excitation energy of the $9/2^{+}$ states in As ($Z=33$), Br ($Z=35$), Rb ($Z=37$), and Y ($Z=39$) isotopes. The solid lines represent experimental values from Refs. [29, 30, 31, 33] and the tentative assignments from this work. The dashed lines represent the predictions of the LSSM calculations.

Figure 12

Valence mirror symmetry in $N=50$ isotones (top) and $Z=28$ isotopes (bottom).

Figure 13

Zuker-Retamosa-Poves diagrams for the pseudo-SU3 proton orbitals (left) and neutron orbitals (right) limits in the case of ${}^{87}\mathrm{As}.$

Figure 14

Potential energy surface (PES) calculated for ${}^{85}\mathrm{As}$ with the treatment of odd-mass nuclei by means of a cranking method following Ref. [39]. The cranked component corresponds to that of the HF minimum $\langle {\widehat{J}}_z\rangle =-3/2$. The yellow diamond symbol represents the HF minimum.

Figure 15

PES for ${}^{87}\mathrm{As}$ obtained with the same method as in Fig. 14.

Figure 16

DNO-SM calculations for ${}^{85}\mathrm{As}$, using 45 deformed HF states (see more details in Ref. [40]).

Figure 17

DNO-SM calculations for ${}^{87}\mathrm{As}$, using 50 deformed HF states (see more details in Ref. [40]).

Figure 18

Wave functions content in $K$-quantum numbers of ${}^{85}\mathrm{As}$. For the various states ${\mathcal{P}}_{\alpha }^{\left(J\right)}\left(K\right)$ (normalized to unity) is the contribution of a component $K$ in the given state $J_{\alpha }$.

Figure 19

Same analysis in $K$-quantum numbers of ${}^{87}\mathrm{As}$ states as in Fig. 18.

Figure 20

Structure of considered states in deformations $(\beta ,\gamma )$ of ${}^{87}\mathrm{As}$. The radius of circles represents the normalized probability to find a deformation $(\beta ,\gamma )$ in the corresponding state.