Low-spin structure of ${}_{35}{}^{86}\mathrm{Br}_{51}$ and ${}_{36}{}^{86}\mathrm{Kr}_{50}$ nuclei: The role of the $g_{7/2}$ neutron orbital

Low-spin excited levels in ${}_{35}{}^{86}\mathrm{Br}_{51}$ and ${}_{36}{}^{86}\mathrm{Kr}_{50}$, populated following ${\beta }^{-}$ decay and the neutron-induced fission of ${}^{235}\mathrm{U}$, were measured using the Lohengrin fission-fragment separator and the EXILL array of Ge detectors at the PF1B cold-neutron facility of the Institute Laue-Langevin Grenoble. Improved populations of excited levels in ${}^{86}\mathrm{Br}$ remove inconsistencies existing in the literature on this nucleus. Directional-linear-polarization correlations, analyzed using newly developed formulas, as well as precise angular correlations allowed the unique $1^{-}$ and $2^{-}$ spin and parity assignments to the ground state of ${}^{86}\mathrm{Br}$ and the 4016.3-keV level in ${}^{86}\mathrm{Kr}$, respectively. Based on these results we propose that the Gamow-Teller ${\beta }^{-}$ decays of ${}^{86}\mathrm{Se}$ and ${}^{86}\mathrm{Br}$ involve the $\nu g_{7/2}\to \pi g_{9/2}$ transition in addition to the $\nu p_{3/2}\to \pi p_{3/2}$ transition proposed earlier. In ${}^{86}\mathrm{Kr}$ we have identified $1_1^{+}, 2_3^{+}$, and $3_1^{+}$ levels, analogous to the mixed-symmetry states in ${}^{94}\mathrm{Mo}$, which in ${}^{86}\mathrm{Kr}$ are from proton excitations, only. Large-scale, shell-model calculations with refined interactions reproduce well excitations in ${}^{86}\mathrm{Br}$ and ${}^{86}\mathrm{Kr}$ and support our interpretations.

Figure 1

Schematic, partial excitation scheme of ${}^{86}\mathrm{Br}$, drawn after Ref. [1]. Energies of $\gamma$ lines and levels are given in keV. Thickness of arrows is proportional to the observed branching ratios. (Porquet 2009) [1], (Negret 2015) [9], (Lundan 1970) [11]. See text for further explanation.

Figure 2

Time spectra gated on the 704.3-keV line of ${}^{86}\mathrm{Se}$, the 48.7-keV line of ${}^{86}\mathrm{Br}$, and the 1564.7-keV line of ${}^{86}\mathrm{Kr}$. See text for more comments.

Figure 3

Level scheme of ${}^{86}\mathrm{Br}$ populated in ${\beta }^{-}$ decay of ${}^{86}\mathrm{Se}$, as obtained in the present work. Energies of levels and $\gamma$ decays are given in keV. Levels and transitions marked with an asterisk were populated in neutron-induced fission of ${}^{235}\mathrm{U}$, measured in the present work. See Tables 1, 2, 3 for more information on excited levels and their decays.

Figure 4

$\gamma$ spectrum gated on 894.2-keV line.

Figure 5

$\gamma$ spectrum gated on 48.7-keV line.

Figure 6

$\gamma$ spectrum gated on 271.9-keV line.

Figure 7

Level scheme of ${}^{86}\mathrm{Kr}$ populated in ${\beta }^{-}$ decay of ${}^{86}\mathrm{Br}$, as obtained in the present work. Energies of levels and $\gamma$ decays are given in keV. See Table 4 for more information on excited levels and their decays. Levels and transitions marked with a star are drawn after Ref. [9] to assist further discussion.

Figure 8

$\gamma$ spectrum gated on the (a) 1389.8-keV line, (b) 1217.2-keV line, and (c) 1465.3-keV line.

Figure 9

Angular correlations analysis for the 2751.4- to 1564.7-keV cascade in ${}^{86}\mathrm{Kr}$, for the (a) $I$ = 1, (b) $I$ = 2, and (c) $I$ = 3 spin assignment to the 4316.1-keV level.

Figure 10

Angular correlation analysis for the 1361.6- to 1564.7-keV cascade in ${}^{86}\mathrm{Kr}$ for the (a) $I$ = 1, (b) $I$ = 2, and (c) $I$ = 3 spin assignment to the 2926.3-keV level.

Figure 11

Schematic directional-polarization correlations for the case of gating (GATE) on the (a) lower or (b) upper transition in the cascade. Polarization (POL) is then measured for the (a) upper or (b) lower transition.

Figure 12

Schematic drawing of the proposed decay scenario in $A$ = 86 isobars, involving the $\nu g_{7/2}\to \pi g_{9/2}$, G-T decay.

Figure 13

Energies of strongly populated $1^{+}$ levels in odd-odd (a) $N$ = 51 and $N$ = 53 isotones and (b) Br, Rb, and Y isotopes.

Figure 14

Comparison of calculated and experimental levels in ${}^{86}\mathrm{Br}$. Calculations are normalized to the experiment at the ground-state level. See text for more comments.

Figure 15

Comparison of calculated and experimental levels in ${}^{86}\mathrm{Kr}$. Calculations are normalized to the experiment at the ground-state level. See text for more comments. The $0_2^{+}, 0_3^{+}, 4_2^{+}, 4_3^{+}$, and $5_1^{+}$ experimental levels are taken from Ref. [9].

Figure 16

Comparison of experimental and calculated multiplets in ${}^{86}\mathrm{Br}$, ($\pi p_{3/2},\nu d_{5/2}{)}_{1^{-},...,4^{-}}$ (connected by solid lines) and ($\pi f_{5/2}^{-1},\nu d_{5/2}{)}_{0^{-},...,5^{-}}$ (connected by dashed lines). Filled circles represent experimental levels and empty circles represent calculations. The “−” symbol in the table means that the contribution is lower than $5\%$.

Figure 17

(a) Partial scheme of low-spin excitation in ${}^{86}\mathrm{Kr}$ drawn to assist the discussion. (b) Occupation of $\pi p_{3/2}$ vs $\pi p_{1/2}$ orbitals for the levels of ${}^{86}\mathrm{Kr}$, shown at the left-hand side. See text for the meaning of the A, B, C, and D labels.