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Low-lying 1/2$^+$ and 5/2$^+$ states, interpreted as resulting from 1p-2h excitation through the N=50 shell gap, have been observed in several neutron-rich N=49 nuclei. In particular, charge radii measured for the ground state and the 1p-2h 1/2$^+$ isomer in $^{79}$Zn provide evidence for a deformed character of the latter [1].
In this context, fast-timing measurements have been performed to investigate the nuclear structure of $^{83}$Se and $^{83}$Br, with the aim of studying the evolution of collectivity and possible shape coexistence in the vicinity of the $N = 50$ shell closure.
The experiment was carried out at the LOHENGRIN recoil mass separator at the Institut Laue-Langevin. Neutron-induced fission products were separated and implanted, and $\gamma$ rays were detected using a fast-timing setup based on LaBr$_3$(Ce) scintillators. Lifetimes were extracted using both Generalized Centroid Difference (GDC) and Advanced Time-Delayed (ATD) techniques, providing sensitivity in the picosecond range.
Three lifetimes were measured for the first time. In $^{83}$Se, the lifetime of the $3/2^+$ state at $E = 963.4$ keV, was determined to be $\tau = 13 \pm 3$ ps using the GDC method and $\tau = 17 \pm 6$ ps using ATD. The corresponding B(E2; $3/2^+ \rightarrow 5/2^+$) transition strength indicates that the $3/2^+$ state is not associated with the deformed structure, while the low B(E1; $3/2^+ \rightarrow 1/2^-$) value makes it impossible to draw any conclusions regarding octupole collectivity in $^{83}$Se.
In $^{83}$Br, the GDC analysis yielded $\tau = 12 \pm 2$ ps for the $5/2^-$ state at $E = 356.7$ keV,and $\tau = 7 \pm 3$ ps for the $7/2^-$ state at $E = 866.9$ keV, with compatible values obtained using ATD. While a regular excitation-energy pattern observed for these states could suggest that they belong to a collective structure built on the $3/2^-$ ground state, the newly obtained B(E2) and B(M1) values undermine this interpretation. Instead, our data combined with the literature lifetimes of the $11/2^-$ and $13/2^-$ states in $^{83}$Br [2] suggest that these four states can be understood as arising from the coupling of a proton hole in the $p_{3/2}$ or $f_{5/2}$ orbitals with neutron configurations of the type $(g_{9/2})^{-2}$, coupled to total angular momenta $J^\pi = 0^+, 2^+, 4^+, 6^+$.
[1] X.F. Yang {\it et al.}, Phys. Rev. Lett. 116, 182502 (2016).
[2] R. Schwengner {\it et al.}, Nucl. Phys. A 584, 159 (1995).