High-spin states in ${}^{\mathbf{127}}$I

In-beam $\gamma$ spectroscopy of the stable nucleus ${}^{127}$I has been studied experimentally using the ${}^{124}$Sn(${}^7$Li, 4$n\gamma$)${}^{127}$I reaction at a beam energy of 32 MeV. The high-spin level scheme of ${}^{127}$I is extended significantly. Negative-parity levels built on the $11/2^{-}$, $\pi h_{11/2}$ particle state are observed up to $(35/2^{-})$ and described as a decoupled band, extending our knowledge of decoupled structures to the most neutron-rich stable iodine isotope. Two $\Delta I=2$ yrast positive-parity sequences are proposed to be associated with the $\pi g_{7/2}$ configuration due to observations of several strong interband transitions, and two weakly populated $\Delta I=2$ positive-parity bands are newly identified and interpreted as arising mainly from the $\pi d_{5/2}$ configuration. Three-quasiparticle configurations are assigned to the $I^{\pi }=15/2^{+}$ and $23/2^{+}$ states according to the existing knowledge in neighboring nuclei; irregular noncollective and regular collective excitations built on these two ($15/2^{+}$ and $23/2^{+}$) states are observed to coexist at similar energies. The observed three-quasiparticle band structures are further interpreted with the aid of configuration-constrained potential energy surface calculations.

Figure 1

Level scheme of ${}^{127}$I deduced from this work. Excitation energies of $0^{+}$, $2^{+},...$,$8^{+}$ states in ${}^{126}$Te [25] relative to $11/2^{-}$ level in ${}^{127}$I are indicated as black dots.

Figure 2

Typical coincidence spectra with gates on selected transitions in band A and level structures D, E, and F as indicated on the panels. Note that there is a 651.0-keV transition depopulating band B, and the 609.7- and 615.5-keV transitions in band B appear in the spectrum gated by the 651.5-keV transition.

Figure 3

Typical coincidence spectra with gates on selected transitions in bands B and C as indicated on the panels.

Figure 4

Band structures based on the $\pi g_{7/2}$ and $\pi d_{5/2}$ orbitals in the odd-$A$ ${}^{127,125,123}$I isotopes (present work and Refs. [7, 8]). The relative intensities are normalized to the $9/2_2^{+}\to 5/2^{+}$ transition.

Figure 5

Comparison of three-quasiparticle bands built on the $I^{\pi }=23/2^{+}$ state in ${}^{127}$I (present work) with that in ${}^{125}$I (Ref. [30]). The relative intensities are normalized to the $25/2^{+}\to 23/2^{+}$ transition.

Figure 6

PES calculations for the $I^{\pi }=23/2^{+}$state (upper left), $I^{\pi }=15/2^{+}$ state (upper right), lowest $I^{\pi }=7/2^{+}$ state (lower left), and $I^{\pi }=5/2^{+}$ ground state (lower right). Minima are indicated by black dots and the contour separation is 200 keV. The calculated deformations and excitation energies are given in Table .