Speaker
Description
Research on the nuclear structure near the doubly magic isotope $^{132}$Sn continues to engage significant experimental and theoretical efforts. Investigating nuclear interactions in the vicinity of the proton ($Z=50$) and neutron ($N=82$) shell closures presents a unique opportunity to enhance our understanding of nuclear structure properties within this exotic region of the nuclear chart. However, experimental data in this area, particularly concerning the half-lives of low-lying excited states, remains limited. Most available spectroscopic data are derived from studies of nuclear states populated through $\beta$-decay of a parent nucleus or $\gamma$-ray spectroscopy following the decay of a long-lived isomeric state. This abstract discusses the in-beam $\gamma$-ray spectroscopy method, which utilizes nucleon knock-out reactions that can populate previously unobserved states. Additionally, this method focuses on prompt $\gamma$-rays emitted after the knock-out reaction, making it an ideal tool for investigating the half-lives ($T_{1/2}$) of low-lying excited states down to a few picoseconds, which often elude fast timing techniques.
Nuclei of interest were produced in the Radioactive Isotope Beam Factory at RIKEN during the HiCARI campaign [1]. Excited states were populated in the $^{9}Be(^{130}In,^{129}In)$, $^{9}Be(^{130}In,^{128}Cd)$, $^{9}Be(^{131}In,^{128}Cd)$, $^{9}Be(^{129}Cd,^{128}Cd)$ nucleon knock-out reactions following in-flight fission of a $^{238}U$ primary beam. In close geometry relative to the $^{9}Be$ target, an array of segmented HPGe detectors [1] was installed for $\gamma$-ray detection. Based on the reconstructed velocity of the ions and position of $\gamma$-ray emission during their de-excitation event-by-event, $\gamma$-ray spectra were obtained for each reaction channel. The line shape of identified transitions carries information regarding their energy ($E$) and $T_{1/2}$. Taking into account precisely measured HiCARI geometry, the response function of every HPGe crystal to the $\gamma$-ray of given $E$ and $T_{1/2}$ was simulated using the Geant4 package [2]. Finally, $E$ and $T_{1/2}$ were extracted by minimizing the $\chi^{2}$ of response functions fitted to the experimental data. To validate more complex reaction channels, a relatively simple case, $^{9}Be(^{132}Sn,^{131}In)$, was used as a benchmark to develop advanced analysis procedures. Further details on $T_{1/2}$ extraction are available in [3].
Taking advantage of multiple reaction channels, the most important scientific output of this analysis is $T_{1/2}$ of 4$^{+}$ and 2$^{+}$ states in $^{128}$Cd, along with observation of two new states and their tentative incorporation into the up-to-date known level scheme. Regarding the case of excited states in $^{129}$In due to a conservative approach to experimental uncertainties, various upper limitations on $T_{1/2}$ of known excited states were extracted, and one previously unobserved transition was tentatively assigned. Results will be interpreted using state-of-the-art shell-model calculations.
[1] K. Wimmer {\it et al.}, RIKEN Accelerator Progress Report 54 (2020) S27.
[2] L. A. Riley{\it et al.}, NIM A 1003, 165305 (2021).
[3] J. Acosta, A. Jungclaus{\it et al.}, Phys. Rev. C 111, 064316 (2025)