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This manuscript reports on the direct observation of a $\beta$-delayed two-neutron emission in a study of ${}^{134}\mathrm{In}$at the ISOLDE Decay Station using neutron spectroscopy. We also report on the first measurement in ${\beta }^{-}$decay of the long-sought $13/2^{+}$excited state in ${}^{133}\mathrm{Sn}$, attributed to be the neutron single-particle $i_{13/2}$orbital. The observation of sequential neutron emission is used to extract the relative population of the $i_{13/2}$state, which was found to be much smaller than the predictions of the statistical model. The experiment was possible because of the innovative use of a neutron array with neutron discrimination and interaction tracking capabilities. This is the first study of the details of the two-neutron emission for a nucleus, which belongs to the $r$-process path. Understanding $\beta$-delayed two-neutron emission probabilities is essential to validate models used in astrophysical $r$-process nucleosynthesis calculations. Observing two-neutron emissions in ${\beta }^{-}$decay paves the way for new experiments to study energy and angular correlations for $\beta$-delayed multineutron emitters. 

Lead-208 is the heaviest known doubly magic nucleus and its structure is therefore of special interest. Despite this magicity, which acts to provide a strong restorative force toward sphericity, it is known to exhibit both strong octupole correlations and some of the strongest quadrupole collectivity observed in doubly magic systems. In this Letter, we employ state-of-the-art experimental equipment to conclusively demonstrate, through four Coulomb-excitation measurements, the presence of a large, negative, spectroscopic quadrupole moment for both the vibrational octupole $3_1^{-}$and quadrupole $2_1^{+}$state, indicative of a preference for prolate deformation of the states. The observed quadrupole moment is discussed in the context of the expected splitting of the $3^{-}\otimes 3^{-}$two-phonon states, due to the coupling of the quadrupole and octupole motion. These results are compared with theoretical values from three different methods, which are unable to reproduce both the sign and magnitude of this deformation. Thus, in spite of its well-studied nature, ${}^{208}\mathrm{Pb}$remains a puzzle for our understanding of nuclear structure. Published by the American Physical Society2025 

The nuclear two-photon or double-gamma ( $2\gamma$) decay is a second-order electromagnetic process whereby a nucleus in an excited state emits two gamma rays simultaneously. To be able to directly measure the $2\gamma$decay rate in the low-energy regime below the electron-positron pair-creation threshold, we combined the isochronous mode of a storage ring with Schottky resonant cavities. The newly developed technique can be applied to isomers with excitation energies down to $\sim 100\mathrm{keV}$and half-lives as short as $\sim 10\mathrm{ms}$. The half-life for the $2\gamma$decay of the first-excited $0^{+}$state in bare ${}^{72}\mathrm{Ge}$ions was determined to be 23.9(6) ms, which strongly deviates from expectations. Published by the American Physical Society2024