Speaker
Description
Nuclear magic numbers arise from large energy gaps between single-particle orbitals and are a defining feature of nuclear structure. When both proton $(Z)$ and neutron $(N)$ numbers coincide with these values, doubly magic nuclei are formed, which provide valuable benchmarks for our understanding of how the strong force arranges nuclei. However, the strength of the shell closures can vary significantly, particularly far from stability. The nucleus $^{100}$Sn $(Z=N=50)$ is the heaviest bound system with equal numbers of protons and neutrons and represents the last doubly magic system along the $Z=N$ line. It serves as valuable testing ground for proton-neutron symmetry at the edge of nuclear existence, where Coulomb repulsion nearly balances the short-range nuclear attraction for the least-bound protons at the Fermi energy. Here we report the first in-beam $\gamma$-ray spectroscopy of $^{100}$Sn. We observed a high-energy transition near 4 MeV, which we assigned to the decay of the first $2^+$ state to the ground state. This excitation energy is among the highest observed in heavy nuclei, comparable to those of established doubly magic nuclei such as $^{132}$Sn and $^{208}$Pb, and significantly larger than the corresponding values in $^{56}$Ni and $^{78}$Ni. These findings demonstrate that $^{100}$Sn exhibits an exceptionally strong $Z=N=50$ shell closure, establishing the robustness of magicity near the proton drip line and providing a stringent test of modern descriptions of nuclear structure.