Mass Measurements Of In99-101 Challenge Ab Initio Nuclear Theory Of The Nuclide Sn-100

The tin isotope Sn-100 is of singular interest for nuclear structure due to its closed-shell proton and neutron configurations. It is also the heaviest nucleus comprising protons and neutrons in equal numbers-a feature that enhances the contribution of the short-range proton-neutron pairing interaction and strongly influences its decay via the weak interaction. Decay studies in the region of Sn-100 have attempted to prove its doubly magic character(1) but few have studied it from an ab initio theoretical perspective(2,3), and none of these has addressed the odd-proton neighbours, which are inherently more difficult to describe but crucial for a complete test of nuclear forces. Here we present direct mass measurements of the exotic odd-proton nuclide In-100, the beta-decay daughter of Sn-100, and of In-99, with one proton less than Sn-100. We use advanced mass spectrometry techniques to measure In-99, which is produced at a rate of only a few ions per second, and to resolve the ground and isomeric states in In-101. The experimental results are compared with ab initio many-body calculations. The 100-fold improvement in precision of the In-100 mass value highlights a discrepancy in the atomic-mass values of Sn-100 deduced from recent beta-decay results(4,5).