  100Sn is of singular interest for nuclear structure. Its closed-shell proton
and neutron configuration exhibit exceptional binding and 100Sn is the heaviest
nucleus comprising protons and neutrons in equal number, a feature that
enhances the contribution of the short-range, proton-neutron pairing
interaction and strongly influences its decay via the weak interaction. Decays
studies in the region of 100Sn have attempted to prove its doubly magic
character but few have studied it from the ab initio theoretical perspective
and none have addressed the odd-proton nuclear forces. Here we present, the
first direct measurement of the exotic odd-proton nuclide 100In - the
beta-decay daughter of 100Sn - and 99In, only one proton below 100Sn. The most
advanced mass spectrometry techniques were used to measure 99In, produced at a
rate of only a few ions per second, and to resolve the ground and isomeric
states in 101In. The experimental results are confronted with new ab initio
many-body approaches. The 100-fold improvement in precision of the 100In mass
value exarcebates a striking discrepancy in the atomic mass values of 100Sn
deduced from recent beta-decay results.
