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Abstract
The properties of exotic nuclei on the verge of existence play a fundamental part in our understanding of nuclear interactions1. Exceedingly neutron-rich nuclei become sensitive to new aspects of nuclear forces2. Calcium, with its doubly magic isotopes ^sup 40^Ca and ^sup 48^Ca, is an ideal test for nuclear shell evolution, from the valley of stability to the limits of existence. With a closed proton shell, the calcium isotopes mark the frontier for calculations with threenucleon forces from chiral effective field theory3-6. Whereas predictions for the masses of ^sup 51^Ca and ^sup 52^Ca have been validated by direct measurements4, it is an open question as to how nuclear masses evolve for heavier calcium isotopes. Here we report the mass determination of the exotic calcium isotopes ^sup 53^Ca and ^sup 54^Ca, using the multi-reflection time-of-flight mass spectrometer7 of ISOLTRAP at CERN. The measured masses unambiguously establish a prominent shell closure at neutron number N = 32, in excellent agreement with our theoretical calculations. These results increase our understanding of neutron-rich matter and pin down the subtle components of nuclear forces that are at the forefront of theoretical developments constrained by quantum chromodynamics8. [PUBLICATION ABSTRACT]