α particles in stable nuclei α Decay is a common mode of radioactivity in heavy elements such as uranium that entails the loss of particles comprising two protons and two neutrons. Despite more than a century of study, when and where these α particles form in stable and unstable nuclei alike remains an open question. Tanaka et al. bombarded a series of stable tin isotopes with high-energy protons and detected ejected α particles at an abundance inversely correlated with mass number (see the Perspective by Hen). This observation, relating α particle accumulation to the neutron skin thickness at the nuclear surface, bears on models spanning radioactive decay to neutron star dynamics. Science , this issue p. 260 ; see also p. 232