Number 135 (Story #3), July 1, 1993 by Phillip F. Schewe and Ben Stein RADIOACTIVE BEAMS, beams in which non-naturally-occurring isotopes with specially tailored ratios of protons to neutrons, are helping scientists study the extremes of nuclear stability. The inventory of stable nuclei is usually portrayed as a zone on a plot whose axes are proton number (y axis) and neutron number (x axis). The upper border of this zone---the proton dripline---reflects the fact that a nucleus becomes unstable because of Coulomb repulsion if there are too many protons. The lower border---the neutron dripline---corresponds to the case of a nucleus having too many neutrons, which taxes the strong interaction's ability to hold the nucleus together. Radioactive beamlines at Michigan State, RIKEN (Japan), Louvain-la-Neuve (Belgium), and elsewhere allow nuclear physicists to better map the driplines, which in turn allow the study of several nuclei phenomena, such as the formation of heavy nuclei in the high-density environment of neutron stars or supernovae. (Science, 25 June 1993.)