The stability of heavy nuclides, which tend to decay by cy-emission and spo
ntaneous fission, is determined by the structural properties of nuclear mat
ter. Nuclear binding energies and lifetimes increase markedly in the vicini
ty of closed shells of neutrons or protons (nucleons), corresponding to 'ma
gic' numbers of nucleons; these give rise to the most stable (spherical) nu
clear shapes in the ground state. For example, with a proton number of Z =
82 and a neutron number of N = 126, the nucleus Pb-208 is 'doubly-magic' an
d also exceptionally stable, The next closed neutron shell is expected at N
= 184, leading to the prediction of an 'island of stability' of superheavy
nuclei, for a broad range of isotopes with Z = 104 to 120 (refs 1, 2), The
heaviest known nuclei have lifetimes of less than a millisecond, but nucle
i near the top of the island of stability are predicted to exist for many y
ears. (In contrast, nuclear matter consisting of about 300 nucleons with no
shell structure would undergo fission within about 10(-20) seconds.) Calcu
lations(3-5) indicate that nuclei with N > 168 should already benefit from
the stabilizing influence of the closed shell at N = 184. Here we report th
e synthesis of an isotope containing 114 protons and 173 neutrons, through
fusion of intense beams of Ca-48 ions with Pu-242 targets. The isotope deca
ys by or-emission with a half-life of about five seconds, providing experim
ental confirmation of the island of stability.