Radioactivity is the transformation of an atomic nucleus by one of several mechanisms. Henry Becquerel observed alpha particles: helium-4 clusters consisting of two protons and two neutrons ejected from heavy nuclei. Meanwhile, in beta decay, a neutron transforms into a proton by emitting an electron and an antineutrino. Gamma radioactivity rearranges the structure of the nucleus by ejecting a photon. Nuclear fission – in which a nucleus splits into two roughly equal chunks – can be regarded as the fourth mode of decay. In 1981 a fifth decay mechanism known as “proton radioactivity” was observed in which proton-rich nuclei with an odd number of protons eject the “unpaired” proton.

In the January issue of Physics World, Bertram Blank of the CEN Bordeaux-Gradignan, France, describes the recent studies by Alfredo Galindo-Uribarri at Oak Ridge National Laboratory in the US of another method of decay – two-proton radioactivity (J Gomez del Campo et al. 2001 Phys. Rev. Lett. at press).