A new mode of radioactive decay
 The two-proton radioactivity

Paris, June 6, 2002

In two recent experiments, one at Grand accélérateur national d’ions lourds (Ganil – CNRS/CEA) in Caen (France) and one at Gesellschaft für SchwerIonenforschung (GSI) in Darmstadt (Germany), an international team of physicists has demonstrated for the first time the radioactive decay of the atomic nucleus Iron-45 by the simultaneous emission of two protons. This radioactive decay mode has been actively sought for 40 years by the nuclear physics community and opens a new avenue for the study of the atomic nucleus.

Stable nuclei are characterised by a balance between the number of their protons and neutrons. When this balance is upset by an excess of one or the other type of nucleons, the nucleus becomes unstable, which usually means that it disintegrates by beta emission, a process which transforms the extra neutron into a proton (emission of an electron, beta- radioactivity) or vice versa (emission of a positron, beta+ radioactivity).

If this imbalance, however, is too great, nuclear forces can no longer bind all the nucleons together and the nucleus ejects the excess nucleon(s). The nuclear theory has predicted, since the 1960’s, that when this cohesive limit is reached for a very proton-rich nucleus it will emit either one or two protons (one-proton radioactivity and two-proton radioactivity), depending on whether it has, respectively, an odd or even number of protons to start with. Single proton emission was observed for the first time at GSI in the early 1980’s, and has since allowed detailed study of the nuclear structure of unstable nuclei. However, this was not the case for two-proton radioactivity.

For the first time, the results from the experiments at GSI and at Ganil demonstrate that the nucleus Iron-45 with a very large proton excess can spontaneously disintegrate by double proton emission from its ground state with a sufficiently long half-life to allow the study of the phenomenon. This discovery should allow to study the mechanism of this new decay mode, thus opening up a new avenue for observing the internal forces governing the atomic nucleus. It is all the more important that there are only a few different types of radioactivity.

These experiments are the fruit of a collaboration between Centre d’études nucléaires de Bordeaux-Gradignan (CENBG – CNRS/IN2P3), Warsaw University, GSI, Ganil (CNRS – CEA), IAP Bucharest, University of Tennessee, University of Liverpool, University of Edinburgh and Michigan State University.

CNRS/IN2P3 Press contact
Dominique Armand
Tel.: 33 1 44 96 47 51