Paris, November 19, 2007

Discovery of hydrogen-7, the most exotic nuclear system ever observed.

A European team(1), among whose members are physicists at GANIL (the French Large Heavy-ion Accelerator IN2P3/CNRS/CEA), has succeeded in characterizing the most neutron-rich isotope ever observed, hydrogen-7. For the first time in a laboratory, physicists have obtained atoms of hydrogen-7 by directing the most intense beam of exotic helium-8 nuclei in the world onto a revolutionary gas target. The results of this research have been published in Physical Review Letters(2).

The experiment, which was carried out at the GANIL facility, turns out to be a world first on two counts: first of all, this is the first time that physicists have managed to characterize atoms of hydrogen-7 in the lab. 'It's as if researchers had created, in the lab, tiny grains of matter similar to those in neutron stars!'   points out Sydney Gales, director of GANIL. In fact, hydrogen-7 is even richer in neutrons (6 for just one proton) than neutron stars. 'The experimental discovery of hydrogen-7 represents a benchmark of excellence for the development of current theoretical models,' Gales goes on. 'It will enable us to improve our understanding of the forces that bind atomic nuclei.' In fact, nuclear forces are still poorly understood today, especially in very neutron-rich systems.

Hydrogen-7 is very short-lived: it only lasts for 10-21 seconds before disintegrating into a bound hydrogen-3 (triton) and 4 neutrons. "When we determined that this excessively neutron-rich system was almost bound we were all astonished," enthuses Hervé Savajols, a researcher at CNRS's IN2P3 Institute and spokesman for the experiment. "The nuclear forces lack a mere 0.5 MeV (million electron-volts) of energy for the exotic nucleus to be bound. 0.5 MeV is absolutely nothing in nuclear physics, where the energies at play are often 100 times greater."


The experiment itself is also unique. The atoms of hydrogen-7 were generated by directing an accelerated beam of helium-8 with an energy of 120 MeV onto a carbon target. 'This beam is an exploit of which the whole world is envious,'  insists Gales. 

The experimental set-up used, dubbed "MAYA" and developed by GANIL, is at one and the same time a gas target and a particle detector. "We use the detector directly as a target, which is what makes the set-up so unusual," Savajols explains. "MAYA enables us both to identify all the products of a reaction and to reconstruct their paths through the gas, while acting as a target. As a result, not one reaction escapes it. This unique set-up enabled us to observe 7 reactions leading to the formation of hydrogen-7."


1) This collaboration brings together physicists from 8 European research institutes:
Universidad de Santiago de Compostela, Spain; GANIL, Caen, France; University of Liverpool, Dept. of Physics, UK; CEA/DSM/DAPNIA, Gif sur Yvette, France; University of Mainz, Institute of Nuclear Chemistry, Germany; PNPI, Gatchina,, Russia; CCLRC Daresbury Laboratory, Daresbury Warrington Cheshire, UK; Henryk Niewodnicza'nski Institute of Nuclear Physics, Krakow, Poland; Flerov Laboratory of Nuclear Reactions, JINR, Dubna, Russia; IFIC, Valencia, Spain.

2) Reference: M. Caamaño et al., Phys. Rev. Lett. 99, 062502 (2007)


Researcher contact:
Hervé Savajols
Tel 02 31 45 46 99

Press contacts
Christine Lemaître

Philippe Chomaz;

Anne-Sophie Paquez
Tel. 01 64 50 17 16

Claire Le Poulennec
Tel 01 44 96 49 88


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