Compressed stars - 28 Janvier 2008

Studies of the compression of stable nuclei found on Earth have already taught us a great deal of invaluable information about the mechanical properties of the dense matter of which they are formed: nuclear matter. For several decades, physicists have been able to compress these stable nuclei by colliding them with light nuclei.

A target/detector : Maya

Until now, however, it has been impossible to compress unstable nuclei, since they exist in the form of beams produced by an accelerator. Information about compressed nuclei has remained locked, so to speak, in the target due to the low-energy reaction product in question. The idea for the experiment performed at the GANIL facility was to use Maya, the only gaseous active target of its kind in the world, which also serves as a detector. Thanks to this instrument, it has been possible to make direct measurements of the compression of exotic nuclei

The compression of an unstable nucleus

The experiment, performed at the GANIL facility using Maya as the gaseous target, made it possible to compress an unstable nucleus, Nickel 56, for the first time ever. It revealed two modes of excitation in unstable nuclei:

• excited nuclei begin to “breathe” according to a process of compression-expansion: giant monopole resonance.
• the nucleus vibrates, changing shape from that of a cigar to the shape of a saucer.

New discoveries to come

This breakthrough opens up the possibility of compressing several hundred exotic nuclei which have, until now, been inaccessible because of their instability¹. The compression of large numbers of nuclei can thus also be measured, by combining this detection system with the exotic nucleus production plants of the future². Neutron-rich nuclei can therefore be used to explore the compressibility of neutron-rich nuclear matter, enabling us to understand how certain stars are compressed before they explode. In a similar vein, this may prove useful in improving our understanding of neutron stars, which are so dense that gravitational attraction presses the nuclei against each other.

These results have been published in Physical Review Letters on 17 January 2008.

* The Institut de Physique Nucléaire (Institute for Nuclear Physics) is a CNRS/IN2P3/University of Paris Sud joint research unit.

¹ Of all the different modes of nucleus excitation, giant monopole resonance is one of the excited states that nuclear physicists are the most keen to explore.

² The exotic nucleus production plants of the future are already being built, for instance the Spiral2 project at GANIL, FAIR at GSI and RIBF at RIKEN.