Paris, January 28, 2008

Compressed stars

Physicists working together on an international project led by the IPN in Orsay(1) and the GANIL(2) have, for the first time ever, successfully compressed an unstable nucleus, Nickel 56. This nucleus is not found on Earth but is present when a star explodes at the end of its life (supernova). This breakthrough opens up the possibility of compressing several hundred exotic nuclei, which have, until now, been inaccessible because of their instability. This will help us develop our understanding of how some stars are compressed before exploding. These results will be published in Physical Review Letters on 1 February 2008 (accessible online from 28 January 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 through collisions with light nuclei. Until now, however, it has been impossible to compress unstable nuclei, since they are found 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 gas. Thanks to this instrument, it has been possible to take direct measurements of the compression of exotic nuclei.

The experiment, performed at the GANIL facility using Maya for 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-dilation: giant monopole resonance.

          the nucleus vibrates, changing shape from that of a cigar to the shape of a saucer.

This breakthrough opens up the possibility of compressing several hundred exotic nuclei, which have, until now, been inaccessible because of their instability(3)
. Compressing large numbers of nuclei may thus also be measured, combining this detection system with the exotic nucleus production plants of the future(4). Neutron-rich nuclei can therefore be used to explore the compressibility of neutron-rich nuclear matter and 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 compresses the nuclei up against each other.


1) The Institut de Physique Nucléaire (Institute of Nuclear Physics) is a CNRS/IN2P3/University of Paris Sud joint research unit
2) The GANIL (National Large-scale Heavy Ion Accelerator) is a major European facility run jointly by CEA and CNRS and located in Caen, in Lower Normandy. Since the early 1980s, scientific and technical advances have made the GANIL one of the four major international centres carrying out research on atomic nuclei, together with GSI in Germany, MSU in the USA and RIKEN in Japan.
3) 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.
4) The exotic nucleus production plants of the future are already being built, for instance the Spiral2 project at the GANIL, FAIR at GSI and RIBF at RIKEN.


“First Measurement of the Giant Monopole and Quadrupole Resonances in a Short-Lived Nucleus:
56Ni”, C. Monrozeau, E. Khan, Y. Blumenfeld, C. E. Demonchy, W. Mittig, P.Roussel-Chomaz, D. Beaumel, M. Caamaño, D. Cortina-Gil, J. P. Ebran, N. Frascaria, U. Garg, M. Gelin, A. Gillibert, D. Gupta, N. Keeley, F. Maréchal, A. Obertelli, and J-A. Scarpaci, Physical Revue Letters, 1 February 2008.

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