New nuclei of superheavy atoms at GANIL - 03 Septembre 2008

Atomic nuclei are composed of protons and neutrons. The larger their number, the "heavier" the atom. Uranium is the heaviest natural element on Earth, with 92 protons.

Beyond this number of protons, atoms are generally highly unstable and may only exist for very short periods of time. However, theory predicts an "island of stability" for atoms whose number of protons is much greater than that of uranium. Various experiments have been performed in several countries to create elements with increasing masses in order to reach this island of stability. The heaviest element synthesized to date has 118 protons.

Highly unstable nuclei

The so-called "superheavy" elements (comprising more than 110 protons) are generally formed by fusion reactions between two lighter nuclei. One of the main difficulties of these attempts to synthesize superheavy elements resides in the excitation which is unavoidably generated within these nuclei, in the form of temperature and deformation, when they are formed by fusion.

However, such nuclei become highly unstable when they are excited and fissioned into two lighter nuclei, long before they reach a detector that would allow their direct observation. Because of this high instability which occurs during fusion, no one was sure whether this method would be able to form superheavy nuclei. An original approach to uncover the existence of such elements, and assess their stability, was developed at GANIL within the framework of a collaboration between different laboratories1: instead of detecting the superheavy composite nucleus (produced by the fusion reaction), it is the fission time required by this nucleus which was measured. 
 

The fission time of a nucleus increases when it approaches stability. During experiments recently carried out at the GANIL facility, physicists have probed very long fission times by means of the so-called "blocking technique in a single crystal"2.

 

Reaching the "island of stability"

Fission events with times greater than 10-18 s (a billionth of a billionth of a second) were observed for nuclei with 120 and 124 protons. These nuclei were formed by bombarding nickel and germanium targets with uranium ions accelerated at the GANIL. They were identified by means of INDRA, a charged nucleus and particle detector, which covers almost the entire space surrounding the targets.

Although this 10-18 s time is clearly very short, on the scale of nuclear lifetimes, it is long enough to sign unambiguously the formation of elements having 120 and 124 protons, and to characterize them as being highly stable with respect to fission, when they are not excited. These results open up new perspectives in the race for superheavy elements, and the quest for the "island of stability".

 

This result has been published in Physical Review Letters. Review. Letter. volume 101, page 072701, 2008
Fission Time Measurements: A New Probe into Superheavy Element Stability, Physical

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¹ This collaboration brings together 6 laboratories:

- GANIL (Grand accélérateur national d’ions lourds), (CEA/CNRS), France;

- Institut de physique nucléaire d’Orsay (CNRS/Université Paris-Sud 11), France ;

- CEA-Saclay, Irfu/Service de Physique Nucléaire, France;

- Institut des nanosciences de Paris, (Université P. et M. Curie/CNRS/Université Paris Diderot) France;

- National Institute for Physics and Nuclear Engineering, Romania;

- Institute for nuclear physics at Lyon (CNRS/Université Lyon1), France.


² The blocking technique in single crystals is based on the atomic interaction between fission fragments generated in the form of positively charged ions, and the regularly ordered atoms lying in a crystal row or plane in which fusion has taken place. This interaction deflects the fission fragments from their original direction. The smaller the distance between the point where the fission fragment was created and the crystal row or plane, the larger this deflection, and therefore, the smaller the fission time of the superheavy nucleus (a detailed description of this technique may be found in: D.S. Gemmel, Review
 of Modern Physics, vol. 46, p. 129, 1974.)