Strontium-76 Is One of the Most Deformed Nuclei

Strontium-76 is one of the most deformed nuclei in its ground state and is the most deformed of all nuclei in which the number of protons (Z) equals the number of neutrons (N). This finding comes out of a new experiment in Switzerland.

The lighter N=Z nuclei, such as He-4, C-12, O-16, and Ca-40, are quite stable and among the most important nuclear species on earth, especially where life is concerned. But as the number of proton and neutron inhabitants of the nuclear abode increases distortion begins; the electric charges on the protons leads to mutual repulsion, and this leads to disintegration of the nucleus.

Nuclei struck by another nucleus can be sent into a rapidly spinning superdeformed state, but what about the quiescent shape of nuclei that haven't been hit? Earlier evidence suggested that Sr-76 should be about as deformed a nucleus as one can have in its ground state. In a new study carried out at the CERN-ISOLDE facility in Geneva, a new method for measuring this deformation has been put into practice.

First, the rare Sr-76 nuclei were made by smashing a proton beam into a target of niobium. The newly made Sr nuclei then diffused out of the target, ionized, and were swept away and sent to the heart of a spectrometer called "Lucrecia." There the fragile nuclei are directed up a slender hole in the world's largest crystal of pure sodium iodide. It is in that sanctum that gamma rays from the disintegration of the Sr-76 nuclei are observed. Not only the lifetime can be deduced, but even the approximate shape of the nuclei can be worked out from the pattern of emergent gammas. Sr-76 was not only shown to be highly deformed, as expected, but its shape is now determined to be highly prolate (its equatorial axis being some 40% less than its longer axis) rather than oblate. (Nacher et al., Physical Review Letters, upcoming article.)