Aug 25, 2009
New neutron-rich nuclei support 'island of inversion' theory
Researchers at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University have succeeded in making and measuring the production rates of 15 new neutron-rich isotopes. Several of these rare isotopes were produced at significantly higher-than-expected rates. The results suggest the existence of a new "island of inversion" – a region of isotopes with enhanced stability in a sea of mostly fleeting and unstable nuclei at the edge of the nuclear map.
Motivation to explore this region of nuclides was provided in part by an earlier experiment at NSCL that produced and measured the production rates of three new isotopes of magnesium and aluminium (CERN Courier December 2007 p37). In particular, the aluminium isotope measured (42Al) was beyond the limit of stability predicted by one of the leading theoretical models. It was therefore logical to ask: how well do existing theories describe the behaviour of heavier, neutron-rich nuclei?
Perhaps not so well, according to the results of continued studies at NSCL, which have investigated the nuclei of elements from chlorine to manganese. Most of the nuclei in this region were expected to be characterized by low binding energies, and thus be exceedingly unstable and difficult to produce. However, the experiments revealed unexpectedly higher production rates for several isotopes of potassium, calcium, scandium and titanium (Tarasov et al. 2009).
The results could imply the existence of a new island of inversion for neutron-rich nuclei. The island would be the result of changes in the interaction strength between protons and neutrons, which is already known to depend on the number of protons and neutrons inside the nucleus. Nearest the stable isotopes, the change is often small enough to go unnoticed, but in very neutron-rich nuclei the effects can be amplified in localized areas, leading to small groupings of isotopes with very distinctive properties.
O B Tarasov et al. 2009 Phys. Rev. Lett. 102 142501.