New neutron-rich nuclei support 'island of inversion' theory

Mapping the properties of nuclei at the edge of the nuclear landscape is one of the fundamental pursuits in experimental nuclear science. Even the simple proof of existence of an isotope, which lives only for a fraction of a second, is sufficient to guide theorists in the development a comprehensive model of nuclear matter. Researchers at the Michigan State University National Superconducting Cyclotron Laboratory (NSCL) have succeeded in producing and measuring the production rates of 15 new neutron-rich isotopes never before produced in a laboratory setting. Several of these new, rare isotopes were produced at significantly higher-than-expected rates. The result suggests 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.

 

One way NSCL researchers are extending the laboratory's experimental reach into unchartered regions of the nuclear chartis to use a unique two-stage process to filter and purify fast beams of fragmented nuclei. Here, the following new nuclei (shown in blue) were produced by fragmenting a beam of germanium-76; chlorine-50, argon-53, potassium-55,-56, calcium-57,-58, scandium-59,-60,-61, titanium-62,-63, vanadium-65,-66, chromium-68, and manganese-70. Asterisks indicate those new nuclei that were produced at higher-than-expected rates.

 

Motivation to explore this region of nuclides was provided in part by a 2007 NSCL experiment that produced and measured the production rates of three new isotopes of magnesium and aluminum. (Nature, 449, October 2007, p. 1022.) In particular, the aluminum isotope measured in that result, aluminum-42, was beyond the limit of stability predicted by one of the leading theoretical models. The logical question that followed: How well do existing theories describe the behavior of heavier, neutron-rich nuclei?

 

Perhaps not so well, according to the 2008 NSCL result. Most nuclei in this region were expected to be characterized by low binding energies, and thus be exceedingly unstable and difficult to produce.  The unexpectedly higher production rates of several isotopes of potassium, calcium, scandium and titanium require a different interpretation, and the possible existence of a new island of inversion for neutron-rich nuclei.

 

The appearance of an island of inversion is attributed to changes in interaction strength between protons and neutrons, which are naturally attracted to each other. This attractive force has been known to change based on the number of protons and neutrons inside the nucleus. Nearest the stable isotopes, the change is so subtle that is often goes unnoticed.  However, in very neutron-rich nuclei, these effects appear to be amplified in localized areas, leading to small groupings of isotopes with very distinctive properties. The amplification of aspects of the nuclear force that are difficult to discern in more stable nuclei is one of the major motivations of the study of rare isotopes.