SYSTEMS

Little is known about the structure of unstable, proton emitting, nuclei. These nuclei can be produced during stellar explosions in the cosmos, and live only for a very small fraction of second before they disintegrate into more stable products. Nowadays, they are also produced and detected in experiments made in large modern nuclear physics laboratories that are equipped with radioactive ion beams. Such elusive nuclei rapidly decay by emitting one (and sometimes two) protons, and for this reason are indicated as nuclei that lie outside of the so-called proton drip line.

A recent publication reports results of an experiment at the GSI laboratory in Darmstdtat, Germany that showed the existence of excited unstable states in two such nuclei, Fluorine-15 and Neon-16. Contrary to the ground level characterized by a broad resonance with short life, these excited states have half-lives sufficiently long that they can be identified as sharp resonances. Of note is that they have excitation energy of several MeV, establishing that such particle unstable systems can have an observable set of levels just as do the many known, particle stable nuclei.

The existence of such narrow resonances in particle unstable nuclei, and in Fluorine-15 in particular, was predicted three years ago by a method of calculation put forward by nuclear theoreticians of the INFN, sez. di Padova, in collaboration with colleagues from Australia, Canada and South Africa. To implement the method (an algebraic solution of systems of coupled equations for the problem of nuclear scattering and reactions) expertise in high performance computing was employed. The existence of sharp resonances in the spectra of radioactive, and specifically of proton (and, may-be, of neutron) emitting nuclei, opens new and interesting perspectives on the way the nuclei, that we observe at present in our Universe, have been formed.