Giant Resonances: Fundamental Modes and Probes of Nuclear Properties

Abstract

To study the properties of nuclear matter, we use nuclear reactions to excite the fundamental modes of the nucleus, which can yield information on the equation of state (EOS) and are also important for understanding nuclear structure aspects of nuclei. Furthermore, it is very important to understand the nuclear processes that precede a supernova event and to understand the properties of nuclear matter in order to explain why stars sometimes explode throwing most of the star material into space leaving a neutron star or a black hole behind.

In the last three decades, the compression modes, the isoscalar giant monopole (ISGMR) and dipole resonances (ISGDR), were extensively studied because of their importance for the determination of the nuclear-matter incompressibility and consequently their implications for the EOS of nuclear matter. Though the nuclear matter incompressibility (K_∞) has been reasonably well determined (~240 ± 10 MeV) through comparison of experimental results on several spherical nuclei with microscopic calculations, the asymmetry term was determined with much larger uncertainty. This has been addressed in measurements on a series of stable Sn and Cd isotopes, which resulted in a value of K _τ  = −550 ± 100 MeV for the asymmetry term in the nuclear incompressibility.

Spin-isospin modes, and in particular the Gamow–Teller (GT) transitions, aside from their interest from the nuclear structure point of view, play very important roles in various phenomena in nature. In nucleosynthesis, the β-decay of nuclei along the s- and r-processes determine the paths that these processes follow and the abundances of the elements synthesised. In supernova explosions, GT transitions are of paramount importance in the pre-supernova phase where electron capture occurs on neutron-rich fp-shell nuclei at the high temperatures of giant stars. Electron capture is mediated by GT transitions. Electron capture removes the electron pressure that keeps the star from collapsing precipitating an implosion followed by a cataclysmic explosion throwing much of the star material into space and leaving a neutron star or black hole behind.