Giant Resonance Overtones: Compression Modes of the Nucleus

Abstract

Giant resonances are high-frequency collective excitations of atomic nuclei, the occurrence of which is a typical phenomenon in many-body quantum systems confined in a potential well [1]. In hydrodynamic models, giant resonances can be described as density and shape oscillations around the ground-state density distribution and shape. Such collective excitations of a nucleus may, similarly to other media with given boundary conditions, exhibit standing wave oscillations with a discrete series of frequencies as solutions of the applied wave equations. In this simple picture, most of the giant resonances studied extensively so far can be associated with the lowest energy, leading-order solutions (main tone). Giant resonances corresponding to higher-order solutions (overtones) have not been identified, except for the special cases of the isoscalar giant monopole (ISGMR) and isoscalar giant dipole (ISGDR) resonances, where excitation of the main-tone components is physically forbidden. Strong evidence for the overtone mode of the isoscalar giant quadrupole resonance, which is denoted as ISGQR2, has been found in our experiments [2,3], which is highly supported by predictions of recent RPA calculations [4]. Moreover, the microscopic structures of high-energy isoscalar giant resonances have been studied through measurements of direct particle decay, and in the case of the ISGDR for the first time in our measurements of such branching ratios.