The Origin of the Giant Dipole Resonance

Abstract: The Giant Dipole Resonance (GDR), which is conventionally described as due to collective motion, is instead shown to be the result of a sudden increase in level density at the 2h{\omega} shell closure. The energy of the GDR closely follows the shell model harmonic oscillator energy model where h{\omega} = 39A^-1/3, for heavy nuclei. A better fit covering the entire mass range is given by h{\omega} = 47:55(0:13)(A^-1/3 - A^-2/3). The GDR is shown to be composed of a lower energy peak, E1, corresponding to the population of levels with oblate deformation and a higher energy peak, E2 corresponding to the population of levels with prolate deformation. The peak energy separation is proportional to the \beta_2 deformation and given by E2-E1 = 11:03(0:22)|\beta_2|. The total photonuclear cross section, sigma = sigma1 + sigma2, populating the GDR is proportional to the level density at the GDR and is given by sigma = 0:483(0:006)A^4/3 where sigma1 = sigma2. The widths of the two GDR peaks are consistent with Nilsson model predictions and found to be Gamma1 = 7:41(0:15)A^-1/6 MeV and Gamma2 = 11:13(0:16)A^-1/6 respectively. The Standard Lorentzian model parameters are fitt to high accuracy as a function of mass and deformation and can be applied reliably to all nuclei. It is shown that the energies of pygmy and spin flip resonances correspond to the E = h{\omega} harmonic oscillator energy and that the giant quadrupole (GQR), giant monopole (GMR), and giant octupole (GOR) resonances coincide with the E = 2-4 h{\omega} harmonic oscillator energies where the level density suddenly increases at the shell gaps.