An Investigation of the Isovector Giant Quadrupole Resonance in Bismuth-209 using Polarized Compton Scattering

Giant multipole resonances are a fundamental property of nuclei and arise from the collective motion of the nucleons inside the nucleus. Careful studies of these resonances and their properties provides insight into the nature of nuclear matter and constraints which can be used to test our theories.;An investigation of the Isovector Giant Quadrupole Resonance (IVGQR) in 209Bi has been preformed using the High Intensity gamma-ray Source (HI g&ar; S) facility. Intense nearly monochromatic polarized g&ar; -rays were incident upon a 209Bi target producing nuclear Compton scattered gamma-rays that were detected using the HI g&ar; S NaI(Tl) Detector Array (HINDA). The HINDA array consists of six large (10"x10") NaI(Tl) core crystals, each surrounded by an optically segmented 3" thick NaI(Tl) annulus. The scattered gamma-rays both parallel and perpendicular to the plane of polarization were detected at scattering angles of 55° and 125° with respect to the beam axis. This was motivated by the realization that the term representing the interference between the electric dipole ( E1) and electric quadrupole (E2) amplitudes, which appears in the theoretical expression for the ratio of the polarized cross sections, has a sign difference between the forward and backward angles and also changes sign as the incident gamma-ray energy is scanned over the E2 resonance energy. The ratio of cross sections perpendicular and parallel to the plane of polarization of the incident gamma-ray were measured for thirteen different incident gamma-ray energies between 15 and 26 MeV at these two angles and used to extract the parameters of the IVGQR in 209Bi.;The polarization ratio was calculated at 55° and 125° using a model consisting of E1 and E2 giant resonances as well as a modified Thomson scattering amplitude. The parameters of the E1 giant resonance came from previous measurements of the Giant Dipole Resonance (GDR) in 209Bi. The finite size of the nucleus was accounted for by introducing a charge form factor in the (modified) Thomson amplitude. This form factor was obtained from measurements of the charge density in inelastic electron scattering experiments.;The resulting curves were fit to the data by varying the E2 parameters until a minimum value of the chi2 was found. The resulting parameters from the fit yield an IVGQR in 209Bi located at Eres = 23.0 +/- 0.13(stat)+/-0.25(sys) MeV with a width of Gamma = 3.9 +/- 0.7(stat)+/-1.3(sys) MeV and a strength of 0.56 +/- 0.04(stat)+/-0.10(sys) Isovector Giant Quadrupole Energy Weighted Sum Rules (IVQ-EWSRs).;The ability to make precise measurements of the parameters of the IVGQR demonstrated by this work opens up new challenges to both experimental and theoretical work in nuclear structure. A detailed search for the missing sum rule strength in the case of 209Bi should be performed. In addition, a systematic study of a number of nuclei should be studied with this technique in order to carefully examine the A dependence of the energy, width and sum rule strength of the IVGQR as a function of the mass number A. The unique properties of the HI g&ar; S facility makes it the ideal laboratory at which to perform these studies.;Such a data base will provide more stringent tests of nuclear theory. The effective parameters of collective models can be fine tuned to account for such precision data. This should lead to new insights into the underlying interactions responsible for the nature of the IVGQR. Furthermore, with the recent advances in computational power and techniques, microscopic shell model based calculations should be possible and could lead to new insights into the underlying properties of nuclear matter which are responsible for the collective behavior evidenced by the existence and properties of the IVGQR.