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

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

Date: 2010

Abstract:

Giant multipole resonances are a fundamental property of nuclei andarise from the collective motion of the nucleons insidethe nucleus. Careful studies of these resonances and their properties providesinsight into the nature of nuclear matter and constraintswhich 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 γ-raySource (HIγS) facility. Intense nearly monochromaticpolarized γ-rays were incident upon a 209Bi target producingnuclear Compton scattered γ-rays that were detected using the HIγSNaI(Tl) Detector Array (HINDA). The HINDA array consists of sixlarge (10''x10'') NaI(Tl) core crystals, each surrounded by anoptically segmented 3'' thick NaI(Tl) annulus. The scattered γ-raysboth parallel and perpendicular to the plane of polarization weredetected at scattering angles of 55° and 125° withrespect to the beam axis. This was motivated by the realization thatthe term representing the interference between the electric dipole(E1) and electric quadrupole (E2) amplitudes, which appears in thetheoretical expression for the ratio of the polarized cross sections,has a sign difference between the forward and backward angles and alsochanges sign as the incident γ-ray energy is scanned over the E2resonance energy. The ratio of cross sections perpendicular andparallel to the plane of polarization of the incident γ-ray weremeasured for thirteen different incident γ-ray energies between 15 and26 MeV at these two angles and used to extract the parameters of theIVGQR in 209Bi.The polarization ratio was calculated at 55° and125° using a model consisting of E1 and E2 giant resonances aswell as a modified Thomson scattering amplitude. The parameters of the E1 giantresonance came from previous measurements of the Giant DipoleResonance (GDR) in 209Bi. The finite size of the nucleus wasaccounted for by introducing a charge form factor in the (modified)Thomson amplitude. This form factor was obtained frommeasurements of the charge density in inelastic electron scatteringexperiments. The resulting curves were fit to the data by varying theE2 parameters until a minimum value of the χ2 was found.The resulting parameters from the fit yield an IVGQR in 209Bilocated at Eres=23.0±0.13(stat)±0.25(sys) MeVwith a width of Γ=3.9±0.7(stat)±1.3(sys) MeV and astrength of 0.56±0.04(stat)±0.10(sys) Isovector GiantQuadrupole Energy Weighted Sum Rules (IVQEWSRs).The ability to make precise measurements of the parameters of theIVGQR demonstrated by this work opens up new challenges to bothexperimental and theoretical work in nuclear structure. A detailedsearch for the missing sum rule strength in the case of 209Bi shouldbe performed. In addition, a systematic study of a number of nucleishould be studied with this technique in order to carefully examinethe A dependence of the energy, width and sum rule strength of theIVGQR as a function of the mass number A. The unique properties ofthe HIγS facility makes it the ideal laboratory at which to performthese studies.Such a data base will provide more stringent tests of nucleartheory. The effective parameters of collective models can be finetuned to account for such precision data. This should lead to newinsights into the underlying interactions responsible for the natureof the IVGQR. Furthermore, with the recent advances in computationalpower and techniques, microscopic shell model based calculationsshould be possible and could lead to new insights into the underlyingproperties of nuclear matter which are responsible for the collectivebehavior evidenced by the existence and properties of the IVGQR.