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

Abstract

<p>&#65279;<p></p><p>Giant multipole resonances are a fundamental property of nuclei and</p><p>arise from the collective motion of the nucleons inside</p><p>the nucleus. Careful studies of these resonances and their properties provides</p><p>insight into the nature of nuclear matter and constraints</p><p>which can be used to test our theories. </p><p></p></p><p><p></p><p>An investigation of the Isovector Giant Quadrupole Resonance (IVGQR)</p><p>in ²⁰⁹Bi has been preformed using the High Intensity &gamma;-ray</p><p>Source (HI&gamma;S) facility. Intense nearly monochromatic</p><p>polarized &gamma;-rays were incident upon a ²⁰⁹Bi target producing</p><p>nuclear Compton scattered &gamma;-rays that were detected using the HI&gamma;S</p><p>NaI(Tl) Detector Array (HINDA). The HINDA array consists of six</p><p>large (10''x10'') NaI(Tl) core crystals, each surrounded by an</p><p>optically segmented 3'' thick NaI(Tl) annulus. The scattered &gamma;-rays</p><p>both parallel and perpendicular to the plane of polarization were</p><p>detected at scattering angles of 55&deg; and 125&deg; with</p><p>respect to the beam axis. This was motivated by the realization that</p><p>the term representing the interference between the electric dipole</p><p>(E1) and electric quadrupole (E2) amplitudes, which appears in the</p><p>theoretical expression for the ratio of the polarized cross sections,</p><p>has a sign difference between the forward and backward angles and also</p><p>changes sign as the incident &gamma;-ray energy is scanned over the E2</p><p>resonance energy. The ratio of cross sections perpendicular and</p><p>parallel to the plane of polarization of the incident &gamma;-ray were</p><p>measured for thirteen different incident &gamma;-ray energies between 15 and</p><p>26 MeV at these two angles and used to extract the parameters of the</p><p>IVGQR in ²⁰⁹Bi.</p><p></p></p><p><p></p><p>The polarization ratio was calculated at 55&deg; and</p><p>125&deg; using a model consisting of E1 and E2 giant resonances as</p><p>well as a modified Thomson scattering amplitude. The parameters of the E1 giant</p><p>resonance came from previous measurements of the Giant Dipole</p><p>Resonance (GDR) </p><p>in ²⁰⁹Bi. The finite size of the nucleus was</p><p>accounted for by introducing a charge form factor in the (modified)</p><p>Thomson amplitude. This form factor was obtained from</p><p>measurements of the charge density in inelastic electron scattering</p><p>experiments. </p><p></p></p><p><p></p><p>The resulting curves were fit to the data by varying the</p><p>E2 parameters until a minimum value of the &chi;² was found.</p><p>The resulting parameters from the fit yield an IVGQR in ²⁰⁹Bi</p><p>located at E_res=23.0&plusmn;0.13(stat)&plusmn;0.25(sys) MeV</p><p>with a width of &Gamma;=3.9&plusmn;0.7(stat)&plusmn;1.3(sys) MeV and a</p><p>strength of 0.56&plusmn;0.04(stat)&plusmn;0.10(sys) Isovector Giant</p><p>Quadrupole Energy Weighted Sum Rules (IVQEWSRs).</p><p></p></p><p><p></p><p>The ability to make precise measurements of the parameters of the</p><p>IVGQR demonstrated by this work opens up new challenges to both</p><p>experimental and theoretical work in nuclear structure. A detailed</p><p>search for the missing sum rule strength in the case of ²⁰⁹Bi should</p><p>be performed. In addition, a systematic study of a number of nuclei</p><p>should be studied with this technique in order to carefully examine</p><p>the A dependence of the energy, width and sum rule strength of the</p><p>IVGQR as a function of the mass number A. The unique properties of</p><p>the HI&gamma;S facility makes it the ideal laboratory at which to perform</p><p>these studies.</p><p></p></p><p><p></p><p>Such a data base will provide more stringent tests of nuclear</p><p>theory. The effective parameters of collective models can be fine</p><p>tuned to account for such precision data. This should lead to new</p><p>insights into the underlying interactions responsible for the nature</p><p>of the IVGQR. Furthermore, with the recent advances in computational</p><p>power and techniques, microscopic shell model based calculations</p><p>should be possible and could lead to new insights into the underlying</p><p>properties of nuclear matter which are responsible for the collective</p><p>behavior evidenced by the existence and properties of the IVGQR.</p><p></p></p>