Browsing by Subject "Polarized Compton Scattering"
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Item Open Access An Investigation of the Isovector Giant Quadrupole Resonance in 209Bi using Polarized Compton Scattering(2010) Henshaw, Seth
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 γ-ray
Source (HIγS) facility. Intense nearly monochromatic
polarized γ-rays were incident upon a 209Bi target producing
nuclear Compton scattered γ-rays that were detected using the HIγS
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 γ-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 γ-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 γ-ray were
measured for thirteen different incident γ-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 χ2 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 Γ=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 (IVQEWSRs).
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 HIγS 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.