Browsing by Subject "HIGS"
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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.
Item Open Access Feedback Systems for Control of Coupled-bunch Instabilities in the Duke Storage Ring(2012) Wu, WenzhongThe Duke storage has been developed as a dedicated driver for the storage ring based free-electron lasers (FEL) and a high flux Compton gamma-ray source, the High Intensity Gamma-ray Source. The storage ring can be operated from about 250 MeV to 1.2 GeV, which can produces FEL lasers over a wide range of wavelengths and gamma-rays with a tunable energy from 1 MeV to 100 MeV. The Duke light source facility conducts world-class researches across a wide range of scientific disciplines and technological applications.
In a storage ring, beam instabilities can cause a signifcant degradation in machine performance. In the Duke storage ring, coupled-bunch instabilities (CBIs) are the main source which limit ultimately achievable beam current in multi-bunch operations. In order to to suppress CBIs in the Duke storage, we developed a bunch-bybunch longitudinal feedback (LFB) system which is based on a field programmable gate array (FPGA) embedded system. During the design and implementation of the LFB system, several novel methods and techniques are developed in numerical analysis of feedback control and kicker cavity design/fabrication. High current are realized at low energies by using the LFB system. In addition, after the successful commissioning of the LFB system, a analog transverse feedback (TFB) system has been upgraded to a digital one using the same technique as the LFB system.
The LFB system has been routinely operated for HIGS. Additional,the LFB and TFB feedback systems become an useful diagnostic tools in researches of electron beam dynamics, FEL lasing process, and background of HIGS. The control of CIBs in different operation modes are studied using the feedback system. Furthermore, based on the TFB system, a novel bunch cleaning method has been developed to reduce the background of gamma-ray.
Item Open Access Measurements of the Absolute Cross Section of the Three-body Photodisintegration of Helium-3 Between E[gamma] = 11.4 MeV and 14.7 MeV at HIGS(2010) Perdue, Brent AndraeMeasurements of the three-body photodisintegration of 3He were performed at the High Intensity &gamma-ray Source (HI&gammaS). Neutrons emitted in this reaction inside a 3He gas target were detected with seven 12.7 cm diameter liquid scintillator detectors. Time-of-flight (TOF) and pulse-shape discrimination (PSD) techniques were used to identify neutron events. The absolute differential cross sections for the 3He(&gamma, n)pp reaction as a function of outgoing neutron scattering angle and energy were determined from the measurements at the incident &gamma-ray energies of 11.4, 12.8, 13.5, and 14.7 MeV to within a precision better than +/- 6 %.
The absolute cross sections at each incident energy are compared to the results of Gorbunov [Gor74], phase space calculations, and state-of-the-art three-body calculations. The inclusion of the Coulomb interaction in the three-body problem has been a long-standing challenge in theoretical nuclear physics. The present experimental data were found to be in good agreement with the state-of-the-art theory, which includes a full treatment of the Coulomb interaction between
the protons in the final state [Del05].