Aflatoxins, toxic secondary metabolites produced by some Aspergillus species, are a universal agricultural economic problem and a critical health issue. Despite decades of control efforts, aflatoxin contamination is responsible for a global loss of millions of tons of crops each year. We show that host-induced gene silencing is an effective method for eliminating this toxin in transgenic maize. We transformed maize plants with a kernel-specific RNA interference (RNAi) gene cassette targeting the aflC gene, which encodes an enzyme in the Aspergillus aflatoxin biosynthetic pathway. After pathogen infection, aflatoxin could not be detected in kernels from these RNAi transgenic maize plants, while toxin loads reached thousands of parts per billion in nontransgenic control kernels. A comparison of transcripts in developing aflatoxin-free transgenic kernels with those from nontransgenic kernels showed no significant differences between these two groups. These results demonstrate that small interfering RNA molecules can be used to silence aflatoxin biosynthesis in maize, providing an attractive and precise engineering strategy that could also be extended to other crops to improve food security.

<p>Measurements of the three-body photodisintegration of <super>3</super>He were performed at the High Intensity &gamma-ray Source (HI&gammaS). Neutrons emitted in this reaction inside a <super>3</super>He 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 <super>3</super>He(&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 %.</p><p>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</p><p>the protons in the final state [Del05].</p> / Dissertation

<p>The 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.</p><p> 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. </p><p>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.</p> / Dissertation

<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> / Dissertation