Session K7: Applications of Nuclear Physics

Actinide Studies with Ultracold Neutrons

Understanding the effects of sputtering due to nuclear fission is crucial to the nuclear industry and has wide-reaching applications, including nuclear energy, space science, and national defense. A new program at the Los Alamos Neutron Science Center uses ultracold neutrons (UCN) to induce fission in actinides such as uranium and plutonium. UCN are an ideal tool for finely controlling induced fission as a function of depth in an actinide sample. The mechanism for fission-induced surface damage is not well understood, especially regarding the effect of a surface oxide layer. We will discuss our experimental strategy for studies of UCN-induced fission and the ejected material, and present preliminary data from enriched and depleted uranium.   

Distinguishing Fissile From Non-Fissile Materials Using Linearly Polarized Gamma Rays

Photofission of $^{232}$Th, $^{233,235,238}$U, $^{237}$Np, and $^{239,240}$Pu was induced by nearly 100\% linearly polarized, high intensity ($\sim$10$^{7}$ $\gamma$s per second), and nearly-monoenergetic $\gamma$-ray beams of energies between 5.3 and 7.6 MeV at the High Intensity $\gamma$-ray Source (HI$\gamma$S). An array of 12-18 liquid scintillating detectors was used to measure prompt fission neutron yields. The ratio of prompt fission neutron yields parallel to the plane of beam polarization to the yields perpendicular to this plane was measured as a function of beam and neutron energy. A ratio near unity was found for $^{233,235}$U, $^{237}$Np, and $^{239}$Pu while a significant ratio ($\sim$1.5-3) was found for $^{232}$Th, $^{238}$U, and $^{240}$Pu. This large difference could be used to distinguish fissile isotopes (such as $^{233,235}$U and $^{239}$Pu) from non-fissile isotopes (such as $^{232}$Th, $^{238}$U, and $^{240}$Pu). The measured ratios agree with the results of a fission calculation (FREYA) which used with previously measured photofission fragment angular distributions as input.   

Ultra-High Rate Measurements of Spent Fuel Gamma-Ray Emissions

Presently there are over 200,000 irradiated spent nuclear fuel (SNF) assemblies in the world, each containing a concerning amount of weapons-usable material. Both facility operators and safeguards inspectors want to improve composition determination. Current measurements are expensive and difficult so new methods are developed through models. Passive measurements are limited since a few specific decay products and the associated down-scatter overwhelm the gamma rays of interest. Active interrogation methods produce gamma rays beyond 3 MeV, minimizing the impact of the passive emissions that drop off sharply above this energy. New devices like the Ultra-High Rate Germanium (UHRGe) detector are being developed to advance these novel measurement methods. Designed for reasonable resolution at $10^{6}~s^{-1}$ output rates (compared to $\sim1-10$e$3~s^{-1}$ standards), SNF samples were directly measured using UHRGe and compared to models. Model verification further enables using Los Alamos National Laboratory SNF assembly models, developed under the Next Generation Safeguards Initiative, to determine emission and signal expectations. Measurement results and future application requirements for UHRGe will be discussed.   

Geant4 predictions of energy spectra in typical space radiation environment

Accurate knowledge of energy spectra inside spacecraft is important for protecting astronauts as well as sensitive electronics from the harmful effects of space radiation. Such knowledge allows one to confidently map the radiation environment inside the vehicle. The purpose of this talk is to present preliminary calculations for energy spectra inside a spherical shell shielding and behind a slab in typical space radiation environment using the 3D Monte-Carlo transport code Geant4. We have simulated proton and iron isotropic sources and beams impinging on Aluminum and Gallium arsenide (GaAs) targets at energies of 0.2, 0.6, 1, and 10 GeV/u. If time permits, other radiation sources and beams ($\alpha $, C, O) and targets (C, Si, Ge, water) will be presented. The results are compared to ground-based measurements where available.   

Dynamic Allocation of Sugars in Barley

Allocation of carbon and nitrogen is a key factor for plant productivity. Measurements are carried out by tracing $^{\mathrm{11}}$C-tagged sugars using positron emission tomography and coincidence counting. We study the mechanisms of carbon allocation and transport from carbohydrate sources (leaves) to sinks (stem, shoot, roots) under various environmental conditions such as soil nutrient levels and atmospheric CO$_{\mathrm{2}}$ concentration. The data are analyzed using a transfer function analysis technique to model transport and allocation in barley plants. The experimental technique will be described and preliminary results presented.   

Development of A Modular Plant Imaging PET System and Its Use In Evaluating Corn Plant Root Systems

A modular high-resolution positron emission tomography (PET) system has been developed at the Thomas Jefferson National Accelerator Facility to study physiological processes that influence the biodistribution of various substances in plants. This system is used to investigate sugar transport under varying environmental conditions using $^{11}$C-tagged sugars. The positron-emitting radiotracer $^{11}$C is produced in the tandem laboratory at TUNL. Initial evaluation of the PhytoPET system to image differences in the biodistribution of $^{11}$C-tagged sugars in corn plants due to fungal-root interactions is underway at Duke and preliminary results are presented.