Improved understanding of the nature of photon scatter at a high energy radiographic imaging facility

Morneau, Rachel
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The U.S. Stockpile Stewardship Program was designed to sustain and evaluate the nuclear weapons stockpile while foregoing underground nuclear tests and requires complex computer calculations. The Dual Axis Radiographic Hydrodynamic Test (DARHT) facility at Los Alamos National Laboratory performs hydrodynamic tests which mimic a nuclear weapon implosion. These implosions happen very quickly and involve very large areal masses, so high energy X-rays are necessary to successfully penetrate the hydrotest that in turn produce radiographs which are numerically analyzed using model fitting and tomographic reconstruction techniques to find material edges and density distributions. One of the areas that can be improved in these computational models is the modeling of scatter at the DARHT facility. The large areal masses present at the radiographic facility cause large amounts of scattered photons to be produced when the X-rays interact with the material, up to 50% of the direct signal in simulations and up to 200% of the direct signal in experiments. With such large amounts of scatter, it is imperative to model the scatter accurately in order to reconstruct density fields. Using static characterization objects, an improved understanding of the scatter field, particularly the Compton scatter field, is developed which can be applied to experimental data. This research will investigate aspects of scatter at the DARHT accelerator using MCNP, a particle transport code ideal for modeling complex systems. Detailed MCNP calculations provide scatter fluxes at specified locations in the DARHT beam line. These calculations will then be used to form a physicsbased reduced-order model of the scatter field. This model employs a kernel that can be convolved with the direct transmission to represent a component of scatter correlated to the direct signal, and an additive object/environmental scatter field that is uncorrelated. The physics-based model of the scatter field provides several benefits, the first eliminating the need for optimization of arbitrary high-order polynomials to simulate the scatter field in the density reconstruction as was done historically. It diminishes the need for continued MCNP calculations for minor changes in the DARHT configuration; it also prevents MCNP calculations from needing to be optimized in conjunction with the density reconstruction, which requires significant computational power and time.