Design, fabrication and testing of a large anti-scatter grid for megavolt /spl gamma/-ray imaging

Despite a century-old patent, and wide application at lower energies, the fabrication of thick anti-scatter "Bucky" grids for application at megavolt energies has been an elusive goal. We discuss the design, fabrication and testing of a 45 cm-diameter grid with a focal length of 525 cm for use at the 20 MeV DARHT flash radiography facility. The predominant difficulties overcome were: (1) understanding the performance of such grids prior to fabrication, and (2) the precision fabrication itself. To understand the former, we employed a specially modified version of the MCNP Monte Carlo code run on the ASC blue-mountain supercomputer at Los Alamos to explore and optimize the parameter space. These calculations were benchmarked at the Los Alamos microtron facility using a small prototype grid. To have useful scatter rejection at megavolt energies, grid ratios of several hundred-to-one are required. State-of-the-art grids used in medical radiography have grid ratios of ten-to-one. We demonstrate a unique casting technology capable of accurately manufacturing large grids with very high grid ratios. Promising initial test results are also presented. It is our belief that similar grids will prove useful in a wide range of potential applications including: megavolt NDE radiography, Compton scatter radiography, balloon-born gamma-ray astronomy, medical therapy, and in the intended application of flash radiography at Los Alamos' DARHT facility