X-RAY OPTICS: Novel optics improve nuclear-weapon safety
The U.S. Department of Energy’s National Nuclear Security Administration (NNSA) continues to pursue a smaller, safer, more secure, and less-expensive nuclear-weapons arsenal.
The U.S. Department of Energy’s National Nuclear Security Administration (NNSA) continues to pursue a smaller, safer, more secure, and less-expensive nuclear-weapons arsenal. To monitor the performance of aging weapons in its stockpile, the NNSA is using the computed-tomography (CT) Confined Large Optical Scintillator Screen and Imaging System (CoLOSSIS) designed and built by Optics 1 (Westlake Village, CA) for the NNSA BW-Pantex Facility in Amarillo, TX. Lawrence Livermore National Laboratory (LLNL; Livermore, CA) developed a single-camera prototype system and provided technical assistance to BW-Pantex during the development of the four-camera CoLOSSIS system.1 Consisting of a scintillator that emits light when struck by x-ray radiation, a pyramid-shaped central mirror, four turning mirrors, and four high-resolution, low-intensity visible-light CCD cameras, the system has software that gathers digital radiographs into a large 3-D image that scientists can use to discover problems or anomalies in nuclear weapons (see figure).
But the road to the four-camera design of the current system was not easy. “We want to see features as small as 2 or 3 mils (thousandths of an inch), equivalent to 50 to 75 µm, in very high-density components,” says Livermore chemist Pat Allen, deputy program manager of the Laboratory’s enhanced surveillance effort. “The best resolution for 9 MeV x-ray CT has until recently been 6 to 8 mils, or 150 to 200 µm.” To improve on the single-camera prototype system created in 2000, the LLNL engineers decided an 8000 × 8000-pixel field-of-view camera would be needed to achieve the resolution required. Because such a camera was not commercially available at that time (and could have cost some $25 million to develop), LLNL scientists turned to Optics 1 for a solution.
Divide and conquer
The approach to achieving the desired imaging resolution was to move from a single-sensor device to an architecture that uses four 4096 × 4096-pixel digital cameras and a novel optical configuration for collecting and transferring images. “This architecture represented the optimum balance between lens magnification, image numerical aperture, and ensured that the required resolution was pixel-limited,” says William McGuigan, Optics 1’s director of engineering. Similar in design to those used for terrestrial astronomy, the cameras are cooled to -100°C. Optics 1 designed the custom lenses, while the cameras were built by Spectral Instruments with help from the University of Arizona (both in Tucson, AZ).
Each lens consists of eight elements arranged into six groups that image light onto each camera’s 16-megapixel CCD chip. The lens is close to diffraction-limited and has high modulation transfer function at the sensor’s Nyquist sampling limit. Mirrors in the system are adjusted so that the light falls upon the four CCD cameras in perfect registration. By using two mirrors in each optical chain, the CCD cameras are shielded from the direct x-ray beam. “The CCDs have been pulled out of the main beam path with a neat optical trick,” says Allen.
The remote access requirement resulted in the need for remote control of the lens and CCD to precisely set focus and magnification. The mirrors are also under remote gimbal control to finely align the four images to a resolution of within a pixel. The need to protect the system components from harmful x-rays also resulted in a lead-shield exoskeleton structure that pushed the mass to almost 13 tons. “Given the mass of the system we felt that CoLOSSIS was a truly an apt acronym,” says Shawn Mulcahey, senior mechanical engineer at Optics 1.
Digital versus film
Software developed at Livermore stitches the four separate images into one, creating an 8000 × 8000-pixel radiograph. The complete data set is then transferred to LLNL or Los Alamos, where the individual images are reconstructed, again using Livermore software, into an approximately 1 Tbyte 3-D data file. To view the 3-D image, computer scientists combine four 3200 × 2300-pixel monitors to create one large monitor for detailed analysis.
CoLOSSIS is currently undergoing final shakedown prior to beginning scheduled inspections. Over the coming years, the system will provide a vital opportunity for the NNSA to inspect stockpiled weapons more efficiently and thoroughly than was achieved prior to CoLOSSIS. “We’re seeing more than ever before,” says Livermore’s Allen.
- A. Heller, Lawrence Livermore National Laboratories’ Science and Tech. Rev. online, https://str.llnl.gov/JulAug09/allen.html (July/August 2009).