IMAGINE a very powerful x-ray machine, several billion times more powerful than the one your dentist aims at your jaw. X rays can penetrate more than a foot of steel and record the motion of materials moving at ultrahigh speeds, making it an excellent tool for peering into the interior of a nuclear weapon's imploding primary stage.
Non-nuclear hydrodynamic experiments reveal the behavior of a nuclear weapon from ignition to the beginning of the nuclear chain reaction. These experiments consist of wrapping inert (nonfissile) material in a high explosive that is then detonated. The resulting explosive compression deforms the material, makes it denser, and even melts it. This process replicates the effects in the core of a nuclear device. High-speed radiographic images of the implosion process are taken with the powerful x-ray machine known as the Flash X Ray, or FXR, which was developed by scientists at Lawrence Livermore National Laboratory in the early 1980s'.
Data from the FXR's x-ray images are used to verify and normalize Livermore's computer models of device implosions. In the absence of nuclear testing, scientists must rely on these computer calculations to develop the judgment necessary to certify the safety and reliability of nuclear weapons, a critical part of the Laboratory's role in the stewardship of our nation's nuclear stockpile.
To improve capabilities for science-based stockpile stewardship, Lawrence Livermore has been upgrading many diagnostic facilities at Site 300, the Laboratory's experimental test site. The FXR was already the most sophisticated hydrodynamic flash radiography system in the world. In response to the need for data supporting ever more exact computer modeling codes, it has been made more powerful and capable of producing sharper, more useful radiographs.

The FXR in Action
The FXR is an induction linear accelerator specifically designed for diagnosing hydrodynamic tests and radiographing the interior of an imploding high-explosive device. Its x rays penetrate and are scattered or absorbed by the materials in the device, depending upon the density and absorption cross section of the various interior parts. The x rays that are neither absorbed or scattered by the device form the image on photographic emulsions or on the recording surface in a gamma-ray camera.
An injector introduces an electron beam into the FXR accelerator. After passing through the accelerator, the beam enters a drift section that directs it toward a 1-millimeter-thick strip of tantalum, called a target. As the high-energy electrons pass through the target, the electric field created by the stationary charged particles of the heavy tantalum nuclei causes the electrons to decelerate and radiate some of their energy in the form of x rays. The product of this slowing process is called bremsstrahlung (braking) radiation.
The x-ray photons travel toward the exploding device, where most are absorbed. The photons that make it to the camera are the image data.






A Better Radiographic Process
The upgrades to the FXR centered on improving the quality of the beam and adding a new gamma-ray camera system that is 70 times more sensitive than radiographic film. In this camera, designed by Livermore scientists, the beam hits an array of bismuth-germanate crystals with which the x rays interact to generate visible light. This light is recorded on photographic film.
The first task in increasing FXR beam quality was to improve the magnetic field that transports the electron beam through the accelerator. New focus solenoids and printed-circuit magnetic steering coils were installed in each of the accelerator and injector cells. Transverse magnetic forces that had been pulling the beam out of alignment were reduced by a factor of 10 to 20.
The next task was to double the injector beam voltage from 1.2 megavolts to 2.5 megavolts. At the same time, the injector electron beam current was increased from 2.2 kiloamperes to 3 kiloamperes. The number of cells in the injector was increased from six to ten, and the electron diode and the injector magnetic transport solenoids were redesigned.
With the completion of these upgrades, the FXR is producing a higher overall x-ray dose and a smaller spot size. Today, the central portion on the x-ray spot is twice as intense compared with pre-upgrade levels. Because tuning the FXR is an ongoing process, improvements in performance are expected to continue.
Prior to the addition of the gamma-ray camera, the size of the beam where it hits the tantalum target was a major concern; a smaller "spot size" increases the sharpness and clarity of the radiographs. Achieving a smaller effective spot size was accomplished by passing the x rays through a small hole in a thick plate near the target, a process known as collimation. But because x rays emitted outside the collimation diameter are lost to the radiographic process, collimating the beam meant that thicker materials could not be studied.
Today, however, the increased sensitivity of the gamma-ray camera and the increased current density of the central portion of the electron beam combine to more than compensate for the losses due to collimation. The gamma-ray camera can produce much sharper, clearer images than before even with a lower available dose. The camera's sensitivity combined with the newly increased x-ray dose at the target means that collimation can be used for experiments involving even higher density materials. Preliminary results indicate that the FXR upgrade--in conjunction with the gamma-ray camera--have significantly improved the radiographic capability at Livermore.
In the near future, the Laboratory will be adding a double-pulse feature to the FXR to provide two radiographs of a single explosion- implosion separated by 1 to 5 microseconds. Researchers can use this information to follow the time evolution of an implosion and learn more about how an implosion progresses. Restoring single-shot, full-energy operation will require simply setting the pulse interval to zero. Livermore scientists are also developing a two-frame gamma-ray camera to capture the fast successive images of double-pulsed FXR radiography and record them on a charged-coupled device camera. Work on the double-pulse feature and the two-frame camera is expected to be complete in 1998.
--Katie Walter

Key Words: flash x radiography (FXR), gamma-ray camera, hydrodynamic testing, induction linear accelerator, pulsed electron beam, pulsed x-ray source, stockpile stewardship.

For more information contact Ray Scarpetti (510) 422-8502 (scarpetti1@llnl.gov).


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