|
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. |