WITH a name like MEGa-ray, for mono-energetic gamma-ray, it is tempting to imagine this light source being used as a weapon by Luke Skywalker, especially when one considers that its beams will be the most powerful and brightest ever. In fact, the MEGa-ray light source is much like a laser, but it generates gamma rays rather than a visible-light beam, and the device is certainly not a weapon.
Project leader Chris Barty, chief technology officer for the National Ignition Facility and Photon Science Principal Directorate, says, “This new technology will produce photons at extremely high energies with the brightness and the spectral, spatial, and temporal density needed to study the nuclei of individual isotopes.” Backscattering is key to MEGa-ray brightness and density. In laser-Compton backscattering, short laser pulses collide head-on with short bunches of electrons moving at relativistic speeds, or almost the speed of light. The collision creates photons that backscatter, or move in the original direction of the electron beam. This scattered gamma radiation is Doppler upshifted in energy by more than a million times and directed forward in a narrow, polarized, laserlike beam that can be “tuned,” or adjusted, to different wavelengths. “We have spent almost a decade optimizing Compton backscattering to achieve such high-brightness, narrow-bandwidth gamma rays,” says Barty.
By tuning the MEGa-ray light source to very precise energy levels, researchers can detect, image, and even assay specific nuclei in objects containing a variety of isotopes. In 2008, proof-of-principle experiments demonstrated a first-generation MEGa-ray machine’s ability to detect isotopes of low-density lithium shielded behind high-density lead and aluminum. Lead can effectively shield many materials from the prying eyes of conventional radiation detectors, so the MEGa-ray capability is truly remarkable.
MEGa-ray experimental systems can occupy an entire large room, but making one small enough to fit into a portable truck trailer is high on Barty’s agenda. With a MEGa-ray device and its accompanying detector in a truck trailer, it can be moved, say, to a port for examining the contents of cargo containers or to a nuclear power plant for measuring precisely how much usable fuel remains in fuel rods.
Far Beyond X Rays
However, x rays have their limits. Distinguishing weapons-grade uranium from depleted uranium requires higher energy and more sophisticated photon beams than conventional x-ray machines can provide. Synchrotron beams are likewise lower in energy and incapable of characterizing heavy elements such as uranium. In contrast, the vastly increased brightness and monochromatic nature of a MEGa-ray beam can efficiently excite, or fluoresce, specific isotopes of both light and heavy elements to identify an object's isotopic contents. In addition, the higher energy of these gamma rays allows scientists to see objects more deeply buried. At 2 million electronvolts, a MEGa-ray beam has roughly 50 times the penetration capability of a conventional chest x ray.
Fluorescence is the emission of radiation by a substance during exposure to external radiation. The most common form of fluorescence is visible light that comes from exciting electrons in an atom. Nuclear resonance fluorescence (NRF) goes deeper. As its name implies, NRF's gamma rays excite the nucleus, which fluoresces as it relaxes.
Smaller, More Powerful
The very name of the next-generation light source indicates its smaller size. Anyone who saw the movie Jurassic Park will remember the tough, nasty little velociraptors. Livermore's VELOCIRAPTOR won't be tearing scientists limb from limb but will, over just a few meters, produce about a million times higher peak brightness than its predecessor. Collaborators include the Department of Homeland Security’s Domestic Nuclear Detection Office and SLAC National Accelerator Laboratory. The latter contributes its advanced accelerator technology to the project.
The DINO Detector
In practice, one would evaluate the ratio of NRF signals obtained from the resonant and nonresonant DINO witness foils, because this ratio is, in principal, sensitive only to the resonant isotope of interest in the object being inspected. The method should thus allow for very clean and accurate isotopic measurements.
An object would be exposed to a continuous flux of MEGa-ray photons whose energy had been tuned to the NRF absorption resonance in the isotope of interest. The interrogating photons, whose energies might range from 1 to 8 megaelectronvolts, would be highly penetrating and able to “see” through many centimeters of steel. DINO detector systems are being designed to require minimal operator intervention and deliver minimal dose to the object, while also providing high throughput at commercial seaports, airports, and other points of entry.
A New Science
“We have created a new science, one we call ‘nuclear photonics,’” says Barty. “We believe that MEGa-rays have the potential to do for isotopes what the laser did for the atom.” VELOCIRAPTOR will serve as the cornerstone for the Laboratory’s unique Nuclear Photonics Facility.
Key Words: Compton backscattering, Dual Isotope Notch Observer (DINO), mono-energetic gamma-ray (MEGa-ray) light source, nuclear resonance fluorescence (NRF), VELOCIRAPTOR.
For further information contact Chris Barty (925) 423-8486 (email@example.com).
Lawrence Livermore National Laboratory
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