X-rays and gamma rays have a wide range of applications including scanning suspicious maritime shipping containers for illicit materials, industrial inspection of materials and processes, and medical diagnostic and therapeutic procedures.
Current technologies, however, are not ideal. X-rays produce a continuum of energies that limit their inspection and diagnostic performance, and gamma rays can only be produced at specific energies unique to a given radioactive isotope.
Defense Advanced Research Projects Agency (DARPA) recently announced its Gamma Ray Inspection Technology (GRIT) program. GRIT seeks novel approaches to achieve high-intensity, tunable, and narrow-bandwidth sources of gamma ray radiation in a compact, transportable form factor that would enable a wide range of national security, industrial, and medical applications. A Proposers Day webinar describing the goals of the program is scheduled for July 8, 2019.
“What we’re trying to do in GRIT is transform the use of x-rays and gamma rays,” said Mark Wrobel, program manager in DARPA’s Defense Sciences Office. “Current sources of gamma rays, like Cobalt-60 or Cesium-137, are not very flexible. They require special licenses to possess and only emit gamma rays at very specific energies. What we desire is a source of very high-energy photons that we can tune to match the application we need. This ranges from more effective detection of illicit cargo, to a more informative medical x-ray.”
GRIT aims to provide a source of tunable, pure x-rays and gamma rays from tens of keV (kilo-electron volts) up through over ten MeV (mega-electron volts). Currently, tunable and narrow bandwidth gamma ray sources only exist at highly specialized user facilities best suited for basic research and are not able to support broad practical applications. Shrinking these photon sources to a transportable system is a major goal and challenge of the GRIT program.
GRIT technology could make possible a range of new inspection and diagnostic protocols. In medical and industrial radiography, for example, GRIT could enable revealing specific elemental and material content, such as calcium in bones or specific metals in cargo. A typical x-ray only shows differences in density in the object being inspected – whether a piece of luggage at an airport, or an individual at a doctor’s office. If successful, a GRIT x-ray source could be tuned to detect and quantify the concentration of specific elements of interest, such as the amount of calcium in a given bone x-ray, enabling radiologists to actually see bone composition.
Tuning energy between 10s of keV to over 100 hundred keV would allow detection of specific elements that might be of interest in characterizing novel materials and processes at micron scales. These techniques would be relevant to defense applications including non-destruction inspection of novel additively manufactured materials and alloys for their elemental composition.
At energy levels in the MeV range, gamma ray photons have high enough energy to actually interact with the nuclei of atoms. Whereas x-rays work by interacting with the shells of atoms, GRIT would be able to stimulate the nucleus of an atom to bring about an effect called nuclear resonance fluorescence, a sort of “fingerprint” that is unique to each isotope of every element in the periodic table.
“With GRIT, you could probe and detect specific isotopes of interest by fine-tuning the photon energy to minimize background noise and take advantage of the nuclear resonance fluorescence phenomenon,” Wrobel said. “Those isotopes could be found in rare-earth elements of interest or special nuclear materials. To be able to definitively say, ‘Yes, there’s highly enriched uranium in this object’ and be able to characterize how much is present would be a significant leap forward over our current capabilities.”
GRIT’s focus on new, compact photon sources for inspection complements DARPA’s Intense and Compact Neutron Sources (ICONS) program, which is developing compact neutron sources. The two technologies would work in tandem, yielding a very robust inspection capability.