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Manufacturing Bits: July 3

Gamma-ray inspection; nonlinear scattering; simulating stars.

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Gamma-ray inspection
The Defense Advanced Research Projects Agency (DARPA) has started a program to develop gamma-ray inspection techniques.

The effort, called the Gamma Ray Inspection Technology (GRIT) program, is aimed to develop gamma-ray radiation sources in compact form factors for use in national security, industrial, and medical applications.

Source: DARPA

An agency of the U.S. Department of Defense, DARPA is seeking expertise from third parties for a range of technologies in the GRIT program, which is still on the drawing board. One day, the technology could be used in medical and industrial radiography, which could reveal elemental and material content, such as calcium in bones or metals in cargo. It could also detect elements and materials at the nanoscale.

A gamma ray is an electromagnetic radiation, which comes from the radioactive decay of an atomic nuclei. They have energy ranges from a few kiloelectron volts to 8 megaelectronvolts with wavelengths less than 10 picometers.

Typically, X-ray inspection is used for many applications. A form of electromagnetic radiation, X-rays have wavelengths from 0.01nm to 10nm with energies from 100eV to 100keV. X-ray has some limitations, however. “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,” according to DARPA.

So, DARPA is seeking to develop various technologies in the GRIT program. DARPA hopes to develop tunable sources for both pure X-rays and gamma rays. Currently, gamma-ray sources exist, but they can only be found at specialized facilities for R&D.

“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.

“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.”

Nonlinear scattering
Russia’s Skolkovo Institute of Science and Technology (Skoltech) and others have developed a new way of generating X- and gamma-ray radiation.

The method is based on a technology called nonlinear Compton scattering. The technology could one day increase the brightness of synchrotron sources for research in medicine, nuclear physics and material science.

In simple terms, the Compton Effect transforms the wavelength of incoming photons from the visible range to X- and gamma rays. The technology could generate a narrow bandwidth of radiation. The wavelengths are tunable by changing the energy of the electrons.

Sergey Rykovanov, a professor from Skoltech’s Center for Computational and Data-Intensive Science and Engineering, said: “Such spectral line broadening is parasitic since we want to obtain a narrow bandwidth photon source with a well-defined wavelength. Together with Vasily Kharin from the Research Institute in Moscow and Daniel Seipt from the University of Michigan in the USA we invented a very simple method to remove the parasitic Compton line broadening for intense laser pulses and significantly increased the number of generated X and gamma-ray photons. To do this one has to carefully tune the frequency of the laser pulse (in other words to chirp it) so that it corresponds to the laser pulse intensity at each moment of time. For optimal effect, we proposed to use two linearly and oppositely chirped laser pulses propagating with a certain delay to each other. In my opinion, the beauty of our work is in its simplicity. To be entirely honest, we were very surprised how simply and smoothly everything worked out.”

Simulating stars
The University of California at San Diego has made use of supercomputers to enable simulations of gamma-ray generation.

Researchers demonstrated a strong magnetic field, which is similar to the surface of a neutron star. The field can be detected using an X-ray laser inside a solid material. The simulations were conducted on the Comet supercomputer at the San Diego Supercomputer Center (SDSC) as well as Stampede and Stampede2 at the Texas Advanced Computing Center (TACC).



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