The ability to non-destructively detect and identify concealed nuclear materials in goods that cross our borders is critically important to our security. Surrounding materials and shielding can make passive signals difficult to distinguish from background radiation. High energy X-rays and neutrons may be used to actively interrogate special nuclear materials or weapons of mass destruction. Characteristic emissions from pulsed X-ray and neutron interrogation improve detection and characterization of nuclear materials. Our investigations are focused on the physical principles supporting the development of ultra-bright, short-pulse, fast neutron and high energy X-ray sources.
Gamma Source
The focused fields of a Petawatt-class laser can accelerate electrons to relativistic energies with the most energetic electrons having energies that exceed 50 MeV. As the electrons interact with the nuclei of the background atoms they slow down and emit bremsstrahlung, literally meaning “breaking radiation”. The resulting photons can be extremely energetic or “hard”. The photon emission is approximately along the direction of the electron’s trajectory. The yield is maximized by maximizing the background target material density and atomic number. Using tungsten and gold targets we are conducting research to develop an ultra-bright X-ray source with photons whose energy exceeds 10 MeV.
Neutron Source
The generation of energetic neutrons from solid targets irradiated by short-pulse, high-energy lasers, is an area of intense experimental activity. Most strategies employ a double target system in which ions accelerated from the rear surface of the laser-irradiated primary target bombard a secondary target that is positioned immediately behind the primary target. The subsequent nuclear reactions in the secondary target lead to the generation of neutrons. For example, the bombardment of lithium by protons whose energy exceeds 1.881 MeV leads to neutrons with a distribution of energies through the reaction 7Li(p,n)7Be.
The ions are accelerated by fields that appear on the rear surface of the primary target as a result of a charge imbalance that is caused when energetic, laser-generated, electrons escape the target. The field ionizes the target rear surface and subsequently accelerates the newly formed ions to high energy. If left untreated, the rear surface of the target is coated with a contamination layer of proton-rich hydrocarbons and water. The over whelming majority of the accelerated ions originate from this layer. Because the acceleration mechanism preferentially favors the acceleration of the lightest ions with the highest charge-to-mass, the most numerous and energetic ion species are protons. We have developed a technique to suppress the proton acceleration that involves coating the rear surface of the target with a layer of heavy water just prior to firing the laser. The subsequent ion acceleration is dominated by deuterons.
The ability to accelerate only deuterons adds a new level of versatility to neutron production. We can now produce neutrons from deuteron bombardment. For example, the bombardment of deuteron-rich materials such as deuterated plastic (CD), produces neutrons with a Q-value of 2.45 MeV while the deuteron bombardment of lithium produces neutrons with a Q-value of ~15 MeV.
Applications
While X-rays are attenuated by high-Z materials such as lead, neutrons are attenuated by low-Z materials like plastic. Used in combination, they can provide contrast to perform imaging of multiple density materials. In addition to the non-destructive interrogation of special nuclear materials, the neutrons can be used to identify hydrogen build-up in air-plane components such as engine blades that leads to material embrittlement, and in the development of fission and fusion reactor technology.
