Improved Active Interrogation Methods for Nuclear Nonproliferation and Homeland Security Applications

Highly enriched uranium is arguably the most difficult material to detect in the realm of nuclear security and safeguards, but is of great concern for its possible role in developing nuclear weapons. Uranium-235 emits very few neutrons, and the low energy photons it emits are easily shielded, making passive detection of highly enriched uranium very difficult. Actively interrogating the material with neutron or photon sources can provide a much more prominent detection signal. These sources of radiation can be used to either induce detectable emissions in the material, or radiograph the material to distinguish it from possible shielding. Active interrogation presents detection challenges in signal quality and operational feasibility, especially because currently-available sources mostly emit photons that can be easily shielded and are below photonuclear energy thresholds. My research will focus on addressing these challenges by demonstrating advantages of photon interrogation based on recent enabling technologies, both from the perspective of the interrogating source and the detection system. Inverse Compton scattering quasi-monoenergetic photon sources using a laser-driven plasma accelerator are a developing technology that has strong potential to advance photon interrogation methods. These sources use the laser wakefield phenomenon to accelerate electrons to very high energy. Photons from a secondary laser beam interact with these electrons through inverse Compton scattering, producing a photon source highly focused in energy and space. The energy of these photons can be tuned to penetrate shielding and induce photonuclear reactions. The work presented here is based on quasi-monoenergetic photon sources at the University of Nebraska Lincoln and the Lawrence Berkeley National Laboratory. Through Monte Carlo simulations, I have demonstrated the capability to image heavily shielded nuclear material, validated by experiment. These studies showed increased accuracy for hidden nuclear material detection over traditional bremsstrahlung sources. A 9-MeV linac has been installed at UM, which outputs a high intensity of photons capable of inducing photonuclear reactions. This high photon intensity makes neutron detection and identification challenging, but we are developing methods to detect prompt neutrons in-pulse with organic scintillators. These methods incorporate high throughput data acquisition, active background reduction, and collaboratively developed neural-network based pulse discrimination and recovery. Initial experiments interrogating lead and tungsten surrogates for highly enriched uranium have identified elevated neutron counts for the cases with target present over active background. Compared to a quasi-monoenergetic photon source, the bremsstrahlung source produces many low-energy photons that only contribute to surrounding dose rates. To demonstrate this dosimetric advantage, and verify shielding for the operation of various accelerators, a method for measuring dose rates was required. An organic scintillator based strategy was developed to provide a replicable and dual-particle dose rate detection method. This method has been used to simultaneously measure neutron and gamma dose rates from isotopic sources; these results show reasonable agreement with traditional instruments. Future experiments will demonstrate the method with active interrogation sources. The results of my research will enable the use of organic scintillators and novel photon sources for use in an active interrogation scenario to prevent the spread of nuclear material.PhDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/155307/1/cmillera_1.pd