Increasing the distance for standoff detection of nuclear materials, beyond that capable with direct radiation detection, using “alternative signatures”, such as the concentration of ionization in the surrounding atmosphere, improves the ability to locate, track and monitor this material. Ionizing radiation generates free elections, ions, and exited states in the nearby atmosphere, even when the sources are shielded, which can be detected from a distance. The power-law decrease in the signal as a function of distance, and the activity of the source, limit the measurement of the unamplified excitation and ionization signatures from the source. The Phase-1 effort evaluated different approaches for detecting exited states or ionization created by radiation from nuclear materials. The most promising approach, developed at the University of Michigan, involves mapping the concentration of ions detected by the generation of a plasma in the air from a laser pulse whose power is below the threshold required to ionize air without charge from ionizing radiation. The Phase-1 effort demonstrated the feasibility of extending the range of the ionizing laser to achieve a standoff range of ~100 m, and proposed a design concept for a prototype system. The Phase-2 effort will design, construct, and test a prototype system.
