Active interrogation involves directing nuclear radiation into a closed container and measuring secondary radiations to gain information about the contents of the container.
Typically neutrons or high-energy photons are used as the impinging radiation. Active interrogation has a greater potential for detection of small quantities of Special Nuclear Material than passive detectors. It also holds the promise of detection and identification of non-nuclear materials, such as hazardous chemicals and explosives, including Vehicle-Borne Improvised Explosive Devices (VBIED). NIST has lead the drafting committee that wrote and successfully balloted ANSI Standard: N42.41 - Minimum Performance Criteria for Active Interrogation Systems used for Homeland Security.
Active interrogation is a highly active area of research and development. The selection of correct techniques for further development, and, ultimately, the selection of appropriate systems, requires a consistent set of standards for comparing the various techniques.