"Radiation detectors have two applications toward nuclear and radiological weapons of mass destruction. The first is to intercept nuclear materials and devices prior to a terrorist attack. The second is for assessment and attribution after an attack. Such detectors represent relatively mature technologies. Portable radiation instruments were developed over fifty years ago in response to the use of nuclear weapons in WW II. Modern electronics and detector materials have made them much more capable. Now, new materials are being developed to enable even more efficient detection of gamma rays and neutrons with simpler devices. In recent years, large systems for imaging of gamma rays from nuclear materials and devices have been developed. Many passive and active, fixed and portable instruments for the detection of gamma rays or neutrons are available commercially. Geometry, air attenuation, and background radiation from natural and man-made sources determine the limits of detection of these materials. For realistic source strengths and available gamma-ray and neutron detectors, nuclear materials and devices can be detected at ranges of a few meters up to a few tens of meters, at most. Detection from fixed-wing aircraft or satellites is impossible. Examples of detection limits for generic detectors are given in this report. It is necessary to quantify the limits of utility for specific nuclear radiation detectors under the circumstances in which they are likely to be used."
Defense Technical Information Center (DTIC): http://www.dtic.mil/dtic/