Detecting nuclear warheads

Steve Fetter, Valery A. Frolov, Marvin Miller, Robert Mozley, Oleg F. Prilutsky, Stanislav N. Rodionov, Roald Z. Sagdeev, "Detecting nuclear warheads," Science & Global Security, 1, no. 3-4, (1990): 225-253.
In the absence of shielding, 'ordinary' nuclear weapons--those containing kilogram quantities of ordinary weapon-grade (6 percent plutonium-240) plutonium or uranium-238--can be detected by neutron or gamma counters at a distance of tens of meters. Objects such as missile canisters can be radiographed with high-energy x-rays to reveal the presence of the dense fissile core of any type of nuclear warhead, or the radiation shielding that might conceal a warhead. If subjected to neutron irradiation, the fissile core of any type of unshielded warhead can also be detected by the emission of prompt--or delayed--fission neutrons at a distance on the order of 10 meters.

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