Detecting nuclear warheads

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.