Can the radiation from illicit nuclear warheads be detected from space?

Quick answer: If a satellite can pick up a meaningful radiation signal from 500 km over the warhead, then please do not sign me up to be the technician standing 10 meters from the warhead and receiving 2.5 billion times as much radiation.

Longer answer: Plutonium-239 is primarily an alpha-emitter, and a cardboard box would be sufficient to stop all the alpha particles, but plutonium also emits a much smaller number of gamma rays which will go through the box and could be detected. And in fact, gamma ray detectors may be used by weapons inspectors, or the guards at the door of a nuclear weapons facility.

However, the gamma radiation is not very intense. By way of comparison, if you go everywhere carrying a plutonium bomb around in a cardboard box, you're giving yourself a radiation dose in roughly the same ballpark as the cosmic ray dose you would get from flying in an airplane for the same period of time.

So, detecting this level of radiation from space is beyond ludicrous. Suppose you are trying to detect 6.1 kilograms of weapons-grade plutonium, the same amount dropped on Nagasaki. This plutonium is putting out about 915,000 gammas per second with energy greater than 1 MeV. You have a satellite orbiting 500 kilometers overhead, carrying a detector which is 3 square meters in area and is 100% efficient. You would expect the detector to see a gamma ray from the plutonium warhead-- one click-- about once every fourteen months. (I'm including here the fact that the satellite is only passing over the warhead for at most 3% of every day.) In order to take your case before the Security Council, you just have to separate out this once-every-fourteen-months click, to a statistically significant degree, from the background noise, which if you have incredibly good background rejection, might be on the order of ten clicks per second.

It's actually worse than that, of course, since we haven't yet considered any shielding of the warhead. At a bare minimum, there will be some self-shielding: that is, since plutonium is itself a heavy metal, the plutonium in the middle of a lump of plutonium is partly shielded by the plutonium on the outside. If the warhead owner is actually interested in avoiding detection, of course, he'll replace that cardboard box with a nice lead-lined safe and hide the plutonium completely.