Loren Pechtel

Yeah, they are reasonable in cargo handling areas. They're not meaningful for passenger areas, though--nobody's going to be hauling around a dirty bomb (they won't survive to do so) so all they'll end up catching is people like my wife (nuclear heart scan. Two detectors at PVG reacted to her.)

Paplover

Weapons grade plutonium also emits neutrons (or else it would not be useful for a nuclear weapon) and gamma rays. All nuclear decay emits some gamma rays even if it emits other particles. It is not at all hard to detect, especially as a neutron emitter.

Loren Pechtel

You need a neutron emitter to set it off but the requirement for going boom is fission when struck by a neutron of the same energy level as it tends to release when it fissions. It would be possible to transport the neutron emitter separately. If you don't care about portability you could even build a bomb with a little particle accelerator to provide the neutrons.

Of course in the real world there will be *SOME* emissions from your fissionables but it's not inherently required.

relangford

I was talking about unexploded Pu, not fission products. Pu-238 decays by alpha emission to U-234; Pu-239 - the most common weapon form - by alpha to U-235; Pu-240 by alpha to U-236; Pu-241 by positron emission to Am-241; and Pu-242 by alpha to U-238. The Am-241 is a powerful gamma emitter, and is present in some fuels, but weapon grade Pu-239 is an alpha emitter, with only low energy x-rays or gamma radiation from impurities. While you may say it is "not at all hard to detect"; I have found, in my 40+ years of experience, that Pu isn't that easy to detect, especially at a distance. Pu isn't a neutron emitter; it fissions when hit by neutrons (one isotope can fission when hit by alphas and one isotope absorbs neutrons, but those are other topics). Neutron detection is not all that easy, either. Yes, I was formerly in nuclear R&D.

B1

Remember to stop your vehicle before the detectors, at the place so indicated. If you idle in the space between them the engine gives off enough heat to set off the detector (this happened to a friend). Also, if you've been to a hospital for a diagnostic procedure that uses radioactivity (such as a PET scan), you will set them off (also happened). Perhaps they are set at a level of detection that is over-sensitive.

Loren Pechtel

The problem is that there is no middle ground. Nuclear medicine patients are hot for a little while--there's no level that will exclude them and yet catch the stuff they're looking for.

Perhaps they could make a detector sophisticated enough to figure out the energy levels involved and exclude the normal nuclear medicine isotopes but I don't think such a device can be built for use in a screening role. (It's going to be nowhere near as sensitive as a detector that's simply recording the energy level.)