Comment by throwway120385
20 hours ago
Strontium-90 is a beta emitter, apparently, so you could just measure Alpha, Beta, and Gamma and detect false positives due to elevated background radiation that way. If you're in the air you're probably much closer to your Beta source as well so you could detect it above background due to distance. Gamma is essentially light so it falls off with the square of distance. But Alpha and Beta are probably much worse than Gamma in that respect, so I'm guessing this is fine most of the time and then they rule out false positives by looking at the total background in each type of radiation.
All nuclear things fall off much faster than square of the distance unless you're in vacuum. The atmosphere is nowhere near transparent to any of them. An addition to the inverse square issue the behavior of the various types is complex:
Gamma: Halving distance in the low hundreds of meters.
Alpha: Goes a few centimeters.
Beta: Goes a few meters per million electron volts. Most sources don't go above a few million electron volts.
Neutron: The only one with good penetration, halving distance around a kilometer for high energy neutrons. But note that there are few sources of neutron radiation other than fission.
Thus unless you're flying into fallout it's not much of an issue.
Gamma is technically a kind of electromagnetic emission, so it falls off with the square of the distance. The others fall off as you say.
Everything falls off as inverse square in 3D. His point is that the atmosphere attenuates it as well.