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Comment by vova_hn2 | Hacker News Reader

What if the helicopter would have to operate in a contaminated area? Wouldn't it cause false positives?

> The CH-53 Sea Stallion first entered service in 1966

1966 is Cold War era, so, I think, an idea that a military helicopter would have to fly in an area that has been recently nuked wasn't totally inconceivable...

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Assuming only one rotor cracks, then the radiation is from one rotor IBIS and the detector electronics could detect radiation levels that ripple synchronously with the rotor speed.

The Strontium-90 is a beta radiation emitter.

  The external radiation level of the IBIS [is] 75 mR/hr at 12 inches in the failure (extended) mode.

I imagine that if you're flying through an active nuclear war, false positives from the blade crack detector are pretty far down your list of things to worry about.

At that point a blade crack might be the best option

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.

Exposure to a nuclear blast doesn't necessarily make things radioactive. Hiroshima and Nagasaki are not radioactive wastelands. Even in Chernobyl, the other reactors continued to operate for a time after the accident, and nature is thriving.

But, the radiation detector is on the upper surface of the rear fuselage. Hypothetical Beta particles from the ground will strike the underside of the aircraft and not the detector.

> Exposure to a nuclear blast doesn't necessarily make things radioactive

Isn’t this neutron activation?

It is, but a) it's a probabilistic process and b) most of the products decay rapidly, down to the background level in a matter of days.

Generally they are going to airburst to cause maximum damage over an area which means what free neutrons hit are gasses which dispurse pretty quickly. Ground detonation does cause more fallout though.

Strontium-90 is a major component of fallout and strongly in the ‘really not good for you’ category.

Nothing I wrote refutes that

fallout is present in dust in the air. it’s radiation would not be coming only from below the helicopter.

I guess in this extremely contrived hypothetical where a Navy helicopter flies through heavily-irradiated dust cloud from a nuclear detonation, yes, the blade alarms might go off spuriously. Well done.

what helicopter do you expect EXCEPT a navy helicopter to be flying through the fallout from a nuclear explosion?

especially with all the nuclear tests they were doing for decades?

Ambulance and rescue helicopters, if there are any left, and the crew didn't just take the helicopter and run as far as possible...

Which will almost certainly be military, because civilian helicopters and staff aren’t equipped or trained for that kind of environment except for perhaps a every couple of years hazmat/mass casualty training.

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