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...

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.

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.

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.