True about the gamma energy dependence, of course, but alpha and beta won't really get to the sensor unless the decay is inside the camera. And even gamma doesn't have that long range, even in air. You pretty much have to have contamination nearby.
For reference, you can comfortably detect, measure and map ground produced gamma radiation from natural elements from a distance of 50 - 80 metres in the air above.
There's also a respectable number of cosmic ray events in the atmosphere all the way down to ground level leading to high energy gamma counts as a matter of course.
Radioactive Iodine and Ceasium particles (in small numbers admittedly) made their way across the Pacific and became gamma emitters caught up in the air scrubbers of the University of Washington in Seattle.
Well, cosmic rays can have energies far, far higher than you get from typical radioactive decay, so they (or gammas produced by them when they hit the atmosphere) have a lot more penetration.
