[Sharlin]

That's because there's gaseous radon-222 continuously released from soil and rock, as a decay product of natural uranium. But it's an alpha emitter so is stopped by anything, including a few cm of air, or a sheet of paper – or the glass filter in front of a digital camera sensor.

[kimixa]

But the lens and camera assembly isn't a vacuum, if there's radon in the air around the camera I'd expect radon to be inside the camera too, right by the CCD.

[Sharlin]

Yes, but that amount of air is going to contain a rather insignificant amount of radon atoms. And as I said, even alpha particles emitted inside the camera have little hope of actually making it to the sensor, which is sandwiched between several layers of filters and the backside electronics and the camera chassis. (4He nuclei from space, making up around 10% of cosmic rays, on the other hand can have enough energy to penetrate the atmosphere and up to tens of meters of solid matter, so you might certainly capture some of those, besides the muons.)