Coin-sized nuclear 3V battery with 50-year lifespan enters mass production
(www.techspot.com)
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No, not by detecting RADIATION but by detecting smoke or anything that interferes with the radiation tripwire. Imagine a photoelectric cell that is constantly on (except that this is a field generated by a small radioactive element) and a person walks through the light beam and sets off the alarm. Same with smoke in this case, it interferes with the field emitted by the Americium contained in the smoke detector. So it's not really detecting radiation, but for the lack of it. That's the best analogy I can make.
So, they DO detect radiation ---- and look for a drop.
No, the radiation ionizes the air inside the device and creates an air flow allowing the air to constantly flow through the detector to be sampled. The smoke is detected by other means such as breaking a photoelectric beam.
AKA radiation ---- smoke stops Alpha radiation
Smoke detectors work by detecting Alpha radiation changes.
Gamma rays don't give a shit and go through everything, but Alpha rays get stopped easily.
...thus it detects the absence of radiation...
To detect the absence of radiation you have to detect radiation.
No one said where the radiation came from.
It works by detecting radiation.
Photocells work by detecting light --- sun or flashlight or infrared LED, doesn't give a shit