You'll also see radioactive sources in household smoke alarms on a smaller scale to estimate smoke load in air, and in mineral exploration in downhole detectors mapping density of surrounding materials, in oil and gas measuring the density profiles of fluids as they flow past, etc.
Flashlights are visible light spectrum sources, gamma emitters are similar in function ( Cs-137 emits a 32 KeV x-ray peak and the 662 keV gamma peak ) as they "shine" through materials opaque in the visible spectrum and can be measured after for absorption.
> Those household smoke detectors were in the 80s, the ones sold now are optical.
No, ionization detectors are still around. They're better for detecting flame fires than optical. Think of when you've seen a campfire and there's been little smoke. There was definitely a fire, but an optical detector wouldn't have detected it.
In contrast, optical is better for smoking fires. Best practice is to have some of each. In fact, I believe some models have both in the same unit.
Another PSA: while you're replacing your old smoke detectors, if you have combustion heat, consider replacing some of them with ones that also detect CO. (You'll have to replace them in 5-7years, though.)
