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.

Better to get another one if you still have one of those, they are not very good: https://m.youtube.com/watch?v=DuAeaIcAXtg

But of course there's many still around, sadly

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.

http://www.nfpa.org/Public-Education/Staying-safe/Safety-equ...

> But of course there's many still around, sadly

I hope not.

PSA: most smoke detectors have a lifespan of 10 years. Smoke detectors from the 80s would be mostly useless now, 40+ years later.

https://www.allstate.com/resources/home-insurance/how-long-d...

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.)