What would be the best sensor one could get so that it could pick up random radiation sources like this? Are there any good ones in keychain form?
The thought process is that if there is random steel laying around releasing unhealthy amounts of radiation, would it be nice to know before going to the doctor?
No. The sensing element needs to have a fair bit of volume, to have a decent chance to detect rare impact events. (Remember, the characteristic feature of gamma radiation is that it penetrates, easily passing through normal matter. A geiger tube has to be big.)
Additionally, geiger tubes consume a fair amount of power. It needs to be recharged daily, like a cell phone. Not something easily carried around just in case.
Additionally, additionally, the 500+ has two tubes, which lets it measure higher doses of radiation. A cheaper single tube geiger counter designed to detect low levels of radiation will saturate when exposed to more dangerous levels of radiation. (This was a fun detail of the Chernobyl disaster: all the dosimeters the staff had access to maxed out at 0.001 R/s, which is far below any threatening level. This lead them to underestimate how bad the explosion was for many hours.)
Apparently wristwatch-sized detectors also exist, and there was a case in Czech Republic that an walking-by engineer accidentally found a missing radiation source in the wild using his watch, it was buried in a playground [0], unbelievable!
You can buy (or build from a kit if you can solder) a functional geiger counter that will detect gamma radiation (what cobalt-60 emits) for under 100 USD. They're a bit too big to put on a keychain, but you can easily keep one in a backpack.
The thought process is that if there is random steel laying around releasing unhealthy amounts of radiation, would it be nice to know before going to the doctor?