Endwall 11/15/2019 (Fri) 02:36:19 No.1498 del
Any radiocative isotope of an element should work. In the video he's using Americium from a smoke detector. You just need some random gamma and beta radiation from a decay event to set off the Geiger counter. Any radio isotope will do. Radioactive decay times and quantities are random and unknowable before the event occurs.