The Radiacmeter makes a very very brief appearance in Ghostbusters. Seen on Egon’s belt, it is used for measuring background radiation. You can see Egon using it in the Sedgwick scene, right after they first fire their proton streams. Egon uses it again on the rooftop battle at the end of the film. The Radiacmeter is a real world radiation detector and was used by the US navy. They are fairly difficult to find and usually pricey though you can get lucky when searching on eBay. A few sellers on GBFans have offered resin cast copies of the Radiacmeter for very reasonable prices.