I found this interesting story about traveling with a Geiger counter: https://sites.google.com/site/diygeigercounter/traveling-with-the-geiger It has to be noted that the Geiger was home made, and hence the suspicions. I suppose that our neat and tidy GMC-XXX devices should be treated like any other electronic device. The only concern that I have is about the following sentence in the story: "The SBM-20 (geiger tube) particularly resembles a detonator. Not much you can do about that." If they can see that in the X-ray scanning inside your hand luggage, maybe it depends on the mood/decision of the security guy on how to proceed.
Could you comment on your experiences when travelling with the Geiger counter in the airplane cabin?
Years ago I carried a kit type detector with just an LED and a speaker on several flights. The home-made enclosure made it look suspicious. I caught worried looks from a seat-mate once, so I made it a habit to talk about the topic before pulling it out if I thought they might get scared. One time I was in a hurry through arrival security in Japan and got some stern looks about it. Through the language barrier I used mainly hand motions to convey that it can measure x-rays, and to listen for the clicks as it went through. Once demonstrated, the security official called over all nearby inspectors to see it; thankfully they were more interested than suspicious by then, and sent me on my way.