Can you board a plane with a geiger counter?

Answer 1

You would have to contact the airline and explain why you wish to carry a Geiger counter with you. With current security and paranoia, they will probably only let you on board if you are a researcher and have a formal letter of request from your university or institute. There's no safety reason why they wouldn't allow it during the flight.

However, the last thing the airline want is someone scaring fellow passengers by telling them they are getting irradiated by cosmic radiation whilst they fly, without being educated enough to explain how minor the effects of this radiation really are...

For example, the annual dose (from cosmic radiation) for staff working on commercial aircraft is around 200mrem (2mSv). The annual dose for someone at ground level (from radioactive rock, cosmic radiation etc) is ~300mrem (3mSv). Add to this that people at ground level, in cities are probably breathing in carcinogenic fumes from cars, factories and smoking/passive smoking, it is probably more healthy to be in an aircraft! :)

Answer 2

Yes, you can typically bring a geiger counter on a plane as part of your carry-on luggage. However, it's always a good idea to check with the specific airline and airport security guidelines to ensure they allow it and to understand any specific regulations or restrictions that may apply.