Loads of options out there mate, I've got a Gamma Scout and love it. It measures Alpha, Beta and Gamma and you can download your hourly averages for the past 36k hours into a text file. You can't switch it off - the battery is soldered in and lasts 10 years.
Lots of people have the ecotest terra-p.
Personally I'd stay away from the Chinese ones but they might be OK..