Rent a geiger counter and bring it with you to the stone yard. Good chance you'll be wasting your money on the rental, but if it'll make you feel better, that's the way to go. As for reputable independent studies, there are none. Either they've been backed by the solid surface industry, or they've been backed by the Marble Institute of America (MIA). Either way, you're not going to get an impartial study.

I WILL say this, though-- this issue HAS been blown way out of proportion by the SSA. Not that it's not something that should be kept in mind while shopping for your stone, but it's not something that's going to eliminate the need for a microwave, either. I DO find it interesting that although this side is just for off topic conversations, and that you have to go through the proper forum to get to this one, you posted this over here where it'll stay up top of the list for a much longer time. If this is really a bonafide question, I apologize for the suspicion. But we've had a couple who are solid surface contractors come in here talking about how bad granite is under different names for the last couple of years, just to scare people away from it, thereby boosting the solid surface industry.

If you're really interested, check out the following thread from a couple of weeks ago:

Here is a link that might be useful: Is granite dangerous? Have heard it emits radon?

One other you might check out. Pay particular attention to posts by a gentleman named Caoimhín P. Connell (his first post is post #12). He's got no "horse in the race", and this is what his profession is all about:

Here is a link that might be useful: Inspection News Forum