[Greenbuilding] radon question

[Clarke Olsen]

A ground floor room tests positive for Radon - not high, but more then nothing.
The floor is 4+" of concrete on 2" ext. styrofoam, on 6 - 8" of gravel.
Big (1/2") joint between foundation and slab is the likely source.
Should I: 
                 a) seal-up the joint with non-shrinking mortar?
                 b) seal it with expanding foam? With caulk?
                 c) arrange some form of fat baseboard vent?

Clarke Olsen
373 route 203
Spencertown, NY 12165 
USA
518-392-4640
colsen at fairpoint.net