However according to James from support at radiation-watch.org, here is how you can use the pocket Geiger to test crab: A) first soak a paper towel in your tap water and place the ring side of the pocket Geiger down on the wet paper towel and measure (10 minutes or so) next B) steam or boil the crab, soak a fresh paper towel in water from the cooked crab and once again measure with the ring side down of the detector (about 10 minutes).
Subtract readings of test A from test B and that is a ball park way of detecting radiation in food with pocket Geiger counter. The procedure is similar for soil. Place the dirt in an air tight container (like Tupperware) , place ring side down measure for 10 minutes. I found no measurable radiation in this crab from Bodega Bay California with these methods.
I put the crab meat in a small container and actually got trace readings (safe but slightly higher than background), the paper towel method, yielded no change from background radiation readings