Radiating chemical suggestion?

Given a flat and thin object laying on a table, such as a small piece of silver paper or a small piece of cardboard, I want to find a way to detect which face of it is facing up. The solution should be able to detect the faces even when the object is placed underneath other object (so that camera/vision based approaches are not feasible). 


What I am thinking is to find a **radiating chemical using which we can paint on one side of the object and paint the other side of the paper piece with a chemical or substance that blocks the radiation**, we should be able to tell if the object is up or down. 


My question is: Is there such substance/chemical and associated detector on the market that we can purchase and use?


