I am designing an experiment to investigate how the number of beta particles scattered backwards depends on the thickness of a foil and the proton number of the foil material. I have an idea of what I will do, i.e. I will have the beta particles fired at the foil and place a GM tube next to the source the get the number of particles scattered backwards. i will change the thickness of the foil, and would also change the type of foil (aluminum, tin and some other material). What source can i use for emitting the beta particles?