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 decided on the source Sr-90 for beta emissions.i know what to do to. I will have the beta particles fired at the foil and place a GM tube next to the source to 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). but my only problem is that when beta particles are going to scatter backwards they are going to collide with the ones being emitted so therefore will not be recorded properly as they will scatter in all directions how will i overcome this problem???.