It is basically shielding a magnetic field. This can be done with rubber or any other material that is thick. Normally small magnets already block the shield.
When an atom has a high neutron-to-proton ration, that atom is very unstable and thus, radioactive. The more neutrons an atom has, the more radioactive it is. So,it is only logical that the element with the greatest number of neutrons is Uranium. It has 140 neutrons.
