Henri Becquerel’s discovery of natural radioactivity in 1896 opened a whole new world of physics and introduced new and exciting opportunities for physics research that eventually developed into important branches of modern physics such as nuclear physics and particle physics. While the early investigators explained the macroscopic kinetics of radioactive decay soon after 1896 starting with the work of Marie and Pierre Curie, Ernest Rutherford and Frederick Soddy, it took several decades until the various radioactive decay modes were fully understood on a microscopic scale.