1906 Charles Barkla discovers that each element has a characteristic X-ray and that the degree of penetration of these X-rays is related to the atomic weight of the element
1909Hans Geiger and Ernest Marsden discover large angle deflections of alpha particles by thin metal foils
1942 Enrico Fermi makes the first controlled nuclear chain reaction
1942 Ernst Stuckelberg introduces the propagator to positron theory and interprets positrons as negative energy electrons moving backwards through spacetime
1953 R. Wilson observes Delbruck scattering of 1.33 MeV gamma-rays by the electric fields of lead nuclei
1954Chen Ning Yang and Robert Mills investigate a theory of hadronic isospin by demanding local gauge invariance under isotopic spin space rotations---first non-Abelian gauge theory
1957 Richard Feynman, Murray Gell-Mann, Robert Marshak, and Ennackel Sudarshan propose a variational approximation (VA) Lagrangian for weak interactions
1977 S.W. Herb finds the upsilon resonance implying the existence of the beauty quark
1982 A. Aspect, J. Dalibard, and G. Roger perform a polarization correlation test of Bell's inequality that rules out conspiratorial polarizer communication