Group portrait (1932) of physicists and chemists who greatly contributed, in the first half of this century, to improve the understanding of the atomic structure. Identified here from left to right and seated are: Sir James Chadwick, the discoverer of the neutron (1932), Hans Geiger, who invented a counter for charged nuclear particles, and Sir Ernest Rutherford who is considered to be the father of nuclear physics. Standing from left to right are: Gyorgy Hevesy, who introduced the use of radioactive tracers to biological systems, Mrs Geiger, Lise Meitner and Otto Hahn. The last two, in 1939, obtained the first nuclear fission.