In the early decades of the twentieth century, scientists were uncovering the structure of the atom. They discovered the existence of negatively charged particles, electrons, orbiting positively charged protons in the nucleus — the center — of an atom. In 1932, two British physicists, Ernest Rutherford and James Chadwick predicted and found the neutron — the last puzzle piece in the atomic structure. The neutron, along with a number of protons, formed the nucleus of an atom. The neutron turned out to be the key particle that would unlock the secret of atomic energy.

Because the neutron was electrically neutral and would not be deflected by the charged particles in the atom, scientists could use neutrons to bombard the nucleus of atoms and further explore their structure. Enrico Fermi, an Italian physicist, began bombarding elements with neutrons just after Rutherford and Chadwick’s...