Hitting the nucleus, or centre, of an atom may be compared to firing a
BB gun at a fly buzzing around somewhere in an acre of ground
surrounded by chicken wire. The chance of hitting it is not great!
But if a million BBs could be fired at a swarm of flies, some hits would surely be scored. For bombarding the atom, scientists chose the neutron. There are some neutrons in the nuclei of most atoms. A neutron is heavy (for its size) and it has no electrical charge that might affect its direction. So scientists began to bombard atoms with streams of neutrons.
Every now and then a neutron would hit the nucleus of an atom and would smash it. In 1933 an Italian scientist, Enrico Fermi, discovered that sometimes the nucleus is not smashed by the neutron but instead "captures" it. The neutron becomes part of the nucleus, thus creating a heavier atom. Bombarding uranium, which has the atomic number 92, Fermi produced a new element, neptunium, with an atomic number of 93. Other experimenters joined in to learn what else happens in the neutron bombardment.
Tliey included O. Hahn and F. Strassman in Germany, O. R. Frisch, and Lise Meitner (who fled from the Nazis in Austria). One of their discoveries was that neutron bombardment sometimes splits a uranium nucleus into two nearly equal parts. Most of the uranium found in the earth has the atomic weight 238, but about seven uranium atoms in a thousand arc lighter, with the atomic weight 235. This lighter atom is called U-235. It is fissionable by neutron bombardment; and the effect of a hit on U-235 was found to be that it broke the nucleus into two parts, at the same time releasing a few free neutrons. In January, 1939, the Danish scientist Niels Bohr visited Princeton University and told Fermi, Albert Einstein and other American scientists about these discoveries.
