Nuclear fission – the physical process by which very large atoms like uranium split into pairs of smaller atoms – is what makes nuclear bombs and nuclear power plants possible. But for many years, physicists believed it energetically impossible for atoms as large as uranium (atomic mass = 235 or 238) to be split into two.
That all changed on Feb. 11, 1939, with a letter to the editor of Nature – a premier international scientific journal – that described exactly how such a thing could occur and even named it fission. In that letter, physicist Lise Meitner, with the assistance of her young nephew Otto Frisch, provided a physical explanation of how nuclear fission could happen.
It was a massive leap forward in nuclear physics, but today Lise Meitner remains obscure and largely forgotten. She was excluded from the victory celebration because she was a Jewish woman. Her story is a sad one.
What happens when you split an atom
Meitner based her fission argument on the “liquid droplet model” of nuclear structure – a model that likened the forces that hold the atomic nucleus together to the surface tension that gives a water droplet its structure.
She noted that the surface tension of an atomic nucleus weakens as the charge of the nucleus increases, and could even approach zero tension if the nuclear charge was very high, as is the case for uranium (charge = 92+). The lack of sufficient nuclear surface tension would then allow the nucleus to split into two fragments when struck by a neutron – a chargeless subatomic particle – with each fragment carrying away very high levels of kinetic energy. Meisner remarked: “The whole ‘fission’ process can thus be described in an essentially classical [physics] way.” Just that simple, right?
Meitner went further to explain how her scientific colleagues had gotten it wrong. When scientists bombarded uranium with neutrons, they believed the uranium nucleus, rather than splitting, captured some neutrons. These captured neutrons were then converted into positively charged protons and thus transformed the uranium into the incrementally larger elements on the periodic table of elements – the so-called “transuranium,” or beyond uranium, elements.
Some people were skeptical that neutron bombardment could produce transuranium elements, including Irene Joliot-Curie – Marie Curie’s daughter – and Meitner. Joliot-Curie had found that one of these new alleged transuranium elements actually behaved chemically just like radium, the element her mother had discovered. Joliot-Curie suggested that it might be just radium (atomic mass = 226) – an element somewhat smaller than uranium – that was coming from the neutron-bombarded uranium.
Meitner had an alternative explanation. She thought that, rather than radium, the element in question might actually be barium – an element with a chemistry very similar to radium. The issue of radium versus barium was very important to Meitner because barium (atomic mass = 139) was a possible fission product according to her split uranium theory, but radium was not – it was too big (atomic mass = 226).
Stefan-Xp/Wikimedia Commons, CC BY-SA
Meitner urged her chemist colleague Otto Hahn to try to further purify the uranium bombardment samples and assess whether they were, in fact, made up of radium or its chemical cousin barium. Hahn complied, and he found that Meitner was correct: the element in the sample was indeed barium, not radium. Hahn’s finding suggested that the uranium nucleus had split into pieces – becoming two different elements with smaller nuclei – just as Meitner had suspected.