Nuclear fission, the splitting of uranium atoms to produce energy, was one of the great physics advances of the twentieth century. It answered one of the great physics puzzles of the age and opened the door to the atomic age. This discovery is the basis for nuclear power and nuclear weapons.
For her discoveries, Lise Meitner has been called “the most significant woman scientist of this century.” Enrico Fermi deserves credit for many major discoveries in the field of atomic physics. But Fermi is most famous for creating the world’s first self-sustained nuclear reaction. Fermi put Meitner’s discovery to practical use and is thus the founding father of nuclear power.
Lise Meitner and Otto Hahn were researchers at the Kaiser Wilhelm Institute in Berlin, Germany. As part of their study of radioactive elements, Meitner and Hahn had struggled for years to create atoms heavier than uranium (transuranic elements). They bombarded uranium atoms with free protons. It seemed obvious that some would hit the nucleus and stick, creating an element heavier than uranium. But it never worked.
They had tested their methods with other heavy metals. Each performed exactly as expected. Everything worked as Lise’s physics equations said it should, until they reached uranium, the heaviest known element. Throughout the 1930s, no one could figure out why the experiment always failed with uranium.
There was no physical reason why heavier atoms couldn’t exist. But in over 100 tries, it had never worked. Obviously, something was happening in their experiments that they did not understand. They needed a new kind of experiment to show them what really happened when they bombarded uranium nuclei with free protons.
Finally Otto conceived a plan using nonradioactive barium as a marker to continuously detect and measure the presence of radioactive radium. If uranium decayed into radium, the barium would detect it.
Three more months were consumed with preliminary tests to establish how barium reacted to radioactive radium in the presence of uranium and to remeasure the exact decay rates and decay patterns of radium.
Before they could finish and conduct their actual experiment, Lise had to flee to Sweden to escape the rise of Hitler’s Nazi party. Otto Hahn had to conduct their grand experiment alone.
Two weeks after Hahn completed this test, Lise received a lengthy report describing his failure. He bombarded uranium with a concentrated stream of protons. But he didn’t even get radium. He detected only more barium, far more barium than he started with. Bewildered, he begged Lise to help him figure out what had happened.
One week later, Lise took a long snowshoe walk through the early winter snows. A flash image appeared in her mind of atoms tearing themselves apart. The picture was so vivid, so startling, and so strong that she could almost feel the pulsing atomic nuclei and smell the sizzle of each atom as it ripped itself apart in her imagination.
Instantly she knew that she had just been given their answer. Adding extra protons must have made the uranium nuclei unstable. They had split apart. One more experiment confirmed that, when radioactive uranium is bombarded with free protons, each uranium atom split in two, creating barium and krypton. In the process immense amounts of energy were released.
Meitner had discovered the process of nuclear fission.
Almost four years later, at 2:20 P.M. on December 2, 1942, Enrico Fermi flipped the switch that raised hundreds of neutron-absorbing cadmium control rods out of stacks of graphite blocks laced with several tons of uranium oxide pellets. Fermi had stacked 42,000 graphite blocks in an underground squash court situated under the west bleachers of Stagg’s field, the University of Chicago football field. It was the world’s first nuclear reactor, the product of Meitner’s discovery. The 1945 creation of the atomic bomb was the second application of Meitner’s fission.
After Lise Meitner’s death, the 109th element on the Periodic Chart of Elements was named after her: “meitnerium.”
