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Enrico Fermi: Atomic Energy, 9147

Slow Neutrons

Natural radioactivity was discovered by Becquerel in 1896 and characterized by Pierre and Marie Curie two years later in their isolation of polonium and radium. Artificial radioactivity was discovered some thirty years later by their daughter, Irene, and her husband, Frederic Joliot. In one experiment, they bombarded boron with alpha particles converting some of it to nitrogen then confirming that the radioactivity from the boron transferred to the nitrogen. This method was also effective on aluminum but not on heavier elements due to the low energy of alpha particles. Alpha particles have drawbacks as "bullets." Being positively charged helium nuclei, they are slowed by electrical interference from surrounding electrons and meet resistance from the positively charged target nuclei. Their speed, power, and effective distance are diminished.

From this information, Enrico Fermi speculated that neutrons, with neutral polarity and higher speed, would make more effective "bullets" than alpha particles. His experimental approach was to move methodically through the periodic table, subjecting each element to neutron bombardment.

The first hurdle was to devise a reliable source of neutrons which are obtained from the collisions of alpha particles with certain elements. Fermi took radon from the disintegration of a radium source and mixed it with beryllium powder and sealed it in a glass tube. The tube was his neutron source. He built the Geiger counter used to measure the radioactivity results and gathered chemical procedures to separate and identify the elements created by disintegrations.

Fluorine, the ninth, was the first element to show radioactivity from neutron bombardment, the aim was to examine as many as possible of all 92 naturally occurring elements in the periodic table.

The method was to bombard a sample, measure the resulting radioactivity, chemically separate the irradiated sample, and measure the radioactivity of each separated element. It was shown that the element present after disintegration was close in atomic number to the original target sample. This outcome held true until the target sample was uranium; the post-bombardment mixture contained a number of elements, including one of atomic weight 93. A new element—even an unstable one—had apparently been created.

The team's next breakthrough came when a scientist noticed that placement of the sample and objects around it influenced the radiation outcome. Intrigued by this, the team began a new path of investigation, varying the materials between the neutron source and a silver target and measuring the resulting radioactivity.

During the experiments Fermi suggested trying a light material, such as paraffin wax, be used as the intermediate material around the neutron source in place of the heavy metal, lead. The results were astonishing—the silver's radioactivity increased a hundredfold. Fermi's theory from these results introduced the slow neutron concept.

Paraffin wax, a solid mixture of hydrocarbons, contains a high percentage of hydrogen atoms. The nuclei of these atoms, single protons, are equal to neutrons in mass. When neutrons enter the wax, the high hydrogen content assures a large number of collisions and the similarity in particle size slows the neutrons' speed when collisions occur. The "slow" neutrons striking the target will be more likely to collide with silver atoms; the increased collisions result in higher radioactivity.

In these experiments, the attention of Fermi's team was on testing for periodically adjacent elements from decomposition rather than more distant elements in the periodic table. The possibility of other decomposition products went unnoticed. It was left to Hahn, Strassman, and Meitner, two years later, to discover that the irradiation of uranium causes nuclear fission. The inference was that the new element thought to have been discovered was in fact a mixture of uranium's decomposition products.