3: Playing with Neutrons

The new radioactive isotopes were separated from the target materials by radiochemical methods. Yields of the radioactive isotopes were small because in bombarding the nucleus with positive alpha particles, most of the alpha particles were repelled by the positively charged nuclei before impact.

Enrico Fermi (Courtesy of the Los Alamos Archives)

In 1934, it occurred to Enrico Fermi, an Italian physicist, to use neutrons to produce radioactivity instead of alpha particles, which are repelled by the positive charge of the target nuclei. In Rome, his group obtained a strong radon/beryllium neutron source and began to bombard the elements in order of increasing atomic number, beginning with hydrogen. Bombardment of the first elements in the periodic chart produced no radioactivity, but finally a fluorine (Z = 9) target gave a radioactive product. In the next 3 years, the group identified 40 new radioactive isotopes. This work was important to the advancement of nuclear theory and provided radioactive tracers for practically all of the elements. The use of these tracers would later revolutionize chemical and biological techniques.

The group observed that when the target nuclei absorbed neutrons, they emitted particles (alphas, betas, positrons, or protons) as well as energy, in the form of gamma radiation. Thus, these nuclear reactions produced isotopes of elements in the neighborhood of the target element. When Fermi's group reached the heaviest known element, uranium, they expected that neutron bombardment would produce transuranic elements (new elements heavier than uranium) with properties similar to rhenium, osmium, iridium, and platinum. Beta radiation from the products and an absence of products that could be assigned to elements of atomic number between lead and uranium validated their hypothesis. Thus, the products were assumed to be transuranium elements.

In 1935, Otto Hahn, Lise Meitner, and Fritz Strassmann, in Berlin, confirmed the uranium experiments of Fermi's group and also identified the presence of U-239 as a beta emitter with a half-life of 23 minutes. The Joliot-Curies also confirmed Fermi’s work and, in 1938, identified a 3.5-hour radioactive isotope that had the chemical properties of lanthanum, an element with an atomic number much lower than that of uranium. They did not realize that it was, indeed, La¹⁴¹, a fission product. Another Nobel Prize blown!

These results indicated that the nuclear reactions were complex and that the Italians’ speculation of transuranium products might not have been entirely correct. Furthermore, in 1934, Ida Noddack (a German chemist who, with her husband Walter, had discovered the element rhenium) had warned Fermi’s group to compare the chemistry of the "new" transuranium elements with all known elements, not just those in the immediate neighborhood of uranium. Noddack suggested that "when heavy nuclei are bombarded by neutrons, it is conceivable that the nucleus breaks up into several large fragments, which would, of course, be isotopes of known elements, but would not be neighbors." The physics community ignored her warning, and she herself did not follow up with experiments.

Fermi's Italian group made one more important discovery in 1934. They found, by chance and intelligent observation, that neutrons passed through a paraffin block before reaching the target element were more effective in producing nuclear reactions than those emerging directly from the neutron source. Fermi concluded that the neutrons were slowed by elastic collisions within the paraffin and that these slow neutrons were more effective than faster ones in producing certain nuclear reactions.

It is ironic that in 1938, while Fermi was accepting his Nobel prize in Sweden, in Berlin, Hahn and Strassmann were discovering nuclear fission. This work clarified the mysterious results obtained from neutron bombardment of uranium and was the genesis of the atomic bomb. It is interesting to reflect on what world history might have been had the Italians recognized uranium fission in 1934.

Complete Bibliography on Neutrons from the Alsos Digital Library for Nuclear Issues

Contributors

• Frank A. Settle (Washington and Lee University)