Enrico Fermi’s impact on science BY JOHN H. MARBURGER III Fermi’s work with slow neutrons prepared NRICO FERMI IS the father of mod- ern nuclear physics and a physicist’s the way for the discovery of nuclear fission, the E physicist, whose legacy was one of key to extracting energy from nuclear reactions. style as well of substance—a style so at- tractive and so productive for science that work follows in a direct line from that of from your own experience with colliding it became substantive in itself. In his 1938 Fermi and his many students. things, energy transfer is maximized in col- Nobel Prize speech, Fermi acknowledged lisions among particles of equal mass. By the grandfather of this field, Lord Ruther- Fundamental discoveries interposing various thicknesses of modera- ford, who a generation earlier had begun In these early experiments, Fermi made tors between the neutron source and the tar- bombarding substances with alpha parti- two fundamental discoveries cited by the get, Fermi and his students could map out a cles, the positively charged nuclei of helium Nobel Committee. The first was the discov- spectrum of the neutron speeds required to atoms that shoot out from uranium, radium, ery of new elements created from targets activate the target, with well-defined peaks and other heavy elements. Those early ex- whose nuclei trapped the bombarding neu- of activation energy. This was the begin- periments bore much fruit for physics, in- trons in a fashion that is easy to visualize but ning of nuclear spectroscopy, an essential cluding today’s atomic model of a tiny mas- hard to calculate. Fermi’s laboratory was the tool for the production and application of sive nucleus orbited by electrically bound isotopes in chemistry, medi- lightweight electrons. But while they eluci- cine, and materials sci- dated the overall structure of the atom, ence—a field that remains Rutherford’s experiments did not penetrate important today. the mysteries of the nucleus itself. Fermi was an ingenious ex- Fermi realized that the electrostatic re- perimenter who obviously pulsion on the positive charge of Ruther- took pride in the details of his ford’s bombarding alpha particles prevent- apparatus. The following ex- ed them from entering the positively cerpt from his Nobel speech charged nucleus. He perceived that the neu- reveals his enthusiasm, as tron, discovered by James Chadwick in well as his talent for clever 1932, having no charge at all, would be the experiments: ideal probe for nuclear studies. Starting in 1933, Fermi and his students conducted In order to measure, di- systematic studies of the effect of neutron rectly at least, the order of irradiation of the chemical elements. This magnitude [of the time neu- was the beginning of our knowledge of nu- trons remained free to diffuse clear matter. in a moderator], an experi- Neutrons were produced for these stud- ment was attempted by my- ies by the collisions between alpha parti- self and my collaborators. cles from a radioactive source (an alpha The source of neutrons was emitter) and a light element such as beryl- fastened at the edge of a ro- lium. The two elements were mixed to- tating wheel, and two identi- gether in pellets to form compact “neutron cal detectors were placed on guns.” Later beryllium targets were bom- the same edge, at equal dis- barded by deuterium ions energized in an tances from the source, one in accelerator. The most modern sources pro- front and one behind with re- duce neutrons in a similar way, by direct- spect to the sense of rotation. ing a high-energy proton beam onto a tar- Fermi: A lifetime of achievement The wheel was then spun at a get, dislodging a spray of neutrons in what very high speed inside a fis- is called a spallation reaction. The Depart- first to create and identify elements with sure in a large paraffin block. We found ment of Energy’s huge Spallation Neutron atomic numbers greater than 92 (uranium), that, while, with the wheel at rest, the two Source, now under construction at Oak the highest naturally occurring element. In detectors became equally active, when the Ridge, Tenn., is designed to provide neu- honor of this work, the artificial element wheel was in motion during the activation, trons for thousands of scientists whose number 100 is named “Fermium,” abbrevi- the detector that was behind the source be- ated Fm. Its longest lived isotope (257) has, came considerably more active than the one John H. Marburger III is Director of the Office coincidentally, a half-life of 100 days. in front. From a discussion of this experi- of Science and Technology Policy, in the Exec- The second important discovery was a ment was deduced, that the neutrons re- utive Office of the President, in Washington, more subtle effect, in which the trapping main inside the paraffin for a time of the or- D.C., and Science Adviser to President George process was found to depend on the energy der of 10-4seconds. W. Bush. This article is based on a speech he or speed of the incoming neutrons. To per- presented on November 27 at a symposium, form these experiments, Fermi exploited Years before these famous experiments, “The Legacy of Enrico Fermi in America: Sci- the slowing effect of “moderating” sub- Fermi had already proven himself to be a ence, Energy, and International Collaboration,” stances like paraffin and water that contain powerful theorist. He was the first to apply on the occasion of Fermi’s centennial birthday, hydrogen nuclei of nearly the same mass as the Pauli exclusion principle to systems of at the Italian Embassy, in Washington, D.C. the neutrons. As you are probably aware multiple electrons not attached to atoms. February 2002 NUCLEAR NEWS 27 The Pauli principle states that for a certain cation and shape determine many important so the approach is called the Thomas-Fer- class of particle known collectively today as properties of the material. The surface does mi method. Fermi performed many calcu- “fermions,” no two can be in exactly the not appear in real space, but rather in the lations using this method, laboriously exe- space of labels that cuting the necessary repeated arithmetical define the quantum operations that today are done so effort- Fermi contributed important states that are filled lessly by computers. or unfilled. It is most These theoretical manipulations occurred technical ideas to the theory convenient to label at the time when quantum theory itself was states of electrons still being invented by the great frontiers- of quantum electrodynamics, that can move freely men of the early 20th century. Fermi con- about by their mo- tributed important technical ideas to the the- notably in the quantization of menta, so the Fermi ory of quantum electrodynamics, notably in surface is a surface the quantization of the electromagnetic the electromagnetic field. of constant energy in field. His 1930 paper on this subject is a momentum space. I model of lucid exposition wherein deep re- do not mean to imply sults appear almost effortlessly. His treat- same quantum state. Crudely speaking, two that Fermi was responsible for the electron- ment remains to this day a standard way of such particles cannot be found in the same ics industry, but every electrical engineer to- introducing the subject. place. By contrast, another class of particle day knows what a Fermi surface is. exists, known as “bosons,” which rather pre- In this same category of work appears Weak interactions fer to be in the same state. The 2001 Nobel Fermi’s treatment of the properties of Of greater importance has been Fermi’s Prizes in physics were awarded to scientists atoms with many electrons. Every physics theory of weak interactions, a topic of pro- who demonstrated “Bose-Einstein Conden- student learns how to apply the equations of found interest even decades after his death sation” of bosonic atoms. Similar experi- quantum theory to derive the spectrum of in 1954. Some historical context is neces- ments on fermionic atoms are also being hydrogen, which with one electron is the sary to appreciate its importance. Until the conducted, but of course they behave com- simplest atom. But a similarly direct ap- discovery of the neutron in 1932, progress pletely differently because they prefer to be proach for heavier atoms is hopelessly in the understanding of matter was toward in different states, even at low temperatures. complicated. Fermi regarded the many simplification. By the end of the 19th cen- Electrons are fermions, and when metal- electrons surrounding such atoms as form- tury, chemists had nearly exhausted the lic atoms condense to form electrical con- ing a gas that moves in an effective poten- search for different kinds of elementary ductors or semiconductors, their electrons fill tial whose form could be derived in a self- atoms. The result, summarized in the lowest unoccupied energy states up to a consistent way. A similar result was Mendeleev’s periodic table, included 92 va- surface called the Fermi surface, whose lo- obtained independently by L. H. Thomas, rieties, from hydrogen, the lightest, to ura- 28 NUCLEAR NEWS February 2002 FERMI’ S IMPACT ON SCIENCE nium, the heaviest. After Rutherford which except for its charge closely resem- We now understand more about what is demonstrated the general shape of the atom bles the tiny massive proton, when the elec- happening, of course, and the complicated in 1911, there was reason to believe that all tron circling the proton in a hydrogen atom interaction Fermi postulated is now known of the 92 varieties could be made up of only keeps its distance at 100 000 times the pro- to involve two simpler interactions, both two particles, electrons and protons.