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Home , Discovery of the neutron, Enrico Fermi, James Chadwick

Enrico Fermi’s impact on BY JOHN H. MARBURGER III Fermi’s work with slow prepared NRICO FERMI IS the father of mod- ern nuclear and a ’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- speech, Fermi acknowledged lisions among particles of equal . 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 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 that shoot out from , , 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 in chemistry, medi- lightweight . But while they eluci- cine, and materials sci- dated the overall structure of the , 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 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 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 targets were bom- the same edge, at equal dis- barded by 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 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 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 during the activation, trons for thousands of whose number 100 is named “,” abbrevi- the detector that was behind the source be- ated Fm. Its longest lived (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 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, 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- “,” 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 , 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 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 . Crudely speaking, two that Fermi was responsible for the - 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 “,” 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- 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, 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 , 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 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 . Some ton radius. By what process is the electron very similar to the electromagnetic interac- electrons were assumed to be bound with- held within the neutron, and how do we de- tion. In place of the , a new kind of in the nucleus by a mysterious force, others scribe its release during neutron decay? particle is involved (one of the “massive orbited far away from the nucleus. This ex- Fermi answered this question with a the- vector bosons” of the weak gauge field), un- traordinary reduction of all to only ory of neutron decay that became the mod- known to science before the 1970s. But Fer- el for all future theo- mi’s theory of weak interactions remains an ries of particle accurate and useful approximation. More- Fermi’s theory of . He did over, the entire subsequent development of not assume that the what is now called the em- interactions remains an decay products were ploys quantum fields and interactions in present in the neu- much the way Fermi first introduced them accurate and useful tron before the de- into physical theory. cay, but rather com- Fermi’s work with slow neutrons pre- approximation. pared the situation pared the way for the discovery of nuclear with the emission of fission, the key to extracting energy from light from an elec- nuclear reactions. He might well have made two components was a great triumph for tron within an atom. A photon is created this discovery himself, but others did it first, science, but it created a psychological prej- “on the spot” out of the energy available in Germany. When he came to the United udice against the introduction of additional when the electron passes to a lower state. States directly after the 1938 Nobel cere- particles to explain new phenomena. Fermi postulated that the proton, electron, mony, the clouds of war were gathering in When Chadwick discovered the neutron, and were similarly created on the . In the following year, Hitler’s it was only natural to think of it as a com- spot, and wrote down a mathematical ex- troops overran Poland. The intellectual bination of a proton and an electron, espe- pression for the interaction that was similar community in America that cially since it was found to decay—in about to the basic interaction in quantum electro- later described in her famous book Illustri- 10 minutes, when outside a nucleus—into dynamics, but more complicated. This the- ous Immigrants included who an electron and another particle that we now ory was the first to apply quantum fields to pondered with foreboding the unfortunate call the “.” It was difficult particles other than the electron, and has coincidence of Hitler’s rise to power and to understand, however, why an electron guided the development of our understand- the discovery of nature’s most awesome should be bound so tightly in the neutron, ing of weak interactions to the present day. source of energy. It was natural that Fermi

February 2002 NUCLEAR NEWS 29 FERMI’ S IMPACT ON SCIENCE should join the effort first to understand, ment of a new at Argonne, the others. Even though the purpose was and then to enlist, in the in . This work ultimately led to the grim and terrifying, it was one of the great- cause of war. Within two years, Fermi was construction of large production est physics experiments of all time. Fermi building the world’s first nuclear reactor reactors at Hanford, Wash., by the DuPont completely immersed himself in the task. under the University of ’s Stagg Company. In 1943 and 1944, and again af- At the time of the test, he was one of the Field. ter the war, Fermi also used the Argonne re- very few persons (or perhaps the only one) actor for pure research, and showed how who understood all the technical ramifica- Nuclear reactors neutrons could be useful for solid-state tions of the activities at Alamogordo.” Fermi’s talents were well-suited to this physics. The test—the actual ignition of task. No one understood the interaction of By that time, X-ray had be- the device—occurred on July 15, 1945, and neutrons with matter as well as he, and no come an established technique for examin- Fermi observed it from a distance of nine one else had the wide range of skills and ing crystal structures. When any wave with miles. Soon after the bomb exploded, he re- knowledge that would as quickly to a a well-defined falls upon a leased small pieces of paper from his hand. working reactor. Fermi’s team did build a structure that has a pattern that repeats it- In still air they would fall at his feet, but successful reactor that first went critical on self on the scale of the wavelength, the in- when the shock wave arrived (many sec- , 1942. The reactor was a “pile” coming wave is strongly redirected at a def- onds after the brilliant flash of light), the of moderator and uranium fuel, inite angle related to the spacing of the bits of paper were blown some distance with cadmium control elements. Fermi had structure. Since, according to quantum the- away from him. Using a table of numbers found cadmium to be a strong neutron ab- ory, particles like neutrons have wave prop- he had prepared in advance, he was able to sorber, and the combination of components erties, they can be used in diffraction ex- estimate the energy released by the bomb was designed to produce neutrons with just periments just like X rays. Unlike X rays, from the displacement of air and the known the right energy to be captured by the ura- however, which interact with the clouds of distance from the source. As usual for Fer- nium nuclei, which subsequently break electrons within a crystal structure, neu- mi, his answer closely approximated that of apart, or fission, creating additional neu- trons interact directly with the nuclear cores the elaborate official measurements, which trons to keep the reaction going. Many new of the crystal lattice, which possess nearly took several days to analyze. The bomb’s elements are created in this process, not all the mass of the structure. Neutrons are energy proved to be near the high end of the only in the direct incorporation of neutrons also microscopic magnets, and therefore range of estimates that had been made by in the uranium nuclei, but also among the they can be used to probe the magnetic scientists at Los Alamos. fission fragments. properties of materials. This is why we go Fermi’s work with reactors makes him Nuclear reactors can produce three to so much trouble to produce neutrons in the father of nuclear energy, and it is for this things: new chemical isotopes, heat, and facilities like research reactors, or the up- reason, as well as for his important contri- neutrons. For the war effort, reactors could coming Spallation at Oak bution to the war effort, that the United produce the new element plutonium, useful Ridge. These facilities are complementary States Congress gave him a special award for nuclear weapons, from an old chemical, to the fabulously productive synchrotron just prior to his death in November 1954. natural uranium, which is not (a rare isotope light sources, which produce intense X rays. Two years later an award was established of uranium is useful for weapons, but that is in his honor, given annually by the Depart- another story). For practical applications, Variety of problems ment of Energy and the President of the the production of heat is most important, Back to the : Everyone . because it can drive steam generators to knows the story of the secret laboratory es- I have mentioned only the high points of produce electricity. For science, the most tablished at Los Alamos, N.M., under the Fermi’s contribution to science. Those who important applications are the production direction of J. Robert Oppenheimer. Here worked with him held him in the greatest es- of isotopes for medical and materials stud- the atomic bomb itself was to be designed teem. C. N. Yang, another great physicists’ ies, and the production of neutrons to use as and built. The laboratory got under way in physicist, wrote this in his autobiography: probes to image the atomic structure of mat- mid-1943 and Fermi went occasionally as ter. Reactors optimized for one application a consultant until August 1944, when he . . . my taste in physics was largely will not in general be optimized for anoth- moved there full-time. Throughout these formed . . . when I was a student in Kun- er. Research reactors, for example, are op- years, he helped and engineers ming. It was in those years that I learned timized for neutron or isotope production, to solve a wide variety of physical prob- to admire the work of Einstein, Dirac, and and not for heat, so they “run cool” and tend lems, ranging from hydrodynamics to elec- Fermi. They have, of course, very differ- to be much smaller than power reactors. tronic circuit design. As an associate direc- ent styles. Nonetheless, they share the abil- Over the next few years, Fermi came to tor, Fermi had responsibility for a new ity to extract the fundamentals of a physi- be viewed as an oracle by people working division, which included theory, and later cal concept, a theoretical structure, or a on the Manhattan Project. His long experi- the hydrogen bomb, and a homogeneous re- physical phenomenon and to zero in on the ence with neutrons and remarkable mastery actor in which Fermi took an active inter- essentials. Later, when I came to know of physics gave him what seemed an intu- est. In this reactor, uranium salts were dis- Fermi and Dirac, I realized that they spoke itive knowledge of neutron behavior. In his solved in water, which served as moderator. and thought about physics very much in biography of Fermi, Emilio Segrè remarked It was made to operate at 5 kilowatts. the way that I had imagined them to do that when an engineer needed a piece of in- In the preparations for testing the bomb, from studying their papers. formation, the regular procedure was to ig- Fermi’s help was invaluable. As Segrè says, nore Fermi’s protests that that quantity had “This was one of those occasions when Fer- Yang’s scope of accomplishment in not been measured and could not be pre- mi’s dominion over all physics, one of his physics is also broad and brilliant, his style dicted. The engineer would just recite slow- most startling characteristics, came into its elegant and more austere than Fermi’s, and ly a series of numbers while watching Fer- own. The problems involved in the Trinity his words testify to the power of Fermi’s ap- mi’s eyes closely, and “the correct number test ranged from hydrodynamics to nuclear proach to science. Even through the dry lan- would produce an involuntary twinkle in physics, from to thermodynamics, guage of technical reports, dense with math- his eyes.” from geophysics to nuclear chemistry. Of- ematical equations, Fermi’s spirit reaches In 1943, Fermi provided both scientific ten they were closely interrelated, and to across oceans of time and space to inspire and engineering guidance for the develop- solve one it was necessary to understand all us still to zero in on the essentials.

30 NUCLEAR NEWS February 2002