Hofstadter Discovers That Protons and Neutrons Have Structure

Date 1951

Robert Hofstadter began to use the high-energy electrons from the Stanford Linear Accelerator to probe the structure of the nucleus and its constituents, the proton and neutron. He discovered that these components of atoms were not “basic building blocks” of matter, but instead were themselves composed of more basic entities.

Locale Palo Alto, California

Key Figures

• Robert Hofstadter (1915-1990), American physicist
• Edward Leonard Ginzton (1915-1998), Soviet physicist
• William Webster Hansen (1909-1949), American physicist
• Robert Wallace McAllister (b. 1919), American physicist and student of Hofstadter
• Mason Russell Yearian (b. 1932), American physicist and student of Hofstadter

Summary of Event

At the end of World War II, physicists who had been working in industrial and national laboratories returned to the universities eager to undertake research on the basic structure of matter. The atomic nucleus offered unexplored territory amenable to many of the new experimental techniques developed to support war-related technologies, such as radar and the atomic bomb. In particular, William Webster Hansen, who had worked during the war on microwave technology for radar, returned to Stanford University and renewed his interest in accelerating electrons to high energies by using microwave frequencies. The electrons are pushed along a series of electrodes in a linear accelerator by an electric wave at microwave frequencies in phase with their motions like surfers pushed along by a water wave.

By the end of 1947, Hansen had constructed a working 37-meter accelerator, the Mark I, which delivered electrons with energies of 6 million electronvolts. Hansen’s death might have ended work on linear accelerators for electrons, but Edward Leonard Ginzton, also a professor at Stanford, took up accelerator construction and built two other electron linear accelerators, the Mark II and the Mark III. In 1952, the Mark III stretched 24 meters and delivered electron beams of 200 million electronvolts. Since linear accelerators could be built in sections, the Mark III grew slowly as funds were available and delivered electrons at 1 billion electronvolts by 1960. The intensity of the electron beams and the reliability of the accelerator also increased, making it a powerful tool waiting for exploitation.

At the beginning of the 1950’s, high-energy electrons lacked the glamor of the cyclotrons used to accelerate protons and alpha particles. Electrons could not be accelerated easily in circular machines since their small mass meant that they radiated away most of their energy. As protons from the cyclotrons and synchrotrons crashed into nuclei, they produced new nuclear species and the first of the new fundamental particles that hinted at a new layer of complexity in the structure of ordinary matter. Electrons had long been used to explore atomic structure, but they seemed crude and uninteresting tools compared to the protons.

When Robert Hofstadter moved to Stanford University from Princeton, he realized that the high-energy electron beam from the Mark III provided an ideal method for determining the structure of nuclei. The technique he decided to use is known as scattering. Because the nucleus of the atom and its constituents—the proton and neutron, known generically as nucleons—are too small to examine using visible light, they are studied using the scattering of other probe particles. These particles are projected at a target containing the nuclei of interest, where they interact with the nuclei and bounce off.

If the target nuclei are small hard spheres, the probe particles generally will travel straight through the target without realizing that it is there. If they strike one of the point targets, they will bounce back at large angles. If, on the other hand, nuclei are diffuse distributions of charge, one expects the probes to be slightly deflected as they pass through but are not bounced back at large angles. Ernest Rutherford had used this technique with alpha particles to develop the original nuclear model of the atom.

Like other constituents of matter, electrons are both particles and waves. The wavelength of a particle decreases as its momentum increases. Because electrons have only one two-thousandth the mass of a proton, the momenta of electrons is normally much less than that of a proton, and the electron’s wavelength is longer than that of the more massive particle. Thus, electrons at normal energy are simply too large to explore the interior of the nucleus, which is one hundred thousand times smaller than the atom. Electrons accelerated to the energies available from the Mark III (first 190, then 550 million electronvolts) had wavelengths smaller than the radius of typical nuclei and could be used to probe them. Experiments done in 1951 at the University of Illinois using lower-energy electrons (15 million electronvolts) showed that the technique was feasible, although the electrons used had too long a wavelength to probe nuclear details.

Electrons offer a second advantage over protons and nucleons as probes of the nucleus. Unlike the nucleons, electrons do not experience the strong nuclear force. Therefore, they interact with the nucleus only through the electric and magnetic forces that physicists understand much better than they understand the strong nuclear force. As Hofstadter and his colleagues examined the patterns formed by the high-energy electrons scattered from nuclei, they were able to determine the precise distribution of charge and magnetism within the nucleus without the confusion introduced by the relatively poorly understood nuclear forces.

Hofstadter’s initial experiments probed the structure of large nuclei. He and his colleagues constructed a detector using a huge magnet mounted on a naval gun base so the magnet could be aligned accurately at different angles. This magnet was necessary because the experimenters wished to examine the nucleus in its normal state, not after it had been excited by an interaction with the incident electrons. The very large and expensive magnet was needed to bend the extremely high-energy electrons and separate them into those that had simply bounced off the nucleus (scattered elastically) and those that had interacted with the nucleus. The electrons were detected behind the magnet by a Lucite counter, which flashed light when struck by an electron and whose light was collected by a photomultiplier tube.

Hofstadter and his group were able to determine that the nucleus is not a point particle. Instead, nuclei consist of a central portion of nearly constant density, which increased in size as the number of nucleons in the nucleus increased. The central portion was surrounded by a “skin” of gradually decreasing charge density. As they went to lighter and lighter nuclei, the central core disappeared, leaving only a fuzzy sphere. Finally, Hofstadter and his colleagues, including graduate student Robert Wallace McAllister, extended the measurement to a target of hydrogen, which contained only protons. They found that the proton had a definite charge distribution and could not be considered a point particle.

Next, the group, now including graduate student Mason Russell Yearian, turned their attention to studying the neutron. This measurement was more difficult because free neutrons do not exist in nature. Therefore, the target had to be made of deuterium gas, a form of hydrogen whose nucleus consists of a proton and a neutron. The properties of the neutron were deduced from the scattering data by subtracting out the effect of the proton.

Hofstadter and his associates continued to use the very short wavelength electrons from the linear accelerator to make precise measurements of the distribution of charge and magnetism within both the proton and neutron. By 1961, when Hofstadter was a cowinner of the Nobel Prize in Physics, they had shown that the nucleons were complex particles concealing a new layer in the structure of matter. Any possibility that the proton and neutron might be simple point particles had been effectively laid to rest. High-energy electrons were an established tool for probing the fundamental structure of matter.

Significance

Hofstadter’s experimental proof that the proton and neutron were in fact complex particles provided one of the major clues that led to the modern picture of the structure of matter, the quark model. His experiments were extended to higher energies using electrons with smaller wavelengths. By the end of the 1960’s, the Mark III had been replaced with the 16-kilometer long Stanford Linear Accelerator (SLAC), which produced electrons with energies of 20 billion electronvolts. At these energies, the scattering experiments no longer showed a continuous distribution of charge inside the proton. In 1970, scattering experiments at SLAC showed results that demonstrated electrons suddenly bouncing back from small point particles inside the proton. With the new, very small wavelengths of the extremely high-energy electrons, experimenters finally had seen the entities that combine to form protons and neutrons, the point particles called quarks.

High-energy electron scattering was not the only experimental technique to produce results leading to the discovery of the quark model. Beams of high-energy protons fired at hydrogen targets also revealed evidence of interactions between constituents of the protons in the beam and in the target. In addition, the number of known fundamental particles had continued to grow with each increase in the energy of the existing accelerators. The proliferation of particles gave rise to several theoretical approaches that sought to explain the multitude of new particles in terms of an underlying theoretical concept.

In 1963, Murray Gell-Mann and George Zweig proposed a mathematical model in which all fundamental particles were composed of three subconstituents called quarks. Neither theorist actually supposed the quarks had physical reality. They merely used them as a conceptual device to organize the rapidly expanding numbers of known fundamental particles. They were able to predict the properties and masses of undiscovered particles on the basis of their quark model. Finally, scattering experiments with both high-energy electrons and protons demonstrated clearly that the quarks were not merely a theoretician’s heuristic device but were, in fact, physical particles that interacted with one another and the other family of particles—the leptons—through the major forces.

Physicists have accepted that there are six different types of quarks which, together with their antiparticles, make up all fundamental particles that experience the strong nuclear force. The six quarks are matched by six leptons, which do not experience the strong nuclear force.

Just as Hofstadter probed the interior of the proton with high-energy electrons to understand the forces that hold nuclei together, particle physicists are probing the interior of the proton with much higher-energy beams in order to understand the force—called the color force—that binds quarks together to form nucleons. Hofstadter’s pioneering scattering experiments with high-energy electrons have spawned a generation of physics experiments that continue to deepen an understanding of the fundamental nature of matter.

Bibliography

Hofstadter, Robert. “The Atomic Nucleus.” Scientific American 194 (July, 1956): 55-68. Hofstadter’s own description of his work written for the well-informed nonphysicist, this article provides a succinct introduction to the major ideas and experimental details involved in Hofstadter’s work. This article is the best starting point for the reader intersted in examining Hofstadter’s experiments.

‗‗‗‗‗‗‗, ed. Electron Scattering and Nuclear and Nucleon Structure. New York: W. A. Benjamin, 1963. This volume contains technical reprints documenting the results of Hofstadter’s experiments on nuclear and nucleon structure as well as an introduction to the theory of determination of nuclear structure from scattering experiments. The best introduction to the subject available, this source requires a knowledge of physics and mathematics, although the introductory material is accessible to the beginner in nuclear physics.

Jacob, Maurice, and Peter Landshoff. “The Inner Structure of the Proton.” Scientific American 242 (March, 1980): 66-75. This article describes the eventual results of the experiments Hofstadter undertook in the 1950’s. Although Hofstadter personally was not involved in most of the work described here, his techniques have been extended to provide the modern picture of the structure of the nucleons as composed of substructures called quarks.

Livingston, M. Stanley. “Linear Accelerators.” In High-Energy Accelerators. New York: Interscience, 1954. This summary chapter presents the technology of the electron accelerator that was critical to the study of the structure of nuclei and the proton and neutron using high-energy electrons. The work is aimed at nonscientists and explains some of the complexities of building a large accelerator.

Mackintosh, Ray, et al. Nucleus: A Trip into the Heart of Matter. Baltimore: Johns Hopkins University Press, 2001. Short but comprehensive popular work explaining both nuclear physics and its history for the engaged layperson. Bibliographic references and index.

Riordan, Michael. The Hunting of the Quark. New York: Simon & Schuster, 1987. Written by a physicist with the unusual gift of explaining complex ideas clearly and entertainingly, this volume places Hofstadter’s work in context as part of a long series of experiments that have led to the modern picture of the nucleon. The importance of Hofstadter’s work is clearly presented at the beginning of chapter 5, “The Birth of a Monster.”

Seth, Kamal K. “Nuclear Sizes and Density Distributions.” Physics Today 11 (May, 1958): 24-28. As part of a report on a topical conference on nuclear structure, this brief paper presents a very readable account of the main issues in the study of the structure of the proton and neutron. Because it was written at the time of Hofstadter’s work, it places his work in the context of the current physics research of that time and mentions most of the other major experimental techniques being used in similar studies of nucleons.