Physics: Nuclear Physics
Physics: Nuclear Physics
Nuclear physics became a scientific discipline and the atomic nucleus a subject of inquiry in the period between the discovery of radioactivity in 1896 and the identification of the neutron in 1932. Since then nuclear physics has spawned many fundamental topics of research, as well as a number of applications with formidable social and political implications. These applications have commanded so much attention that they tend to overshadow the scientific areas of the discipline.
Historical Background and Scientific Foundations
In 1886, while investigating the phosphorescent properties of uranium salts, Henri Becquerel (1852–1908) accidentally discovered that they emitted penetrating radiation that could expose (leave images on) photographic plates. Other researchers quickly pursued this finding. Marie and Pierre Curie (1867–1934 and 1859–1906), working at the Sorbonne in Paris, who coined the term “radioactivity,” also discovered new radioactive elements, including radium and polonium.
Ernest Rutherford (1871–1937) found that uranium emitted two types of radiation: one that he called alpha radiation was rapidly absorbed; a second, much more penetrating type, he called beta. Eventually, by observing the mysterious alpha particles' behavior in electric and magnetic fields, Rutherford concluded that they were positively charged helium ions. In similar experiments, Walter Kaufmann (1871–1947) determined that beta radiation was composed of high-energy electrons. A third class of radioactivity, gamma radiation, was discovered by Paul Villard (1860–1934) and eventually shown to be high-energy electromagnetic waves.
Rutherford's most famous and important series of experiments, conducted in 1911 at Manchester University with Hans Geiger (1882–1945) and Ernest Marsden (1889–1970), led to his discovery of the positively charged atomic nucleus. J.J. Thomson (1856–1940) had identified the negatively charged electron in 1897, fueling speculation about the atom's internal structure, especially regarding the distribution of the positive charge. Thomson had proposed a model in which the atom's positive charge was distributed evenly in a sphere, with a diameter on the order of 10-10 m. The negative electrons, he thought, rested within the positively charged blob, somewhat like raisins in a pudding (hence the name “plum pudding model”). Others, such as the Japanese physicist Hantaro Nagaoka (1865–1950) speculated that the atom resembled a tiny planetary system, with the electrons rotating around a small positively charged center in much the same way as the planets rotate around the sun.
To test these hypotheses, Rutherford and his team shone a beam of alpha particles (emanating from a radium source) onto a thin gold foil target, then counted them in painstaking detail by visually observing their faint scintillations on zinc sulfide screens. The team found that most alpha particles went straight through the foil with little deflection. Less than 1% deflected to surprisingly large angles (in excess of 90 degrees). This led Rutherford to conclude that Nagaoka's guess was correct. Because only a small number of incident alpha particles were scattered at large angles, the positive charge had to be a concentrated core at the center of the atom (on the order of 10-14 m). At first, Rutherford referred to this core as the “charged center,” but later he used the term “nucleus.”
In 1919 Rutherford became the head of the Cavendish Laboratory at Cambridge University, a post he held until his death in 1937. One of his most important experiments in these years involved bombarding nitrogen with a beam of alpha particles from a radium source. This transmuted the nitrogen nuclei and produced a stream of unknown particles. To identify them, Rutherford passed them through various absorbing screens to measure either their penetration range or their deflection as they passed through or deflected from magnetic fields of known strength. He identified the unknown particles as hydrogen nuclei—to which he gave the special name “proton”—and determined that the nuclear reaction had transmuted nitrogen into oxygen. A year later, Rutherford postulated the existence of a particle with the same mass as the proton but with a neutral charge. He also suggested that a nucleus might be formed with one proton and one of these “neutral protons,” forming the nucleus of an atom that would behave chemically like hydrogen (since it had a charge of one unit) but have double the mass.
In 1932 Rutherford's speculations bore fruit when the neutral proton was discovered at Cambridge by James Chadwick (1891–1974), who found that beryllium could be made to emit an unknown and highly penetrating radiation. Trying to identify this radiation, Chadwick pursued a variation of Rutherford's disintegration experiments, bombarding beryllium with alpha particles from a polonium source. This increased the intensity of the unknown radiation. He placed an ionization chamber in front of the beryllium sample to detect the penetrating radiation, and found that when thick pieces of lead were put between the beryllium and the ionization chamber, the number of counts per minute did not change significantly. When paraffin wax was placed in the gap, however, there was a huge increase in the count rate.
By measuring the range of this ionizing radiation, Chadwick concluded that it was composed of protons. Although not detected directly, the unknown radiation still knocked hydrogen nuclei (protons) first out of the air and then out of the paraffin. Chadwick deduced that it had the same mass as protons—and therefore that he had found Rutherford's neutral proton, or “neutron.” At almost the same time, Columbia University chemist Harold Urey (1893–1981) and his collaborators announced their isolation of Rutherford's “heavy hydrogen nucleus” with a charge of one unit and a mass of two units. They gave the name “deuterium” to this hydrogen isotope.
Rutherford's proton and Chadwick's neutron made the new results of nuclear physics easier to interpret. We can use one of Rutherford's 1919 transmutation experiments as an example. When an alpha particle (2 protons, 2 neutrons) strikes a nitrogen nucleus (7 protons, 7 neutrons), it produces an oxygen nucleus (8 protons, 9 neutrons) and a hydrogen nucleus (1 proton). The notation adopted to represent this is shown in Equation 1, where the lower number is the atomic number (the number of protons) and the upper number is the mass number (the number of protons plus neutrons, collectively referred to as “nucleons”).
Disintegration experiments such as Rutherford's and Chadwick's created the need for an artificial laboratory radiation source, one that could produce greater numbers of particles at varying energies. A number of researchers built early accelerators, including Robert Van de Graaff (1901–1967) at Princeton University, but the first team to disintegrate nuclei with an artificially produced beam was that of John D. Cockcroft (1897–1967) and Ernest T.S. Walton (1903–1995) working at the Cavendish Laboratory under Rutherford. To accelerate protons, Cockcroft and Walton developed a voltage-multiplying circuit and fed its output into a linear discharge tube. In 1932 they accelerated a beam of protons and disintegrated lithium nuclei into two alpha particles. (See Equation 2.) With the basic model of the nucleus established and the development of laboratory equipment such as ionization chambers and accelerators, nuclear physics had become an established field of study.
The Forces of Nature
To the two fundamental forces of classical physics (gravitation and electromagnetism), nuclear physics added the strong force, which holds together the constituents of the atomic nucleus, and the weak interaction, which is responsible for certain processes such as beta decay. By 1920 Rutherford was convinced that there must be some sort of a strong nuclear force that overwhelmed the repulsive Coulomb (electric) force between the protons of
the nucleus. However, because beta decay was one of the earliest topics studied by the nuclear physics community, the weak interaction attracted greater initial attention.
Beta decay presented a difficult problem. At first scientists did not realize that the electrons were being released from the nucleus and not from the atomic orbitals. Even when this became clear, however, the electrons' energies were hard to understand. According to energy-mass conservation, the initial mass of the decaying nucleus should equal the sum of the mass of the final nucleus, the mass of any particles released, and the kinetic energies. The alpha radiation energy spectra showed well defined peaks at the expected energies. Sharp peaks were also seen in spectra of gamma radiation. In 1914 Chadwick had found that beta decay appeared to violate mass-energy conservation. These spectra did not have well defined energy peaks but instead showed long peaks that extended from zero up to the energy expected from mass-energy conservation.
While Niels Bohr (1885–1962) accepted this possibility, Wolfgang Pauli (1900–1958) sought in 1930 to avoid this conclusion by postulating that a hitherto unobserved particle was also emitted during beta decay, which accounted for missing energy. To account for the fact that the particle had never been detected, Pauli supposed that it had no charge and no mass. For two years, this hypothetical particle was known as “Pauli's neutron,” until Chad-wick identified the massive neutron and Enrico Fermi (1901–1954) renamed Pauli's neutron the “neutrino.”
Although the neutrino was not conclusively observed until 1955 by Frederick Reines (1918–1998) and Clyde L. Cowan Jr. (1919–1974) at the Savannah River Atomic Energy Plant in South Carolina, many nuclear scientists accepted its existence since it could maintain mass-energy conservation in beta decay. With Chad-wick's neutron and Pauli's neutrino, beta decay could be seen as the spontaneous decay of a neutron within a nucleus into a proton, a beta particle (electron), and a neutrino. (See Equation 3.) In present language, the neutrino in this reaction would be an “anti-neutrino.”)
The weak interaction was introduced by Enrico Fermi to explain the interaction of particles during the beta decay process. During its development, nuclear physics used the new quantum mechanics, in which particles were shown to have wave properties and were represented mathematically with wave functions. In a 1933 paper Fermi theorized that the probability of beta decay in a given nucleus was proportional to the product of
four wave functions, one for each particle involved in the process (neutron, proton, electron, and neutrino). By fitting beta-decay data he was able to determine the proportionality value's constant, finding very low values (on the order of 10-14 in dimensionless units); hence the name “weak interaction.”
Fermi's success with beta decay (and some early work by Heisenberg on the strong force), led Japanese physicist Hideki Yukawa (1907–1981) to suggest an influential model for the strong force in 1935. In 1927 Walter Heitler (1904–1981) and Fritz London (1900–1954) had modeled the sudden increase in the force that occurs when two atoms come into contact and exchange electrons between orbitals. From this, Yukawa postulated that the force between nucleons was the exchange of a heretofore unidentified particle. He estimated that to have the correct magnitude and range, this unknown particle would have a mass greater than the electron but less than the proton (it is now called a meson, from the Greek mesos, “middle”) and have an associated coupling constant that was much larger than the proportionality constant in Fermi's model of the weak interaction.
In 1937 C. Anderson (1905–1991) and Seth Neddermeyer (1907–1988) at Caltech found a new particle in earthbound cosmic rays that had the same mass as Yukawa's unknown particle. Further experiments, however, showed that the Anderson-Neddermeyer particle interacted much more weakly with nucleons than Yukawa's theory required. Ten years later, the English cosmic ray researcher Cecil Frank Powell (1903–1969) and coworkers at the University of Bristol used high-altitude balloons to observe cosmic rays in the upper atmosphere; they found that the Anderson-Neddermeyer particle was a decay product of a somewhat heavier meson that interacted strongly with nucleons. This p-meson, as it was called, was recognized as the one that Yukawa had posited, while the Anderson-Neddermeyer particle was named the m-meson.
Fission and Fusion
The idea of mass-energy equivalence, encapsulated in Einstein's famous equation E = mc2 suggested that energy would be released in nuclear reactions when the mass of the products was less than the mass of the reactants. When John Cockcroft (1897–1967) and Ernest Walton (1903–1995) checked this carefully, they found that the mass of the incident proton and the target lithium plus the kinetic energy of the incident proton exactly balanced the (lesser) mass of the product alpha particles plus their (greater) kinetic energy. Such results gave added credence to speculation that nuclear reactions might be harnessed for power generation or a new type of bomb. In 1939 the Hungarian-born American physicist Leo Szilárd (1898–1964), working with Fermi, proved that a nuclear chain reaction was possible.
The discovery of nuclear fission began in the work of chemist Otto Hahn (1879–1968) and physicist Lise Meitner (1878–1968) at the University of Berlin. In their experiments, they bombarded uranium with neutrons to produce new isotopes. These new isotopes, close to uranium on the periodic table, were also unstable and decayed along chains involving beta and alpha decay. Usually, Hahn and Meitner identified the new isotopes by measuring their radioactive properties and isolated them chemically. In 1938 Meitner, an Austrian Jew, left Nazi Germany for Sweden, but Hahn continued his research with physicist Fritz Strassmann (1902–1980). One of the unknown nuclei produced in their neutron-irradiated samples of uranium appeared to be an isotope of radium. This was confusing since it did not seem to fit into any of the decay chains they'd seen. Hahn and Strassmann came to the unexpected conclusion that the unknown substance was not radium but barium. This was surprising, since barium was nowhere near uranium on the periodic table. Hahn and Strassmann speculated
that they had stumbled upon a reaction in which uranium broke up into two pieces.
Hahn immediately wrote to Meitner in Sweden about this finding. Fortuitously, her nephew Otto Frisch (1904–1979), who worked at Niels Bohr's institute in Copenhagen, was visiting. Frisch and Meitner realized that if uranium absorbed a neutron as it was being bombarded, a highly unstable nucleus would result, due partly to the Coulomb repulsion between its protons. Using Bohr's earlier model, which viewed the nucleus as a large drop of liquid, absorbing a neutron would cause the nucleus to become unstable. This vibrating liquid drop would eventually break up into two smaller drops of approximately equal mass (e.g., barium and krypton). The drops (nuclear fragments) would then be repelled by their respective positive charges. This would produce significantly more energy than the two products simply added together. Meitner and Frisch called this process fission.
Upon his return to Copenhagen, Frisch conducted an experiment in which he bombarded uranium with neutrons and found huge bursts of energy. The reaction created a sensation in the physics community, especially when it was confirmed that, in addition to barium, a krypton nucleus and three neutrons were formed. (See Equation 4.)
It was clear that the three neutrons might be used to trigger further fission reactions. Suddenly, it appeared that Szilárd's idea of a chain reaction might be feasible.
The fact that fission had been discovered within Nazi Germany sent chills throughout the physics community. In August 1939 Szilárd and a number of other concerned physicists convinced Einstein (who by now had left Germany and was living in the United States) to send a letter to President Franklin Roosevelt, warning of the danger of allowing the Germans to build such a bomb. During the first three years of the war, the American research effort on a fission bomb was small; it picked up in late 1942, when it became clear that the weapon was a practical eventuality.
The Manhattan Project was organized under the Army Corps of Engineers at a number of installations, each with its own specialized scientific or technical problem. At the University of Chicago, Fermi and coworkers built the first nuclear reactor and demonstrated a chain reaction on December 2, 1942. In Hanford,
Washington, radiochemist Glenn Seaborg (1912–1999) and coworkers labored to find a chemical means of separating plutonium (a second material capable of undergoing nuclear fission), from samples of uranium. A large facility in Oak Ridge, Tennessee, used a process called gaseous diffusion to separate the uranium isotope useful for building a fission bomb, 235U, from natural uranium samples, which contained large amounts of 238U.
Finally, the facility at Los Alamos, New Mexico, was given the job of computing how large a mass of fuel was needed to start a chain reaction (a quantity called critical mass) and building the actual bombs. The Los Alamos effort, headed by J. Robert Oppenheimer (1904–1967), found two ways to assemble the nuclear fuel: the relatively simple gun method, in which two chunks of uranium were held at either end of a sort of gun barrel and then pushed together at the last moment; and the more difficult implosion method, in which plutonium was stored in a thin spherical shell and then pushed to its center by conventional explosives.
By the time the weapons were ready, the war in Europe had ended, but Japan fought on with relentless ferocity. The battle of Okinawa in March 1945 had killed over 12,000 American soldiers, 66,000 Japanese troops, and 150,000 Okinawan civilians—a chilling indication of the casualties that would result from a planned invasion of the Japanese mainland. To force an end to the war, despite an eleventh-hour effort by many scientists to convince the Truman administration not to use the bombs without warning, on August 6, 1945, the United States dropped a gun-type uranium bomb nicknamed “Little Boy” on Hiroshima, Japan. The bomb delivered the equivalent of 12.5 kilotons of TNT, laying waste to the city and killing about 70,000 people immediately. Three days later, the United States dropped “Fat Man,” an implosion-type plutonium bomb on Nagasaki, with similar results. In response, a portion of the American physics community, including members of the Manhattan Project, founded the Federation of Atomic Scientists (now renamed the Federation of American Scientists) to press for arms control.
Immediately after the war, Congress established the Atomic Energy Commission (AEC, now the Department of Energy). The AEC developed and maintained America's nuclear weapons and also explored peacetime uses for nuclear physics, including power generation and medical applications. The AEC gave grants to universities which, in turn, operated the national laboratories. Lewis Strauss (1896–1974), the AEC's second chair, encouraged the development of fission energy, reinforced by the Eisenhower administration's 1954 Atoms for Peace program. At the first International Conference on the Peaceful Uses of Atomic Energy in Geneva, in 1955, the Oak Ridge National Laboratory transported and set up a working light-water reactor (using regular water and not “heavy water” made with deuterium), which lent the United States added scientific prestige on the international stage. A reorganization of the AEC in 1954 made it legal for the commission to enter into contracts with private companies; the first American nuclear power plant, built by the AEC but run by the Duquesne Light Company, began operation in 1957 in Shippingport, Pennsylvania.
In contrast to fission, fusion reactions had been known to the physics community before 1939. In 1920 the chemist Francis Aston (1877–1945) made precision measurements of the masses of light atoms; this showed that the total mass of four hydrogen atoms was higher than a single alpha particle. The British astrophysicist Sir Arthur Eddington (1882–1944) immediately realized that Aston's measurements, along with Einstein's mass-energy formula, implied that the fusion of hydrogen into helium might account for the great energy produced by the sun and stars. Because two protons had to overcome the Coulomb force repulsion before they could feel the short-ranged nuclear attraction, fusion reactions could only occur at the high temperatures and densities found within stars.
During the next two decades, George Gamow (1904–1968), Robert Atkinson (1898–1982), Fritz Houtermans (1903–1966), and others explored the reaction rates and energy release of fusion reactions in the stars. In 1939 physicist Hans Bethe (1906–2005) published a seminal paper in which he specified two processes by which fusion reactions created energy. The first process, the proton-proton chain, was found in smaller stars like the sun, while the second process, the carbon-nitrogen-oxygen cycle, was important in more massive stars.
After World War II it was clear that a bomb based on fusion reactions might be possible. Certain scientists, such as Oppenheimer, opposed the development of the fusion bomb. However, the burgeoning Cold War, punctuated by the first successful Soviet fission bomb test in 1949, gave the day to scientists such as Edward Teller (1908–2003), and AEC chair Strauss. A new national laboratory, Lawrence Livermore, in California, was devoted to its development. The hydrogen bomb,
or H bomb as the new weapon became known, used a small fission bomb as a sort of fuse to ignite the fusion fuel. One such reaction was the fusion of deuterium and tritium. (See Equation 5.) The first successful fusion device, code named “Mike,” was too large to fit into any military delivery vehicle and was detonated on November 1, 1952, in the Enewetak atoll in the central Pacific, unleashing a 10-megaton equivalent of TNT.
One great dream of the postwar years was to harness fusion reactions for power generation—if only the Coulomb repulsion between nuclei could be overcome by means other than a fission bomb. Calculations showed that creating a beam of particles to collide with a target would use more energy to produce than would be gained from fusion reactions. Attention therefore turned to heating gases to high temperatures, in the hopes that thermal energy would enable fusion reactions while the ionized fuel or “plasma” was confined for a short time in carefully designed magnetic field configurations.
Secret research programs on controlled thermonuclear fusion began at British national laboratories during the late 1940s and in the United States and the U.S.S.R. in the early 1950s. In mid-1958 the three countries agreed to declassify their fusion programs in time for the Second International Conference on the Peaceful Uses of Atomic Energy. Strauss pushed the fusion effort as a sort of crash program, modeled after the Manhattan Project, and hoped for a working fusion reactor at the conference. The Project Sherwood scientists—a part of the Atoms for Peace project that focused on fusion reactors—worked until the last possible moment but ultimately disappointed Strauss. In the coming years, practical fusion energy continued to be elusive. The fusion research community obviously has some intersection with nuclear physics but, because most of the fusion devices involve the confinement of hot ionized gases, much of its expertise lies in plasma physics.
Models of the Nucleus and Nuclear Reactions
In the early years of nuclear physics, experimentalists took the lead, but in the 1930s, theorists became increasingly important. As experimentalists continued to amass results, it became clear that the nucleus could behave in astonishingly complex ways. Theorists sought to impose order on the data by proposing models of the nucleus and of the reactions that the nuclei entered into.
The study of the internal structure of the nucleus began with the collation of experimental results. In 1948 the physical chemist turned nuclear physicist Maria Goeppert Mayer (1906–1972), working at the University of Chicago, noticed that certain nuclei have many more stable isotopes than might be expected. She also noticed the dependence of certain nuclear properties on the number of neutrons. For example, the binding energy of the last neutron placed in a nucleus was especially high in certain cases, and this corresponded to those nuclei with many stable isotopes. Mayer used her knowledge of atomic physics to make an analogy between it and a new model for the nucleus. She suggested that nucleons are in a nuclear potential in much the same way that electrons are in a Coulomb potential, and that nucleons fill energy shells of the nucleus in a way similar to how electrons fill shells of the atom. Mayer called the special numbers of neutrons that created particularly stable nuclei “magic numbers” (2, 8, 20, 28 …) and likened them to the closed electron shells of atomic physics. She constructed a nuclear shell model by assuming a simple form for the shared nuclear potential and assigning each of the nucleons in the nucleus an orbital and spin angular momenta. While the results were not as pretty as the ability of atomic physics to reproduce the layout of the periodic table, the nuclear shell model did make sense of a great deal of nuclear data.
IN CONTEXT: GETTING MORE FROM LESS
If all protons and neutrons have the same mass, and if the numbers of protons and neutrons are conserved in many nuclear reactions, how can the mass of the products be less than the reactants?
When nucleons (protons or neutrons) are in bound systems, they lose some of their mass to the binding energies (due to the attractive nuclear force) that holds the nucleus together. There are two ways that the total mass diminishes and releases energy fission reactions: by taking apart nuclei, such as uranium, to give products such as barium and krypton; or with fusion reactions, by assembling nuclei such as deuterium and tritium into nuclei such as helium.
With the publication of Mayer's work, a great deal of research turned to the new field of nuclear structure. Following the analogy with atomic physics, nuclei needed to be characterized in both their ground state and many excited states. Many excited states of nuclei could be described with the single-particle approximation of the shell model. Others, however, reasserted the need to see the nucleons of the nucleus as part of a collective motion; excited nuclei could rotate and vibrate in complex ways, not unlike the way that nuclei deformed in the fission process. In 1953 Danish physicists Aage Bohr (1922–; the son of Niels Bohr) and Ben Mottelson (1926–) found a way to make the single-particle and collective models of the nucleus consistent with one another and to apply them to the available data. Their efforts resulted in a two-volume work, Nuclear Structure, which defined the field for many years.
The study of nuclear reactions is important to the determination of the ground state and excited states of nuclei as well as the modeling of nuclear properties such as a cross section (a measure of the degree to which a reaction occurs). Nuclear reactions often are studied in scattering experiments, in which a beam of projectiles hits a target and initiates a nuclear reaction, after which the reaction products move off at different angles relative to the original beam. There are two general categories of nuclear reactions: compound-nucleus reactions and direct reactions.
The first contribution to a theory of the compound nucleus was made by Niels Bohr in 1936. Bohr imagined that the projectile is entirely captured by the target nucleus, forming a new, compound nucleus. Because the energy of the projectile is soon shared with all of the nucleons, the compound nucleus is an excited system. Therefore, it is short-lived (on the order of 10–16 seconds), and breaks up into reaction products. States of the compound nucleus could be identified when the incoming particles were at an energy such that the “waves” of those particles resonated with the state of interest.
Experimental results showed such resonances clearly. When researchers graphed the cross section against the energy of the projectiles, large peaks were found at well-defined energies. Bohr's model for compound nucleus reactions was strengthened by the work of Eugene Wigner (1902–1995) and Gregory Breit (1899–1981) at Princeton University, who produced a general mathematical formalism for resonant systems that could be used to analyze compound nucleus reactions. The combined effect of Bohr's intuitive model and the Breit-Wigner formalism was galvanizing. The nuclear physics community enthusiastically conducted new experiments identifying ground and excited states of many compound nuclei.
In contrast to compound nucleus reactions, in a direct reaction the projectile does not enter into the target nucleus but instead only grazes the surface. The projectile might scatter elastically or interact with nucleons at the target's surface. In either case, the time for a direct reaction is much smaller than that for a compound reaction, on the order of about 10-22 seconds. Another contrast is found in the pattern made by the scattered particles. Compound nucleus reactions scatter fairly evenly to all angles since the compound nucleus has no “memory” of the incoming projectile after it is formed and so decays in any direction. However, in a direct reaction, the target nucleus “remembers” the initial beam direction; its scattering patterns show high cross sections for small angles near the original beam direction.
The theoretical analysis of direct reactions was pioneered by Stuart T. Butler (1926–1982) at the University of Birmingham, Australia. In 1951 Butler developed mathematical formulae to analyze the stripping reaction, in which a projectile has a nucleon stripped off it as it passes the target. Consider, for example, a deuteron (deuterium nucleus) projectile glancing off of a target such that it loses its neutron, leaving only the proton to scatter away. Butler's analysis was able to deduce the final states (ground state and excited states) in which the final nucleus was left. For stripping reactions in which a single nucleon was stripped off, the identified states were often similar to those of the ideal shell model.
By the 1950s, high-energy or particle physics had established itself as a new area of study and independent research. The field grew from elements of cosmic-ray research and experiments using newer, larger accelerators. By 1932 Carl Anderson (1905–1991) of Caltech had identified the electron's antiparticle, dubbed the positron, in the course of cosmic-ray studies. This discovery was a ringing confirmation of British physicist Paul Dirac's (1902–1984) combination of quantum mechanics and Einstein's special theory of relativity, which had predicted the existence of antiparticles in 1928. Identification of many other particles and their antiparticles followed. The m-meson was discovered by Anderson and Seth Neddermeyer (1907–1988) in 1937 and the p-meson by Powell and coworkers ten years later. In 1947 George Rochester (1908–2001) and C.C. Butler at the University of Manchester identified the K-meson or kaon, a new type of particle with a quizzical new quantum number called “strangeness.”
IN CONTEXT: WOMEN IN NUCLEAR PHYSICS
Until the closing decades of the twentieth century, women were poorly represented in physics—and nuclear physics was no exception. While women faced an uphill battle in this field, however, they made significant contributions nonetheless.
Lise Meitner (1878–1968) was raised in an Austrian Jewish family. She became the first woman to earn a doctorate in physics at the University of Vienna—Austrian restrictions on women's education had precluded even her entry into the university before 1901—working with the theoretician Ludwig Boltzmann (1844–1906). Upon her graduation, she won a position at the University of Berlin, working with Max Planck (1858–1947), despite his disapproval of women in academia. When, in collaboration with Otto Hahn (1879–1968) and Fritz Strassmann (1902–1980), their discovery of fission earned the two men the 1945 Nobel Prize in chemistry, Meitner was ignored.
Historian Ruth Sime highlights a number of reasons why Meitner was not given her due credit, including the prize committee's favoring of chemistry over physics, anti-Semitism in Germany, Meitner's exile to Sweden, and bias against women scientists. Meitner's 1938 flight from Nazi Germany not only separated her from Hahn and Strassmann but also left her with difficult working conditions in Sweden, where she was given little more than laboratory space. Hahn's first paper on fission in 1939 did not even mention Meitner, perhaps out of fear that his German colleagues would be more prone to reject his results if they involved a collaborator who was a Jew and a woman. After the war, Hahn may have tried to rationalize Meitner's exclusion, perhaps worried about his own reputation and the rebirth of German science. Meitner's contribution was not recognized until near the end of her life when the Atomic Energy Commission gave her, Hahn, and Strassman the 1966 Enrico Fermi Award for their discovery of fission.
Maria Goeppert Mayer's (1906–1972) story demonstrates how difficult it was for many women to secure a position, despite producing significant work, and even being married to a fellow scientist. Goeppert Mayer was born in 1906 into a German academic family and received an excellent physics education, interacting with such luminaries as Niels Bohr (1885–1962) and Enrico Fermi (1901–1954). After marrying the physical chemist Joseph E. Mayer in 1930, she embarked on a long search for employment. She and her husband first moved to Baltimore, where Joseph took a position at Johns Hopkins.
In Baltimore and later in New York after her husband changed jobs, Goeppert Mayer kept working and publishing—specializing in chemical physics—despite the fact that she never received a professional position or salary. Her persistent work eventually paid off to some degree. In 1946, after she and her husband moved to Chicago, Goeppert Mayer was given two positions, one at the University of Chicago and another at the nearby Argonne National Laboratory. Although these positions gave her office space, they still did not include a salary. After doing her fundamental work on the nuclear shell model and being elected to the National Academy of Sciences, she and her husband moved to the University of California at San Diego in 1956, where she was finally given a professorship. In 1963 she shared the Nobel Prize in Physics with J. Hans D. Jensen for their work on the nuclear shell model. She was only the second woman to have won this prize (after Marie Curie).
The development of larger accelerators was pursued by many researchers, but the most famous is probably Ernest Lawrence (1901–1958). In 1928 Lawrence invented the cyclotron, a device that was able to accelerate charged particles by making them circle within a magnetic field by having an oscillating electric field that delivered small “kicks” during each revolution. Lawrence continued to build larger and larger accelerators and in 1936 was given a separate laboratory at the University of California at Berkeley called the Radiation Laboratory. In 1955 Owen Chamberlain (1920–2006) and coworkers identified the antiproton at the Radiation Laboratory using the Bevatron accelerator.
After Lawrence's death in 1958, larger and larger machines were built. During the 1960s the laboratories of the European Organization of Nuclear Research (CERN) became among the most important in the world for both nuclear and high-energy physics. In 1967 the National Accelerator Lab was founded in Chicago; it was renamed Fermilab seven years later.
By 1960 high-energy physicists had identified a bewildering array of particles and antiparticles. In 1961, American Murray Gell-Mann (1929–) at Caltech and the Israeli physicist Yuval Ne'eman (1925–2006) independently came up with a new scheme to classify existing particles and, in the case of the omega minus particle, to predict the existence of a missing particle that was later identified experimentally. Gell-Mann referred to this classification scheme as “the eightfold way” since the particles were organized into groups of eight. Three years later, Gell-Mann and, independently, the Russian-born American physicist George Zweig (1937–), suggested the eightfold way could be explained by the fact that hadrons—particles such as protons and neutrons, which experience the strong force—were composed of subparticles, which he named “quarks.”
Perhaps the greatest accomplishment of high-energy physics has been the so-called Standard Model, developed during the 1970s, which joins three of the four forces of nature into one consistent theory. The gluon was proposed as the particle that mediated the strong force between quarks and offered a more fundamental understanding of the strong force than the Yukawa theory of pion exchange. The electromagnetic interactions (between charged particles such as electrons) are mediated by bits of electromagnetic energy called photons.
The Standard Model predicted that the weak interaction was mediated by relatively massive particles called the W and Z bosons, first identified in 1983 by two research groups at the CERN. Efforts to consolidate the Standard Model have focused on the Higgs particle, which is related to the mediating particles for all three of the forces addressed by the Standard Model. Identifying the Higgs particle experimentally will require collisions at energies that are higher than those produced by any presently existing accelerator. In 1993 Congress canceled construction of the so-called superconducting supercollider (SSC). The high-energy community has therefore turned its attention to a machine at CERN, the large hadron collider (LHC), which produces proton-proton collisions and was scheduled to begin operation in May 2008.
Modern Cultural Connections
Nuclear physics continues to be an area of vital research. As in so many other scientific and technical fields, the computing revolution opened new doors. In studies of the strong force, for example, the “few nucleon” community takes precision measurements of three-nucleon systems, such as a nucleon scattering from a deuteron, and compares them to the results of computer simulations. The computer simulations begin with detailed representations of the basic nucleon-nucleon (strong) force and calculate each nucleon's interactions separately. Assuming that the three-nucleon calculation is trustworthy, these precision measurements will allow the basic nucleon-nucleon force to be even better understood.
Studies of the weak interaction continue also. In 1965 American physicist Ray Davis Jr. (1914–2006) and coworkers at Brookhaven National Laboratory set up a giant neutrino detector in an abandoned gold mine at Homestake, South Dakota. According to knowledge of the fusion reactions in the sun, a certain flux of neutrinos was expected at Earth's surface. However, Davis and coworkers found only about half of this flux. This “missing solar neutrino problem” was solved by the discovery that there are actually three different types of neutrinos; all are unstable and oscillate from one type to the other as they travel through space (as the result of having a tiny but non-zero mass). To test these ideas and to study neutrino oscillations, new underground experiments are being proposed for a still-to-be-built National Underground Science Laboratory, one location for which could be Homestake.
As particle physics has continued in its inexorable march to higher energies and bigger machines, nuclear physics has moved into abandoned energy fields with a somewhat different focus. One of the newer nuclear physics facilities, the relativistic heavy ion collider (RHIC) was completed in 1999 at the Brookhaven National Laboratory. The RHIC accelerates protons, deuterons, copper nuclei, gold nuclei, and lead nuclei to over 99.99% the speed of light, creating dense high-temperature nuclear matter. Under such conditions, nucleons lose their individual identity, creating, for a brief moment, a sort of “nuclear plasma” made from quarks and gluons. Such nuclear matter is thought to have existed moments after the beginning of the universe—the so-called big bang.
The military applications of nuclear physics continue to be important. Stockpile stewardship is the effort to insure that the U.S. arsenal of nuclear weapons is reliable, if and when it is needed. In 1963 the United States and the U.S.S.R. signed the Limited Test Ban Treaty, banning nuclear tests above ground, underwater, and in space. This left only underground tests as a means of checking nuclear devices but, in 1992, President George H.W. Bush put a moratorium on these. Since that time, the stockpile stewardship program has used computer simulations, along with the laboratory measurement of reaction cross sections and other nuclear properties. Untested new hybrid weapons, however, designed to be more stable and less vulnerable to both terrorists and accidental deployment, may force the United States to resume underground tests.
After September 11, 2001, concern over the possibility of terrorist attacks gave nuclear military applications renewed attention. One area of research centers on the detection of radioactive materials that might be concealed in baggage. New detection systems are being developed to identify the characteristic radiation of different materials in the relatively short time available at airport check-ins and highway ports of entry. New weapons are also being sought. In recent years, the Pentagon has encouraged the development of relatively small nuclear weapons that could be used in a battle-field situation. One class of tactical nuclear weapon, the so-called “bunker buster,” is designed to penetrate the earth and destroy bunkers of chemical and biological weapons. Some members of the physics community have voiced concerns about the fallout associated with such weapons; supporters claim that newer weapons use far less radioactive material, and that the weapon's subterranean target would limit fallout.
Neither fusion nor fission energy has lived up to the enthusiasm first generated in the 1950s. None of the many fusion experiments that have been attempted have even reached breakeven, the point at which fusion reactions produce energy as great as that which heated the fuel in the first place. The largest experiment currently being pursued, the international tokomak experimental reactor, that will be built in Cadarache, France, hopes to reach breakeven but will not begin operation until about 2016.
Fission power has met with considerable public opposition. The accident at Three Mile Island, Pennsylvania in 1979, and the more serious disaster in Chernobyl, Ukraine, seven years later, combined with concerns about storing spent nuclear waste, turned the American public against the idea of new nuclear power plants. Renewed concerns about energy shortages however, have brought renewed attention to nuclear power. The problem of storing nuclear waste might possibly be solved by the proposed facility at Yucca Mountain, Nevada, a ridge of volcanic rock chosen by the Department of Energy as a repository, though this site has been controversial. Meanwhile, problems with older plants might have been answered by new nuclear technology, such as
gas-cooled reactors (including the so-called pebble bed reactor). Of course, questions and concerns remain with all of these developments.
See Also Physics: Radioactivity.
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G. J. Weisel