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6 Scientific ant! Societal Benefits Nuclear physics presents a remarkable paradox: its awesome tech- nological progeny, nuclear power and nuclear weapons, are among the most well-known and hotly debated topics of our age, yet the physics of the nucleus itself is possibly the least understood of the basic sciences. This is all the more puzzling in light of the profound impact that nuclear physics has had on the development of the other sciences as well as on countless areas of modern technology. From solid-state physics to molecular genetics, from food technology to forensic medicine, from mineral prospecting to cancer therapy, the principles and techniques of nuclear physics are applied in ways far too numerous to survey comprehensively in a book of this size. In this chapter we touch on a few applications of nuclear physics that reflect its broad impact on science and technology. Although these applications cannot evoke the cosmic themes of nuclear astrophysics, discussed in the preceding chapter, the benefits they confer on a technological society are both more immediate and more tangible- even if we tend to take many of them for granted. It is noteworthy that most of these applications are derived from research carried out at low-energy facilities, which have provided much of the basis for our present understanding of nuclear physics. Implicit in our discussion of the impact of nuclear physics, of course, is the realization that it is a two-way street. Many advances in nuclear physics, for example, depend critically on state-of-the-art accelerator 120

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SCIENTIFIC AND SOCIETAL BENEFITS 121 technology, which hinges, in turn, on new developments in solid-state electronics, physical chemistry, materials science, cryogenic engineer- ing, and computer-aided design, to name a few. Theoretical nuclear physics, which contributes much to our understanding of the basic forces that govern all natural phenomena, likewise benefits greatly from the development of physical concepts and mathematical methods in other disciplines as well as of faster, more powerful computers. CONDENSED-MATTER PHYSICS The condensed phases of ordinary matter solids and liquids- exhibit an enormous diversity of form and function, owing in part to the great variety of the chemical elements and the types of chemical bonding that they undergo. Atomic and molecular interactions are purely electromagnetic, which simplifies the description of solids and liquids compared with that of nuclear matter. In analogy with nuclear matter, however, there can be a variety of cooperative motions of large numbers (here, essentially infinity) of interacting particles, whose net effect superconductivity, for example transcends the underlying properties of the particles. Much of the richness of solid-state phenom- ena, in particular, is due to such cooperative effects. In probing the structure and behavior of ordinary solid matter (typically, crystals), physicists have found that accelerated nuclear beams are extremely useful, since nuclei (ions) of almost every element can be implanted into a chosen crystal lattice to any desired depth. The value of this ion implantation technique for solid-state physics research lies in studies of the ensuing hyperfine interactions: subtle interplays between the electromagnetic properties of the implanted ions and the electron configuration of the crystal. Such studies can reveal details of the crystal's vibrational modes and of its microscopic magnetic and electrostatic properties. One can also study aspects of the crystal structure, such as the locations and mobilities of impurities, as well as the radiation damage caused by the implanted ions, the healing of this damage through heat treatment,.and the eject of the ions on the crystal's electrical conductivity. Information obtained by the ion-implantation technique and by other techniques derived from nuclear-physics research, such as perturbed angular correlations, is of great value in developing new materials- magnetic compounds and alloys, for example with properties that are tailor-made for specific purposes. Another phenomenon of solid-state physics that makes use of nuclear physics techniques is the channeling of charged particles in

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122 NUCLEAR PHYSICS 5W _ _ _ _- FIGURE 6.1 Artistes conception of the channeling of a positively charged particle in a diamondlike crystal lattice. The particle typically follows a spiral path that actually consists of a series of oblique ricochets from lattice nuclei along the channel, caused by the repulsive Coulomb force between the particle and the nuclei. The distance traveled by the particle in one turn of the spiral is of the order of 100 interatomic distances. (From W. Brandt, Scientific American, March 1968, p. 91; ~ 1968 by Scientific American, Inc.) crystals. Here the energetic projectiles bombarding the surface of the crystal are found to be channeled through the tunnels formed by adjacent rows of atoms in the lattice structure (see Figure 6.11. Studies of the behavior of charged particles as they are channeled-or some- times blocked inside crystals have yielded much information on surface conditions and the locations of impurities, for example. These studies can reveal a much deeper level of detail than that provided by

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SCIENTIFIC AND SOCIETAL BENEFITS 123 even the best electron microscopes. They are particularly useful in evaluating the effects of radiation damage in solids. Research on channeling is now being conducted at many accelera- tors around the world, including the highest-energy ones. There is apparently no practical upper limit to the kinetic energy of charged particles that can be made to channel in crystals. Relativistic effects associated with extremely high velocities are being exploited in order to measure ultrashort time intervals, in an effort to determine the lifetimes down to perhaps 10-20 second or even less-of some elementary particles. An intriguing offshoot of these experiments was the discovery that by bending the crystal, even the most highly relativistic particles at energies of hundreds of GeV-can be made to follow curved paths; to bend such particle beams through equivalent deflection angles in an accelerator would require immensely powerful superconducting magnets. The positrons emitted by some radioactive elements have been used for many years as a sensitive probe with which to map the charge and energy distributions of electrons in solids. In recent years, however, the intense, high-quality beams of muons, both positive and negative, developed at nuclear-physics laboratories have proved to be even more versatile than positrons in the study of solids. Muons are heavy leptons much heavier than electrons or positrons, but much lighter than nucleons. This intermediate mass alone makes them a valuable probe with which to study solid-state phenomena such as particle diffusion. Their characteristic decay properties are also valuable. In addition, muon beams have the useful property of being almost 100 percent spin-polarized, i.e., their spins are all oriented in the same direction. This property provides the basis for the technique of muon spin rotation, in which the changing direction and gradual degradation of the spin polarization are monitored after the beam is injected into a crystal. The rate and degree of these changes provide information about the muons' local magnetic environment, at any of several kinds of sites within the crystal lattice. As a local probe of solid-state structure and dynamics, muon spin rotation nicely complements several other techniques derived from nuclear physics, such as nuclear-magnetic-resonance spectroscopy, Mossbauer spectroscopy, and neutron scattering. These last three are also used, to varying degrees, by chemists, biologists, geologists, and others in countless analytical applications. The influence that nuclear physics exerts in these sciences is far-reaching and beneficial.

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124 NUCLEAR PHYSICS ATOMIC PHYSICS Although every atom contains a nucleus, many of the physical properties of the atom are determined by its cloud of orbital electrons. The electrons interact not only with each other (through the repulsive Coulomb force) but also with the electric and magnetic fields of the nucleus. As the properties of nuclei vary throughout the periodic table or through an isotopic sequence of a given element, so, to different degrees, do the characteristic features of the associated optical spectra of the atoms, which are determined by the electron energy levels and the transitions between them. For many years, much information about nuclear properties has been deduced from analyses of atomic spectra. Now, however, with nuclear accelerators that can produce ion beams of precisely controllable energy and ionization state, it is possible to create exotic atomic species unlike any that exist under ordinary conditions, and thus to use nuclear beams to study novel aspects of atomic physics. Such experiments and the corresponding atomic-structure calculations are interesting in their own right. They also have a direct bearing on our understanding of the nature of thermonuclear fusion plasmas both in stellar interiors and in terres- trial machines such as tokamak fusion reactors. In collisions between very heavy ions (uranium and curium, for example, for which the combined Z value is 188), a massive nuclear system can be created that exists long enough for the electrons of the two ions to rearrange themselves in a configuration corresponding to the combined Z value. In the formation of this extremely high-, pseudo-atom, however, a vacancy is sometimes created in the lowest electron shell. This shell becomes tightly bound while the nuclei are close together, and if the vacancy is filled during this period, the effect is formally equivalent to the creation of a positron. Recently positrons have, in fact, been detected in heavy-ion collisions at the GSI accel- erator in Darmstadt, West Germany. Surprisingly, one observes a discrete structure superimposed on a continuum spectrum. The origin of the sharp features in this structure is a mystery. Speculations have arisen as to whether it is due to the formation of relatively long-lived giant nuclear complexes or to some hitherto unknown physical phe- nomenon. In a different kind of atomic-physics experiment, accelerated heavy ions are stripped of most of their electrons by passing the beam through thin films or low-pressure gases. Careful stripping can yield heavy nuclei with only one orbital electron (a hydrogenlike ion) or two electrons (a heliumlike ionJ. These species thus exhibit a huge imbal

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SCIENTIFIC AND SOClETA ~ BENEFITS 125 ance between the positive nuclear charge and the surrounding negative electron charge. Studying their atomic spectra affords a unique oppor- tunity for making stringent tests of some of the predictions of quantum electrodynamics (QED), the quantum field theory of the electromag- netic interaction. One of these predictions concerns a fundamental but subtle spectroscopic effect called the Lamb shift, which can be measured with great accuracy. To date, all measurements of the Lamb shift in hydrogenlike ions (e.g., one-electron chlorine) and helium- like ions (e.g., two-electron neon) have confirmed the correctness of QED. It is also possible to strip all the electrons from an accelerated ion, leaving a bare nucleus as the projectile. In 1982 the production of relativistic beams of fully stripped uranium (U92 +) was demonstrated at the Bevalac accelerator at the Lawrence Berkeley Laboratory. Also recently, very-low-energy, fully stripped heavy ions have been pro- duced at the Double Tandem facility at Brookhaven National Labora- tory. Collisions between these slow nuclei and target atoms produce relatively long-lived, very heavy pseudo-atoms. The study of x rays resulting from such collisions is expected to provide a better under- standing of the processes that are critical for the production of superheavy atoms and to enable further tests to be made of the attendant QED phenomena in very heavy atomic species. The experiments described above illustrate only a few of the ways in which the techniques of nuclear physics have expanded the boundaries of atomic physics, thereby both broadening and deepening our under- standing of this vital subject. GEOLOGY AND COSMOLOGY Ancient objects whether man-made or natural, whether of geolog- ical or cosmological origin- are fascinating to scientists in many fields because of the invaluable clues they provide about the nature of the environment in which they were formed. Along with the chemical and, sometimes, microbiological analyses of such objects, their accurate dating is clearly of great importance. The familiar technique of radio- carbon dating (using carbon-14, which has a half-life of 5730 years) was one of the earliest practical applications of nuclear physics. It has proved to be of inestimable value in archeology and paleontology, enabling scientists to date events that occurred as far back as 50,000 years ago. Similar measurements of the decay products of other long-lived radionuclides have extended the applicability of the tech- nique.

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126 NUCLEAR PHYSICS Another dramatic advance in dating technology has taken place in the last few years, again as a spinoff of basic nuclear-physics research. Various heavy-ion accelerators around the world have been modified for use as ultrasensitive mass spectrometers, in which the atoms of long-lived radionuclides in the sample of interest are counted directly, rather than indirectly (and slowly) by detecting the radiations associ- ated with their decay. The immediate result of this ability to circum- vent the tedious process of radiation monitoring of specimens has been a spectacular increase in the sensitivity of dating measurements by a factor of as much as 10'2! This sensitivity, in turn, allows the use of much smaller samples (in the range of micrograms to milligrams) than before. Thus the technique of accelerator mass spectrometry, still in its infancy but developing rapidly, has vastly enlarged our scientific window on the past. Among the growing list of subjects being inves- tigated with this powerful new tool are atmospheric methane, polar ice, lake and ocean sediments, manganese nodules, tektites, meteorites, and long-lived radionuclides produced by cosmic rays. Geophysicists, paleoclimatologists, cosmologists, and others have much to gain from such studies, which reveal new information on changes that have occurred both on the Earth and outside the Earth over periods ranging from thousands to tens of millions of years. Already it has been learned, for example, that some manganese nodules on the ocean floor have grown at a uniform rate (of the order of a few millimeters per million years) for as long as 10 million years, whereas others have grown at sharply different rates during different periods of geologic time. The latter phenomenon suggests that signifi- cant changes in the manganese and iron content of local undersea growth environments have occurred at certain times in the past. Another interesting discovery from the deep is that ocean sediments at plate-tectonic boundaries are not scraped off during the subduction process, in which the edge of one crustal plate is bent downward and very slowly slides beneath the edge of the other. Instead, the sediments are carried down with the subducting plate, eventually to reappear in volcanic eruptions in these geologically volatile areas. The radio- nuclide whose atoms were counted in these studies, as in those of the manganese nodules, was beryllium-10, which has a half-life of 1.6 x 106 years; it thus allows the dating of events that occurred over the last 10 million to 20 million years. Similarly useful in geochronological studies over an even greater time scale are the radionuclides manganese-53 and iodine-129 (half-lives of 3.7 x 106 years and 1.6 x 107 years, respec

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SCIENTIFIC AND SOCIETAL BENEFITS 127 lively), whereas aluminum-26 (half-life of 7.2 x 105 years) is useful over a time scale of a few million years. Of obvious scientific interest are any objects, such as meteorites and cosmic rays, that reach the Earth from outer space. Until recently, it was thought that most tektites- strange, glassy objects that have been found widely distributed on land and under the seas-were of extrater- restrial origin. However, a careful comparison of their nuclidic com- positions with those of terrestrial and extraterrestrial rocks, using accelerator mass spectrometry, has now made it certain that tektites are terrestrial objects after all. Whatever the ultimate significance of this fact may prove to be, its discovery exemplifies part of the excitement of scientific research- knowing that progress will be made, but never knowing exactly from which quarter the breakthrough will come. NUCLEAR AND RADIATION MEDICINE For many years, nuclear physicists have been collaborating with physicians, chemists, pharmacologists, and computer scientists in a highly successful effort to solve some of society's most pernicious health problems. Their efforts have firmly established nuclear medicine as a standard part of modern medical practice. While the most widely applied techniques of nuclear medicine entail the use of radioactive tracers to diagnose diseases and monitor their treatment, radionuclides and accelerated particle beams also play important therapeutic roles. In addition, nuclear physics serves medical science through the devel- opment of exotic materials for use in prosthetic implants. In a typical nuclear-medicine examination, of which many millions are performed annually, a radiopharmaceutical agent is administered intravenously, and gamma rays emitted by the tracer nuclide are recorded with an array of radiation detectors positioned about the patient; this technique is called emission tomography. The tracer compounds are usually chosen for their selective uptake by a particular organ or type of tissue so that the detected gamma rays provide a detailed image of the region of interest. Advances in detector design and in data acquisition and analysis have led to markedly improved instruments for emission tomography of both the photon and positron types (see Figure 6.21. To the trained eye, the images produced can reveal structural or metabolic abnormalities whose recognition can lead to a diagnosis that might otherwise be more difficult or even impossible.

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128 NUCLEAR PHYSICS FIGURE 6.2 A computerized tomographic (cross-sectional) image of the human brain, showing regional metabolic demand for oxygen. A few seconds after the subject inhaled oxygen labeled with the positron-emitting radionuclide i50 (half-life of 122 seconds), the distribution of oxygen in the brain was revealed (bright areas) by gamma rays resulting from annihilation of the positrons with electrons in the surrounding tissue. The technique of positron-emission tomography has become a powerful tool of nuclear medicine. (Courtesy of R. J. Nickles, University of Wisconsin.) The recent development of the radionuclide thallium-201 from the research stage to commercial production for worldwide clinical use provides an illustration of how progress results from multidisciplinary investigations. One out of every six Americans is afflicted with cardiovascular disease, often undiagnosed, and over 70,000 deaths from heart attacks occur in the United States each year. Until about 10 years ago, the available tracer nuclides for early diagnosis of cardio- vascular disease were generally unsatisfactory. Following the demon- stration that after thallium is administered it is rapidly and selectively

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SCIENTIFIC AND SOCIETAL BENEFITS 129 localized in the heart muscle, however, nuclear scientists devised techniques for producing pure thallium-201 (half-life of 73 hours) in commercial quantities at affordable cost. As a result, nuclear cardiol- ogy tests using this nuclide were administered to some 250,000 patients in 1981. An even more impressive example of progress in nuclear medicine is the development of the radionuclide technetium-99m (a metastable excited state of technetium-99, with a half-life of 6 hours) over the last two decades. Radiopharmaceuticals incorporating this nuclide have proved to be invaluable for studying the brain, liver, thyroid, lungs, skeletal system, kidneys, heart, and hepatobiliary system. About 5 million patient studies using technetium-99m were performed in the United States in 1981. Roughly half of the 850,000 new cases of cancer that occur in the United States each year receive radiation therapy, either alone or in conjunction with surgery or chemotherapy. The electiveness of radi- ation therapy can be increased by improving both the dose localization and the biological effect of the delivered dose. Either of these will result in proportionally more damage to the tumor and less damage to normal tissue. Improved dose localization can be achieved by using accelerated beams of charged particles such as electrons, protons, heavy ions, and negative pions. Biological electiveness depends in part on the stopping power of the tissue for the particle in question and can be increased by using the particle in its characteristic stopping region. Nuclear physics contributes in a number of ways to this research. A thorough understanding of nuclear as well as atomic phenomena is required not only for determining the optimal type and energy of the primary beam, the production target material, and the shielding re- quirements but also for calculating dose distributions. Because of the slim margin between the responses of tumors and normal tissues, differences as small as about 5 percent in dose must be carefully monitored and controlled for proper treatment. In therapeutic radiol- ogy, as in nuclear medicine, progress depends on close collaboration among physicists, chemists, and physicians, with the additional re- quirement of coordinated advances in accelerator physics and instru- mentation. For example, the improved design of compact, relatively inexpensive linear accelerators has led to their widespread use in clinical x-ray and electron-beam radiotherapy. A final example in this abbreviated survey of ways in which nuclear physics is contributing to medicine concerns some new work on surgical alloys used for articulating orthopedic implant devices, such as

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130 NUCLEAR PHYSICS artificial hip joints. Over 75,000 hipjoint replacement operations are performed in the United States each year. Unfortunately, with pro- longed use, these joints are subject to gradual deterioration caused by the corrosive effects of normal body fluids; the resulting metallic debris can then poison and inflame the surrounding tissues. This can neces- sitate a second replacement of the joint an obviously undesirable prospect. Recently, however, materials scientists have taken a major step toward solving this problem. Using ion-source and accelerator tech- nology originally developed by nuclear physicists for basic research, they have found that the implantation of nitrogen ions to a concentra- tion of 20 atom percent to a depth of about 100 nanometers (100 x 10-9 meter) into the surface of a typical surgical alloy reduces the wear corrosion by a factor of at least 400. The successful clinical application of these new results could be of great benefit to patients requiring artificial articulating joints. MATERIALS MODIFICATION AND ANALYSIS Armed with ion sources, accelerators, and instruments developed in low-energy nuclear-physics research, investigators in numerous disci- plines are using energetic ion beams to modify and study the near- surface properties of materials in highly selective and often unique ways. When these beams stop in a solid, ion implantation occurs, which can alter or even dominate the electrical, mechanical, chemical, optical, magnetic, or superconducting properties of the material. The results are often dramatic. Perhaps the most impressive application of ion implantation arises in solid-state electronics. Most semiconducting devices require the selec- tive doping of silicon or germanium crystals with impurity atoms, and ion implantation has rapidly become the dominant doping technique in the semiconductor industry. Among its many advantages is that it permits extreme miniaturization; consequently, most semiconductor devices and integrated circuits for watches, calculators, computer chips, and other electronic porducts requiring small components are fabricated by this method. Ion implantation has also been exploited in a myriad of other applications. Controlled ion damage to insulators and semiconductors is used to alter the index of refraction of such materials for the fabrication of optical waveguides and mixers and to modify magnetic- bubble memory devices selectively. Ion implantation holds promise as

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SCIENTIFIC AND SOCIETAL BENEFITS 13 1 a fabrication method for high-temperature superconducting materials, since these require the formation or stabilization of a metastable phase that need exist only within a few tens of nanometers of the surface. Ion bombardment has also recently been discovered to be effective in bonding thin films to substrates. Studies of the dynamic behavior of light impurities such as hydrogen and helium embedded in materials and of the changes in the proper- ties of materials induced by the presence of these impurities have been carried out in recent years with new accelerator-based tech niques. The depth distribution of the impurity can be precisely mapped by making use of the sharp resonance behavior of nuclear reactions as a function of the incident beam energy. These reactions, using ion beams such as lithium-7, boron-11, nitrogen-15, ~9uorine-19, and chlo- rine-35, have very fine depth resolution (about 5 nanometers) and high sensitivity (better than 1 part per million). Problems for which this technique is used include the erosion of thermonuclear fusion reactor walls, the characterization of amorphous silicon solar cells, the embrit- tlement of steels and niobium by hydrogen contamination, and the effects of the solar wind (high-energy hydrogen and helium nuclei ejected by the solar corona) on moon rocks. ENERGY TECHNOLOGY Basic research in nuclear physics has created-and continues to create a fund of advanced technology that pervades energy-related research and development. The most familiar examples, of course, are those of nuclear fission and fusion. Nuclear fission reactors currently satisfy about 13 percent of the electric power demand in the United States, and nuclear fusion holds the promise of satisfying the bulk of this demand in the twenty-first century and beyond. The impact of nuclear physics on energy technology is also felt, however, in other, less-well-known areas. Nuclear techniques are used by the drilling industry to help probe geologic formations and locate hydrocarbons and other valuable resources that are deep underground. Passive forms of nuclear well-logging employ gamma-ray detectors to distinguish regions containing clean sands and carbonates (low natural radioactivity) from the less productive but more radioactive regions containing clays or shale rock. More sophisticated well-logging tech- niques generate neutrons with the aid of miniaturized nuclear acceler- ators that can be lowered into the test bores, which are typically about 10 cm in diameter. The apparatus produces fast neutrons by bombard- ing a tritium target with an accelerated, pulsed deuteron beam, and the

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132 NUCLEAR PHYSICS interactions of the neutrons with the surrounding material provide the logging information. In one application, gamma rays following inelastic neutron scattering are measured, and the well log is inspected for the characteristic yield that indicates the presence of carbon, the main constituent of oil and gas. In another application, neutron detectors are used to measure the duration of the well-defined slow-neutron pulse that results when the initial fast neutrons from the accelerator encounter hydrogen in the surrounding material. Rapid disappearance of the slow-neutron pulse suggests that the hydrogen in the region is accompanied by chlorine, which has a high efficiency for the capture of slow neutrons, and indicates the presence of saltwater. A long-lasting pulse shows that chlorine is not present and provides a good indication of petroleum deposits. The sensitivity of these and related nuclear techniques helps identify oil- or gas-bearing regions that might otherwise be overlooked. Whenever research and development efforts lead to increased effi- ciencies in existing energy technologies, the result is energy conserva- tion. Here too, the impact of nuclear physics is felt in various ways. For example, tracer techniques have been used to study friction and wear in gasoline engines by incorporating radioactive carbon in steel piston rings. Inhibiting friction and wear and hence improving effi- ciency- can often be accomplished by using the ion-implantation method to modify the surface properties of materials. Wire-drawing dies that have been ion implanted with nitrogen, at a cost of only a few dollars per die, can be kept in service about five times longer than ordinary dies, with consequent savings in tooling costs, plant down- time, and other tool-replacement costs. Ion implantation also shows promise for the fabrication of corrosion- resistant surface alloys, the use of which would conserve rare or strategic alloying metals such as chromium, platinum, cobalt, and tungsten. The conservation occurs not only through corrosion reduc- tion but also because nuclear accelerators permit the implantation of these scarce elements selectively into the surface of the material- precisely where they are needed for corrosion resistance. Intimately intertwined with these ongoing studies are efforts by metallurgists and other materials scientists to understand the ejects of intense radiation on the properties of structural materials and to design new materials for service in advanced fission and fusion reactors. Examples of the problems that must be investigated are the stability of waste-containment materials and the embrittlement and damage of reactor materials due to irradiation by neutrons, protons, and alpha particles. Such studies have already helped to identify metallurgical

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SCIENTIFIC AND SOCIETAL BENEFITS 133 A: :] FIGURE 6.3 Saint Rosalie interceding for the Plague-Stricken of Palermo, by Anthony Van Dyck. A conventional photograph of this oil painting is shown at the top left. At the top right is an x-ray radiograph, which reveals traces of a hidden painting underneath. This underlying painting is revealed more clearly in the two neutron autoradiographs shown at the bottom. The hidden painting turned out to be a self- portrait. (Courtesy of the Metropolitan Museum of Art, New York, and the Brookhaven National Laboratory.)

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134 NUCLEAR PHYSICS techniques for minimizing high-temperature swelling and grain- boundary embrittlement. They are also being used to look for possible ways of minimizing radiation damage by annealing the materials, using either controlled preirradiation or the ambient radiation of the reactor itself. THE FINE ARTS Nuclear techniques based on the use of neutron-induced radioactiv- ity in art objects have been used for many years as tools for determin- ing the elemental composition and thus, often, the origin of these objects. Recently, however, the complete neutron irradiation of paint- ings, followed by autoradiography, has proved to be a valuable technique for studying the underlying paint layers, which record the evolution of paintings by the great (and lesser) masters. The technique involves making a series of radiographic exposures over periods of many days following the neutron irradiation. Because of the difference in half-lives of radioactive nuclides of elements such as manganese, sodium, copper, arsenic, mercury, and antimony, it is possible to view selectively the images contained in the many layers of paint present in a typical oil painting. In a program conducted by the Metropolitan Museum of Art in New York, many oil paintings by masters such as Rembrandt, Hats, Van Dyck, and Vermeer have been examined. Many reveal not just one but several previously unknown underlying images (see Figure 6.3), which reveal the compositional evolution of the painting and the thoughts and moods of the artist.

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III Burnt Unhip of clc~ Panics

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