and national security. Beyond these direct applications is the general benefit that arises from pressing forward the frontiers of high-technology development.

Some of the most pervasive applications of nuclear physics are in medicine. Medical imaging techniques now widely used, such as positron emission tomography (PET) and nuclear magnetic resonance imaging (MRI), provide information in three dimensions about the structure and biochemical activity of the human interior. Radioactive isotopes produced by accelerators and reactors are routinely used in medical diagnostic procedures, in treatment, and in medical research. Cancer radiation therapy mainly uses electron accelerators and radioactive sources. Treatment with protons, neutrons, and heavier ions is becoming more widespread and shows great promise for improved selectivity and effectiveness.

Many applications to environmental problems take advantage of the exceptional sensitivity of nuclear techniques such as accelerator mass spectroscopy to obtain information not available by other means. One can determine oceanic circulation patterns, the rate of carbon dioxide exchange between the atmosphere and the land and oceans, and the historic climate record. All of these have major implications for an understanding of climatic change. Studies of groundwater resources and their recharge rates, and of the origin of atmospheric pollutants, also provide unique information.

The assortment of industrial applications reflects the great variety of industrial processes. One common theme is the use of nuclear techniques and accelerators to determine the composition and properties of materials, their structural integrity after manufacture, and their wear in use. Another is the development of techniques for the modification of materials through accelerator ion-implantation, as in the doping of microelectronic circuits, or the introduction of defects to increase the current-carrying properties of high-temperature superconductors.

Safety and national security are areas with broad applications of nuclear techniques. Their use in detection of explosives and weapons has occupied increasing attention as a barrier to terrorism. Diagnostic procedures based on nuclear physics techniques will play a major role in noninvasive monitoring of chemical weapons and in controlling the distribution of enriched uranium and plutonium from dismantled nuclear weapons. Such procedures will also be important in the stewardship of the remaining nuclear stockpile. Intense beams from accelerators may in the future serve a joint role in production of the tritium required to maintain the required stockpile of nuclear weapons and in disposal of radioactive wastes.

Nuclear physics continues to have a profound impact on the production of energy: nuclear fission reactors produce about 19 percent of U.S. electricity (17 percent worldwide), and they provide an option for reducing use of finite hydrocarbon fuels and hence the emission of carbon dioxide into the atmosphere.

A few examples—of successes, of programs in early stages of development,

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