In addition to the pervasive role of nuclear techniques in medical diagnosis, nuclear science also plays a key role in the treatment of disease. There are approximately 800,000 new cases of cancer each year in the U.S. and about one half of these receive radiation therapy, either as the main modality or in conjunction with surgery or chemotherapy. The success rate of radiotherapy could be increased further by (1) improving dose localization, thereby sparing normal tissue and (2) improving the biological effect of the delivered dose. Optimization of either of these factors would result in an increased differential between damage to tumor tissue and damage to normal tissues. Dose localization may be improved by using nuclear charged particles heavier than electrons, e.g.. protons, heavy ions and negative pions. Biological effectiveness depends, in part, on stopping power and can be increased by using neutrons, heavy ions, and pions in their stopping region. Clinical trials are underway with these nuclear particles to gauge their promise for significantly improved treatment of localized cancers via radiotherapy.

Nuclear science contributes importantly to this research. Nuclear cross-section data and models are being developed to determine the primary beam type and energy, production target material, and shielding requirements, and also to calculate dose distributions. Differences of about 5% in dose can be clinically observable due to the slim margin between tumor control and normal tissue complications, and in some cases better crosssection measurements and nuclear models will be required to permit a closer approach to safe limits with consequent improved tumor control. As in the case of diagnostic nuclear medicine, progress will depend on close collaborations among chemists, physicians and

physicists, and coordinated advances in accelerator physics and instrumentation.

Gamma radiation is used in another application of nuclear science to sterilize about 30% of all medical and surgical products used in American hospitals today, and in many cases consumer goods such as talcum powder, milk cartons and cosmetics are routinely sterilized with this procedure. The radiation kills or disables microorganisms, bacteria and insects that otherwise could either spread disease or cause spoilage. Gamma-irradiated foods have been served on the Apollo 17 and space shuttle flights and gamma-treated foods are available to consumers in several countries. The World Health Organization concluded in 1981 that low radiation doses produce no adverse effects on foodstuffs. In the United States, the Food and Drug Administration has proposed new rules that would permit irradiated foods such as spices, fruits and vegetables to be sold to American consumers. Since relief organizations estimate that at least 10% of the crops that are grown in third-world countries are lost either to spoilage or to insects, food preservation via gammairradiation may prove to be a significant factor in reducing hunger on a worldwide basis.

Very recent work on the corrosion and wear of surgical alloys used for devices such as artificial hip-joints provides one final example of health benefits made possible by nuclear science. Each year in the United States more than 75,000 total hip joint replacement (THR) operations are performed. This involves surgical implantation of an alloy "ball" working inside a polyethylene "socket." There are several problems associated with this procedure. First, the rubbing wear of this ball and socket in the presence of corrosive body fluids can eventually lead to a poor fit and the failure of the assembly. Second, metallic flotsam is released to the body and can poison and inflame it, leading to undesirable histological effects. These effects have become increasingly serious as the average life span increases, causing many older people to have two THR's in their lifetime. Using ion source and accelerator technology originally developed by nuclear physicists for basic research, materials scientists have found that the ion implantation of nitrogen into the surface of a typical surgical alloy leads to a substantial reduction of this painful problem. By implanting nitrogen over a depth of 1000 angstroms, R. A. Buchanan and J. M. Williams were able to reduce the wear corrosion by a factor of at least 400. The successful clinical application of these very new results could be of enormous benefit to patients requiring artificial articulating joints.

B. Energy

Basic research in nuclear physics has created, and continues to create, a legacy of advanced technology that pervades energy-related research and development. The impact of this legacy extends far beyond the obvious example of nuclear energy into areas as diverse as fossil fuel prospecting and energy conservation.

Nuclear techniques are used by the drilling industry to help probe geological formations and to locate hydrocar bons and other substances in strata deep underground. Passive forms of nuclear well-logging employ gamma-ray detectors to distinguish regions containing clean sands and carbonates of low natural radioactivity from the less productive and more radioactive regions containing clays or shaly rock. Gamma-ray and neutron detectors operated in conjunction with neutron sources provide more detailed information. The more sophisticated of such logging techniques generate neutrons with the aid of miniaturized nuclear accelerators that can be lowered into the test bores. The apparatus produces fast neutrons, and the 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 log is inspected for the characteristics that indicate the presence of carbon, a major constituent of oil and gas. In another application, neutron detectors are used to measure the duration of the well-defined slowneutron 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 accom. panied by chlorine, which has a high efficiency for the capture of slow neutrons, and indicates the presence of salt water. 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 ail or gas-bearing regions that might otherwise be overlooked.

Oil shales are being investigated as a possible source of domestic energy with the aid of small angle neutron scattering (SANS). This technique measures the sizes of pores or voids that might contain petroleum products, thereby identifying shales that could contain potential sources of energy.

Energy conservation results whenever research and development efforts lead to increased efficiencies in existing energy technologies. Examples of nuclear physics contributions are found throughout this area. Nuclear tracer techniques have been used to study friction and wear in gasoline engines by incorporating radioactive carbon in steel piston rings. Ion implantation, initially a by

product of low energy nuclear physics research, is used to modify the surface properties of materials to inhibit friction and wear. Wire-drawing dies, ion implanted with nitrogen at a cost of only a few dollars per die, can be kept in service about five times longer than non-implanted dies, with consequent savings in tooling costs, plant downtime, etc. Ion implantation also shows promise for fabricating corrosion resistant surface alloys while conserving expensive, rare or strategic alloying materials such as chromium, platinum, cobalt and tungsten. The conservation occurs not only through the reduction of corrosion. but also because ion beam accelerators permit the implantation of these scarce elements selectively into the surface of the material-precisely where they are needed for corrosion resistance.

The development of the new alloys and ceramics that would permit fossil fuel power plants to operate at higher pressures and temperatures, with correspondingly higher efficiencies, is another area where nuclear diagnostic techniques such as neutron and charged particle scattering as well as ion beam implantation, with its ability to introduce essentially any element into any substrate, rake important contributions. These efforts, aimed at increas ing fossil plant steam conditions from the current state-ofthe-art 1000°F at 3500 psig to 1400°F at 7000 psig. would result in annual fuel cost savings of 13 million dollars in a typical (800 megawatt) power plant.

The historic role of nuclear physics in the development of nuclear energy needs little elaboration, but perhaps less well known are the ongoing research efforts in support of national programs to develop advanced nuclear energy resources. Today, in the United States, approximately 13% of our electrical power is supplied by uranium-fuel thermal neutron spectrum reactors. In some regions of the country, such as Chicago, New England, and the Tennessee Valley areas, the percentage is much higher.

The design of advanced fission and fusion reactors capable of meeting the projected future demand for elec. tricity depends upon detailed knowledge of neutron interactions over a wide range of neutron energies. Even though nuclear-data programs for reactor design have been active for many years and many data requirements have been met, several important ones remain unfulfilled and a few have not been seriously addressed.

Several accomplishments in the last few years can be selected to illustrate the diversity of these neutron data. These are (1) a precise 0.2% measurement of the average number of neutrons per fission from spontaneous fission of 252Cf, used as a standard for all related measurements: (2) capture and fission data on several actinides (Np, Am,

Future needs for the liquid metal fast breeder reactor technology include more accurate data on 238U at neutron energies above the resonance region, inelastic capture neutron scattering for 239Pu and 238U, neutron capture in fission products, and measurements of self-shielded cross sections for 238U and 239Pu in the resonance energy region. Optimal design of thermal reactors requires more accurate knowledge of neutron cross sections at energies through 1 eV on 235U, 239.240Pu (and 233U if the thorium cycle is to be thoroughly understood). All fission designs need better-defined neutron spectra from fission, starting with the 252Cf standard. For fusion-energy systems, the higher energy of the primary neutron source at 14 MeV requires extending the energy range of the nuclear-data measurements. Other reactions become important in induced radioactivity in structural materials or in the production of charged particles forming gas inclusions within the structural materials. Adequate nuclear data for shielding materials are yet to be measured. The strongly asymmetric angular distributions, which impact neutron streaming calculations, must be determined.

Intimately intertwined with these ongoing studies are efforts by metallurgists and other materials scientists to understand the effects of intense radiation on the properties of structural materials and to design new materials for service in advanced fission and fusion reactor systems. The nuclear physics and materials science research efforts both depend crucially on the availability of well instrumented, state-of-the-art facilities for nuclear measurements, such as high-flux research reactors and intense-beam pulsed electron and proton accelerators for neutron production.

Low energy Van de Graaff accelerators also contribute in a variety of ways. For example, Rutherford scattering of low energy ions is used to determine the suitability of various nuclear waste containment materials, as well as to characterize the surface properties of new reactor alloys under various conditions. The swelling and other effects that occur when helium is produced in structural reactor

materials are being studied under controlled conditions by bombarding the material simultaneously with helium from a Van de Graaff accelerator and with neutrons from pulsed or steady-state sources. Such studies have helped identify metallurgical techniques for trapping the helium in a high concentration of small cavities or bubbles to minimize high temperature swelling and grain-boundary embrittlement.

C. Semiconductor Doping and lon
Implantation

Armed with ion sources, accelerators, and experimental techniques developed in low energy nuclear physics research, investigators in numerous disciplines are discovering that energetic ion beams can be used to alter and study the near surface properties of materials in a selective and often unique manner. When these beams impinge on 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 and several examples have been discussed earlier in this chapter. Perhaps the most impressive application of ion implantation concerns semiconducting materials. Most semiconducting devices require the selective doping of silicon or germanium with impurity atoms and ion implantation has rapidly become the dominant doping technique in the semiconductor industry. Responsible not only for new devices-such as high frequency transistors, improved MOS transistors, and integrated circuits-ion implantation also increases yields of old devices by orders of magnitude during fabrication and permits tremendous miniaturization. As a result, most semiconductor devices and integrated circuits for watches, calculators, computer chips, etc. are fabricated by ion implantation. The billion-dollar-a-year portable calculator industry is but one consequence of ion implanted integrated circuits; other examples ranging from color television to personal computers could be cited in complete analogy.