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7 Interactions with.Other Areas of Physics and Technology The purpose of research in elementary-particle physics is to inves- tigate the basic nature of matter. energy, space. and time. In the course of this research elementary-particle physics interacts with other areas of physics. Subject matter, instruments, and theories from other areas all have a bearing on elementary-particle research. And elementary- particle research contributes data, theories, and apparatus to other parts of physics. In this chapter we briefly describe the interaction between elementary-particle physics and four other areas: cosmology and astrophysics, cosmic-ray physics, nuclear physics, and atomic physics. Those areas are the subjects of separate volumes in this survey; here we look only at their interaction with particle physics. Elementary-particle physics interacts with technology in two ways. First, the technology that is invented and developed for use in particle physics subsequently finds use in other fields. The foremost example is the particle accelerator itself, some of whose applications are described below in the section on Other Applications of Accelerators. The second way that elementary-particle physics interacts with technology is that technology from outside particle physics is stimulated and developed during the design and construction of particle-physics accelerators and detectors. Prominent examples are superconducting magnets, described below in the section on Large-Scale Uses of Superconductivity, and integrated circuits and computers, described in the section on Support and Stimulation of New Technology. IS7

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158 ELEMENTARY-PARTIC'E PHYSICS COSMOLOGY AND ASTROPHYSICS Recent years have witnessed a growing symbiotic relationship between elementary-particle physics, the science of the very small, and cosmology, the science of the very large. This interdependence has been fostered by the revolution in our understanding of particle physics on the one hand and on the other hand by the existence of and observational support for the big bang model of the origin of the universe. In that cosmology, the universe evolved from an original explosion of a dense, hot mixture of matter and energy. In those first moments of the universe elementary particles were created and de- stroyed at an enormous rate. Their creation and destruction occurred through interactions. For example, if two photons collide they can interact and be destroyed, but as a result of that interaction an electron and a positron can be created. In big bang cosmology the universe is expanding as this happens. the matter and energy cooling ok and becoming less dense. Some particles no longer interact, hence they are not destroyed, and they remain in our present universe. Such particles are called relics. This is the natural interpretation of the observed recession of distant objects in the universe, and it is supported by the observations of the 3-K microwave background radiation, generally interpreted as a relic from the recombination of electrons and nuclei to form electrically neutral matter. Recombination occurred when the universe was thou- sands of times more compressed than it is today. with a temperature of several thousands of kelvins. Such an increase of temperature at earlier times when the universe was denser receives further support from successful calculations of the astrophysical abundances of light ele- ments through cosmological nucleosynthesis when the universe was a billion times smaller than it is today, with a temperature of several billion kelvins. The successes mentioned above illustrate the principles enabling big bang cosmology and particle physics to be interrelated. Our present understanding of particle physics enables us to extrapolate the Hubble expansion of the universe backward to earlier times and higher temperatures and to calculate the abundances of other elementary- particle relics from the big bang. An example is provided by the question of how many kinds of neutrinos exist. This is a critical question because it is one way to find out the number of generations of leptons that exist. Nucleosynthesis calculations impose the most stringent available limit on the number of light neutrino species. They are restricted to

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INTERACTIONS WITH OTHER AREAS 159 three or at most four' which is below the best upper limits currently available from particle-physics experiments. As another example, from present theories it can be calculated that stable neutrino masses must be less than 100 eV, or more than a few GeV, if these neutrino relics are not to decelerate or reverse the present expansion of the universe because of their mass. These constraints on neutrino masses are much more general than those ob- tained from laboratory experiments, and for the muon and tau neutrinos they are more stringent. Another long-standing problem in cosmology has been the existence of dark matter; that is. the amount of visible matter in the universe seems to be far less than the amount of mass that is inferred from the interactions of the visible matter. There seems to be far more matter out there than we can see; in fact, the amount of matter implied by the dynamics of the universe on large scales may be even greater than the limits on the amount of hadronic matter in the universe as obtained from the big bang synthesis of nuclei. Neutrinos with small but nonzero mass might constitute this missing mass, and they could have played a significant role in the formation of galaxies and other structures in the present universe. More speculative particle theories provide other relic candidates for these roles, such as the hypothesized supersymmetric particles and axions. Some limits on the existence of such particles come from astrophysical considerations of the energy flow out of the cores of stars at late stages in their evolution. In view of these interesting exchanges of information, it is not surprising that cosmologists and particle physicists have been inspired to speculate about much earlier epochs of the big bang, when temper- atures and hence particle energies were much higher than present or conceivable particle accelerators could provide. Despite our lack of control of the experimental conditions, the early universe could be a useful laboratory for testing new particle theories. One of the most striking examples has been the realization that grand unified theories that predict baryon decay could also explain the presence of the matter in the universe today. The interactions and decays of superheavy particles with masses above 10' GeV could have assured the present predominance of matter over antimatter. It would no longer need to be assumed as an arbitrary initial condition. Grand unified theories also predict the existence of magnetic mono- poles potential cosmological relics which, if they exist, could inval- idate current cosmological theories. Their masses of 10'6 GeV or more

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. 160 ELEMENTARY-PARTICLE PHYSICS are so high that only the early universe could have produced them. Some simple arguments suggest that unacceptably many grand unified monopoles would have been produced in the conventional big bang cosmology, a difficulty that was a stimulus for the proposal of so-called inflationary cosmology. According to this idea, there may have been an early epoch in the history of the universe during which it expanded exponentially, driven by the energy released when there was a change or transition in the state of matter. A large number of particles would be produced when this transition terminated, and the abundance of monopoles would then be greatly diluted. Such an inflationary epoch could also explain many of the greatest cosmological mysteries of the universe' such as the high degree of homogeneity and isotropy that it exhibits and its great age. Inflationary cosmology also enables one for the first time to relate particle physics to the wide range of the large- scale fluctuations in the present universe. It is a challenge to find a particle theory that naturally leads to inflation and to this wide range. This is one of many areas in which the interaction between particle physics and cosmology will continue to be fruitful in the future. COSMIC-RAY PHYSICS Cosmic-ray physics and nuclear physics are the parents of elemen- tary-particle physics. Many of the fundamental discoveries about elementary particles were made using cosmic rays. This is because cosmic rays are a natural source of high-energy particles. Cosmic rays consist primarily of protons that come from outside the solar system. When they hit the atmosphere they make other particles. The positron. the muon, and some of the strange hadrons were discovered and first studied using cosmic rays. However, with the development of accelerators, the use of cosmic rays in elementary-particle physics has gradually decreased. This is because accelerators provide controllable and much more intense fluxes of particles. It is only the highest energy cosmic rays that are still useful for studies in particle physics energies that cannot be attained try accelerators. Thus at present the field of high-energy cosmic rays acts as a bridge between high-energy particle physics and experimental astrophysics. At and above the highest energies reached by hadron-hadron colliders the energy spectrum, composition, and possible source directions ~ f primary cosmic rays are known to varying degrees. At the same time. the nature of strong interactions at energies above those provided try colliders must be deduced from extrapolations based on known accel-

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INTERACTIONS WIT[l OTHER AREAS 161 erator data and from the largely indirect cosmic-ray data. As the interpretation of these cosmic-ray-data in terms of particle-physics phenomenology depends on knowledge of the identity of the initiating cosmic ray (e.g., proton. carbon. or iron nucleus), our knowledge and understanding of both areas are interrelated, and progress is made in an iterative manner as we move to higher energies. We still do not know the source or acceleration mechanism of high-energy, primary cosmic rays. At the highest observed energies (about 10~ eV), it appears that cosmic rays would be too energetic to be trapped in the known magnetic field of our galaxy or to survive energy loss by photoproduction on the relic blackbody radiation in propagation over intergalatic distances. They might come to us from our own local supercluster of galaxies, or they might come from the core of our own galaxy, bent to the Earth by the (unknown) magnetic fields in a galactic halo. Correspondingly' the only source of information concerning the nature of particle interactions above the highest accelerator energies comes from cosmic rays. Hints of strange, unanticipated phenomena at these energies permeate the cosmic-ray literature. In the past, some cosmic-ray hints, such as evidence for free quarks and monopoles, have not stood up under closer scrutiny. But others, such as the increase of the strong interaction cross section with energy, were later confirmed at particle accelerators. The problem of studying cosmic rays at energies above 10'4 eV (greater than those at the CERN proton-antiproton collider) is dis- cussed in Chapter 6. Above 10'6 eV, the integrated primary cosmic-ray flux is only one per square meter per year. Other areas addressed by cosmic-ray experiments that overlap astrophysics, particle physics, and nuclear physics include the search for antimatter in cosmic rays and the study of nucleus-nucleus inter- actions at very high energy. It is quite certain that our local galaxy is composed of ordinary matter, but if antinuclei as heavy as iron are found in primary cosmic rays, even at a level of 10-7, this would be evidence for entire distant galaxies composed of antimatter. Currently there are no data with which to answer this question. Finally, the study of cosmic-ray neutrinos with proton-decay detectors may portend a new field of neutrino astronomy. NUCLEAR PHYSICS High-energy physics traditionally is closely linked to an area broadly termed nuclear physics. Elementary-particle physics grew out of

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162 ELEMENTARY-PARTIC~E PHYSICS nuclear physics; nuclei can be used as probes of elementary particles and vice versa. But the two disciplines are different because they deal with matter at different levels of elementarily. Accelerators are tools that are common to high-energy and nuclear . physics. Low- and medium-energy accelerators used by the nuclear- physics community include meson factories, which produce the most intense beams of protons, pions, and muons, and reactors, which pro- duce the highest fluxes of neutrinos. Some special questions in particle physics must be explored with beams of these kinds. The nucleus itself is a unique high-density laboratory in which interactions between quarks may be probed, by electron bombardment or by nucleus-nucleus collisions. Conversely, processes such as the production of strange protonlike and neutronlike particlesprocesses usually associated with particle physics may be used instead to study nuclear properties, as when the produced strange particle is a nuclear constituent. An outstanding puzzle is the existence of multiple generations of quarks and leptons. In the case of quarks it is known that the up-down, charm-strange, and bottom-top generations mix, but there is no clear evidence for mixing between members of the electron, muon, and tan lepton families. Searches for muon-electron mixing in muon decay have been carried to exquisite precision with free muons at the LAMPF accelerator at Los Alamos and with muons in the fields of nuclei at TRIUMF (Vancouver) and SIN (Zurich). Mixing of lepton generations in combination with differences in neutrino mass would produce an oscillatory behavior in the composition of neutrino beams. Highly restrictive limits on neutrino oscillations have been set at LAMPF and at reactors in the United States, in France. and in Switzerland, as well as at high-energy physics facilities. Discovery of lepton-generation mixing would be a major achievement, sharpening and expanding our theoretical understanding. Experiments at high sensitivities will continue with this aim. Successful unification of the strong and electroweak interactions depends on identification of the underlying symmetry group. An out- standing question is the parity symmetry of the electroweak interac- tion, which at present appears to be fully left-handed. Under the assumption that the neutrino that would participate in any right-handed weak interaction is light enough to be produced in muon decay. the weak-force-carrying particle WR that would mediate that interaction is required by recent muon-decay data to be at least five times more

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INTERA CTIONS WI TH OTHER AREAS 163 massive than its left-handed counterpart We. If further assumptions including electron nonconservation. are made, much more stringent bounds on the WR mass are set by the limits on neutrinoless double beta decay, e.g., of germanium or selenium isotopes, by direct or geochemical observation. Under the same assumptions, these obser- vations require the electron-neutrino mass to be less than approxi- mately 10 eV. More direct measurement of the electron-neutrino mass is possible through extremely precise study of the endpoint spectrum in tritium beta decay. At present there is unconfirmed evidence from one experiment of finite electron-neutrino mass, in a range that would suggest that neutrinos could account for the dark mass of the universe. The concept of the quark composition of nucleons has had a major impact on nuclear-physics theory and has led to new ideas in the description of nucleons. For nearly a decade, one direction of research at the Lawrence Berkeley Laboratory Bevalac has been based on the idea that medium-energy heavy-ion central collisions would produce an instantaneous temperature and nuclear density so high that hadronic matter would evolve (deconfine) into an as yet unobserved new state of matter, the quark-gluon plasma. Present estimates indicate that the plasma can be made in two different environments. The first is found at lower energies (a few GeV/nucleon usable energy) in which heavy nuclei are still able to stop in each other, building up high-energy densities in a baryon-rich zone. At much higher energies (above 2~30 GeV/nucleon usable energy), the two colliding nuclei are transparent to each other, leaving a hot baryon-free plasma in the central region after the collision process has taken place. In this central region large densities can be found, sufficient for Reconfinement to occur. These large usable energies await the construction of a new accelerator, called by the nuclear science community the Relativistic Nuclear Collider (RNC). Recently, evidence has indicated that nuclei do not behave simply as a collection of nucleons when high-energy muon, electron, or neutrino scattering occurs. CERN and SLAC experiments find a difference in the form of the quark distribution between deuterium and iron nuclei. This observation will be pursued in the nuclear-physics community through the construction of the SURA 4-GeV electron linear ac- celerator/stretcher. These are some examples of the interactions between nuclear physics and elementary-particle physics. Such interactions will con- tinue, not only in the questions that are being studied but also in the accelerators and detectors that are used.

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164 ELEMENTARY-PARTICLE PHYSICS ATOMIC PHYSICS e Effects from new particles are most readily observed at the appro- priate energy required to produce the particle. Historically, however, the effects seen in new energy ranges have often been correctly foreshadowed by extrapolation of small deviations from theory ob- served at lower energies. The extreme precision with which measure- ments are possible by atomic-physics techniques makes conceivable, even today, the exploration of energy ranges beyond those currently obtained. As an example, atomic experiments were recently used to test the electroweak theory predictions for the synthesis of the weak and electromagnetic theories. These experiments were based on slight differences in the absorption of left- and right-circularly polarized light. Only the extreme precision of laser spectroscopy techniques made these experiments possible. A second example is the study of two-particle systems that are simpler than the hydrogen atom. The hydrogen atom has at its core an extended, complex objecta proton. In contrast, the positronium system, composed of a positron and electron combined as a short-lived atom, consists of two simple' pointlike particles that exhibit effects that are masked in atomic hydrogen. Muonium~ composed of a muon and an electron, is another such simple system that can be formed by stopping muons, produced in an accelerator, in noble gases. Investigation of the spectra of atomic systems with exotic constitu- ents can also be used to probe particle structure. Examples are provided by atoms composed of muons and pions and of electrons and pions. Deviation from the results expected from pointlike particles provides insights into the structure and interaction of the pion with leptons. Another use of spectroscopy has been the insertion of muons, pions, and kaons into the innermost electron orbits of nuclei to provide, via x-ray spectroscopy, a measure of the electric-field struc- ture in the neighborhood of the nucleus and also to provide high- precision mass measurements of kaons and pions. A further example is provided by the precise measurements of the magnetic properties of the electron and the muon. The quantum theory of electromagnetism predicts that an electron will act as a small bar magnet and also predicts the strength of that magnet. Atomic-physics experiments have measured that strength accurately and have thus made one of the most careful tests of that theory. One ingenious small-scale experiment has reported positive evidence for fractional electric charge on small pellets of niobium. An interpre- tation of these results might be that free quarks on these spheres were

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/NTERA CT/ONS W/ TH O THER A REA S 1 65 responsible for the observations. However, the results have not been confirmed elsewhere, and the consensus in the community is to postpone accepting these results as evidence for free quarks pending strong confirmation. These classes of experiments investigate properties of matter that are of interest to both atomic and particle physics and are thus an important meeting point for two apparently unrelated areas of physics. CONDENSED-MATTER THEoRETICAL PHYSICS There has been and continues to be ~ fruitful and vigorous dialog between theoretical condensed-matter physics and theoretical particle physics. Here we sketch some of the topics in which concepts and techniques of elementary-particle theory enrich condensed-matter physics and also some of the topics where ideas of condensed-matter physics illuminate particle physics. In the late 1950s, the techniques of quantum field theory used in particle physics started to be employed in condensed-matter physics with outstanding results. Early on. field-theory techniques were used to solve the problem of the energy of an electron gas that forms the starting point for the general discussion of crystals. These techniques were found to be of great importance in dealing with superconductivity and superfluidity. The nature of second-order phase transitions was revolutionized by the use of renormalization group techniques that were developed by both particle and condensed-matter physicists. In turn, the general concepts of phase transitions developed by con- densed-matter physicists have been of much use to particle physics in two areas. On one hand, a strongly first-order phase transition has been invoked to produce an inflationary epoch in the early universe that may solve several outstanding cosmological puzzles. On the other hand, there is much recent interest in the possibility of a phase transition in dense, energetic nuclear matter in which the quarks and gluons become Reconfined and behave as a quark-gluon plasma. This may happen deep in a neutron star or, perhaps, in high-energy heavy-ion collisions. It is hoped that the new theory of quantum chromodynamics (QCD) will explain the mass and structure of the hadrons. The method used is to replace the space-time continuum by a discrete lattice of points, and the techniques of calculation employed are quite similar to those first developed in the condensed-matter context. The QCD calculations are of a large scale. The methods are often checked by first applying them to the simpler models of condensed-matter phenomena. Such checks have often proved to shed light on these models. The QCD calculations

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1 66 ELEMENTA R Y-PA R TI CLE PH YSI CS are so extensive that powerful special-purpose computers are being developed to handle them. This has led to a similar development of special-purpose computers to deal with lsing model calculations in condensed-matter physics. The concept of order parameters. first introduced in condensed- matter physics' now plays an important role in quantum field theory. We give some examples. These order parameters are akin to the Higgs fields of elementary-particle physics. Liquid helium can flow in a circle about a vortex. Such a flow is analogous to a topological knot. and the vortex line is a kind of defect. Similarly. the Higgs field can arrange itself in a pattern like that of the extended spines of a hedgehog. This is again a type of topological knot. The corresponding defect is a magnetic monopole. Such magnetic monopoles are exceedingly heavy. They may have been produced in the early universe, but they have not yet been detected. Polyacetylene is a long molecule with an alternating bond structure. The bonds can have a jump just as a canal can have an extra lump of water. Such objects are called solitons. A similar situation may occur in the quantum field theory of elementary particles with hadrons being described at least approximately. as solitons. The total electronic charge about a soliton in polyacetylene is most pecu- liar it is half the charge of a free electron. If magnetic monopoles do exists they would also behave to some extent as solitons~ and they could induce a fractional electronic charge. OTHER APPLICATIONS OF ACCELERATORS In this section we give some examples of how accelerators have been extended in their applications to other kinds of research and other kinds of technology. Synchrotron Radiation The foremost example of the application of accelerators is the use of circular electron accelerators to produce synchrotron radiation. As shown in Figure 7.1, when an electron moves in a circle it emits electromagnetic radiation in a direction tangent to that circle. That electromagnetic radiation covers a broad range of frequencies, extend- ing from the visible to the ultraviolet to the x-ray region of the spectrum. In addition to the broad frequency spectrum. the intensity of the emitted radiation is much higher than can be obtained by other means. For example' the intensity of the x rays within any given frequency range is much greater than can be obtained from a conven-

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INTERACT/O^'S WITH OTHER AREAS 167 / l Electron Moving Clockwise ~ ~yncnro~ron \ \\ Rodiation ~: . Circulor Poth _ of E lectron - FIGURE 7.1 The most important use of electron storage rings outside of elementary- particle physics is the production of synchrotron radiation. As ~ high-energy electron moves in a circular orbit it emits an intense beam of x rays called synchrotron radiation. Synchrotron radiation is used for research in many scientific and technical fields: for example. solid-state physics. material science. chemistry. and biology. tional x-ray tube. This wide frequency spectrum of intense radiation has found many applications in applied physics. material science, electrical engineering. metallurgy, biology. biochemistry. biophysics. and chemistry. Originally synchrotron radiation was obtained only as a parasitic by-product from circular electron accelerators and from electron- positron colliders. However. as the importance of research based on synchrotron radiation has increased. special dedicated accelerators to produce synchrotron radiation have been built. A list of present-day synchrotron radiation sources now in operation or being constructed is given in Table 7.1. There are more than 20 such facilities. A simple example of the use of synchrotron radiation has to do with the process called photoionization. in which light is used to eject an electron from an atom or molecule or from a solid. The frequency of the light that ejects the electron tells the researcher something about the structure of the atom, molecule, or solid. Photoionization has been known about since the turn of the century, but to do efficient advanced research at present requires intense sources of light with the frequency involved being known precisely. This is exactly what can be done with synchrotron radiation. Synchrotron radiation facilities have now begun to develop special kinds of accelerator technology to enhance their capabilities. Synchro- tron radiation is produced when an electron goes in a circle, because

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168 E! EMENTAR Y-PART/CLE PH YSICS TABLE 7. 1 Storage Ring Synchrotron Radiation Sources Electron Energy Location Ring (lab) (GeV) Notes . China Beijing BEPC (lHEP) 2.~-2.8 Parasitic" Hefei HESYRL (USTC) 0.8 Dedicated England Daresbury SRS 2.0 Dedicated France Orsay ACO (LURE) 0.54 Dedicated DCI ( LU RE) I .8 Partly dedicated SuperACO (LURE) 0.X Dedicated" Germany Hamburg DORIS ( DESY ) 5. ~ Partly dedicated West Berlin BESSY 0.8 Dedicated Italy Frascati ADONE 1.5 Partly dedicated Japan Tsukuba Photon Factory (KKK) 2.5 Dedicated Accumulator (KKK) 6 8 Partly dedicated" Tokvo TRISTAN (KKK) 30 Parasitic" Ol;~~aki SOR (ISSP) 0.4 Dedicated Tsukuba U VSOR ( I MS; 0 6 Dedicated Sweden Lund Max 0.55 Dedicated United States Gaithersberg. MD SURF (NBS) 0.28 Dedicated Ithaca. NY CESR (CHESS) 5.~ - Parasitic Stanford. CA SPEAR (SSRL) 4.() Partly dedicated Stoughton. Wl Tantalus (SRC) 0.94 Dedicated Aladdin (SRC) 1.0 Dedicated Upton. NY NSLS I (BNL) 0.75 Dedicated NSLS 11 (BNL) 2.s Dedicated Soviet Union Karkhov N-100 (KPI) 0.10 Dedicated Moscow Kurchatov 0.45 Dedicated Novosibirsk VEPP-2M (lNP) 0 7 Partly dedicated VEPP-3 (INP) 2.' Partly dedicated VEPP-4 (INP) 5-7 Parasitic " Under construction as of April 1985. any deviation of the path of a particle from a straight line means that the particle is being accelerated. ln fact. any means by which an electron can be made to move off a straight line and thus be accelerated will also produce synchrotron radiation. Therefore modern synchro- tron radiation accelerators have devices called wigglers or undulators introduced in the path of the electrons. These devices shake the electron as it moves through them, causing strong acceleration and the emission of intense synchrotron radiation in particularly desirable frequency ranges.

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INTERACTIONS WITH OTHER AREAS 169 Accelerators in Medicine The electron accelerator is now one of the major tools used in the radiation-therapy treatment of cancerous tissue. The usual way to use such an accelerator is to allow a high-energy beam of electrons to strike a target and then to form the resulting x rays from that target into a narrow, well-defined, and intense beam. The radiation therapist then directs that x-ray beam as carefully as possible onto the tumor that is to be treated. Most treatments involve.x rays in the a- to 6-MeV range, but x-ray energies as high as 30 or 40 MeV are sometimes used. Most of the electron accelerators used for standard radiation therapy are commercially produced linear accelerators. Some circular electron accelerators, based on the betatron principle' are also used. Although the standard method of radiation therapy using accelera- tors continues to be the use of x rays' during the last decade there has been a good deal of research on the use of other kinds of particles to destroy cancer. For example, accelerators have been used to produce beams of charged pions, which are then used to treat the tumor. Work has also been done using neutrons and high-energy heavy ions pro- duced in an accelerator. An interesting new use of accelerators in medicine involves the production of short-lived radioactive materials that produce positrons when they decay. inside the patient these positrons annihilate, and the resulting photons can then be used in tomography. Because these materials have short lifetimes they cannot be stored but must be produced soon before they are used, and cyclotron accelerators are now being used in hospitals to produce such materials. High-Intensity Neutron Sources The scattering of neutrons in mattter has become an important tool in materials science, solid-state physics, polymer chemistry, molecular biology, and other areas of applied and pure science. In the past, nuclear reactors have been the source of the neutron beams used in the scattering experiment. Reactors are still the major source, but spalla- tion neutron sources that use technically advanced proton accelerators are coming into increasing use because they can provide more-intense and higher-energy neutron beams. In a spallation source, a proton beam from a rapid cycling synchrotron bombards a uranium or other heavy-element target, providing a neutron beam.

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170 ELEMENTAR Y-PARTICLE PHYSICS Accelerators and Plasma Physics . There is an increasing interaction between accelerator physics and technology and plasma physics and technology. This interaction takes several different forms. One example is the use of heavy-ion acceler- ators to produce inertial fusion. Another example is the use of accelera- tors to inject charged particles into a plasma to add energy to the plasma as a step toward producing fusion. These ideas are described in detail in the companion volume on plasma and fluid physics. LARGE-SCALE USES OF SUPERCONDUCTIVITY The study o.f superconducting effects and the use of superconducting phenomena play an important part in many areas of physics. Briefly, superconductivity is the absence of electrical resistance that some metals exhibit when cooled to a temperature near absolute zero. That means that an electric current can circulate through the metal without any power loss and therefore could literally circulate forever. While superconductivity has been used on a laboratory scale for a long time, there have been few large-scale uses of this phenomenon until recently. The most striking example is the recent construction of the Fermilab Tevatron accelerator, which uses about 1000 super- conducting magnets. The liquid-helium refrigeration system used to cool those magnets is the largest in the world. In Chapter 5 we discussed the significance of this accomplishment for future construc- tion of very-high-energy proton-proton or proton-antiproton colliders. This accomplishment will also help to lead the way to other large-scale applications of superconductivity. Large-scale applications of superconductivity require large facilities for cooling and refrigerating with liquid helium, control systems for maintaining the temperature of superconducting devices, and emer- gency systems for absorbing the sudden power surges that occur if the material suddenly loses its superconducting properties because it warms up. This kind of technology only becomes practical when there has been a sufficient amount of development and engineering work and sufficient experience with big superconducting systems. This is exactly what has been accomplished with the Fermilab superconducting accel- erator. The construction and operation of the 1000-GeV superconducting proton accelerator at Fermilab is the first large-scale use of su- perconductivity in the world. The technology developed for this

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I N TERA CTl ONS WI TH O THER A REA S 1 7 1 accelerator and the experience gained in using it will be useful for other proposed large-scale uses of superconductivity. Some possible appli- cations are listed below: Rotating electrical machinery with superconducting windings; Superconducting high-power electrical transmission lines; Superconducting current-limiting devices for electrical switch- gear; Superconducting magnet energy storage to smooth peak loads; Superconducting coils for separation of materials via their mag- netic properties; Superconducting magnet systems for fusion reactors: Superconducting magnet systems for magnetohydrodynamic power generators; Electrodynamic levitation systems for trains using supercon- . ductiv~ty. SUPPORT AND STIMULATION OF NEW TECHNOLOGY As described in Chapter 6, experiments in elementary-particle physics depend a great deal on the use of integrated circuits. micro- processors, and large high-speed computers. Since the particle physi- cist uses these devices in an experimental situation, it is often possible to use devices that are not yet fully commercially developed. The researcher will often buy devices or computers that are in the proto- type stage in order to have the advantage of using the newest technology. This supports the development of new technology in integrated circuits and in computers. In addition, there is another valuable effect. The research physicist in elementary-particle physics is often well acquainted with the prin- ciples, both physics and engineering, of the new device. Therefore the researcher can often provide information back to the manufacturer about how the prototype device behaves and how it might be im- proved. Thus elementary-particle physics, through providing for early use of new electronic devices, supports the development of new technology and new devices in electronics and computers. Superconducting magnet technology is another example. These magnets as used in the Tevatron and, as proposed for use in the Superconducting Super Collider (SSC), use large amounts of superconducting wire. This has stimulated the superconducting metals industry to develop better and cheaper ways for refining and fabrica- tion.