Accelerator-Detector Technology and Benefits to Society
From the early scattering experiments of Rutherford to the colliding beam experiments that produced the top quark with a mass almost as large as Rutherford's gold nucleus, particle beams have been the mainstay of elementary particle physics. The tools described in Chapter 6 are among the most technically sophisticated in the world. Elementary-particle physicists are motivated to develop and use these tools by a deep desire to understand how the world works at its most basic and fundamental level. These tools can, however, and often do find application beyond the restricted realm of elementary-particle physics research. This chapter discusses a few examples of the application of elementary particle physics technologies to the more general benefit of society in areas as diverse as biology, medicine, microelectronics, and national defense.
In elementary-particle physics as in most fields of science, advances in understanding are closely coupled to advances in technology. Many technical obstacles exist in the search for fundamental physics, and much of the creative effort of elementary-particle physicists, both experimental and theoretical, is devoted to overcoming these obstacles: Higher-energy accelerators are needed to cross thresholds for suspected new phenomena, and machines of higher luminosity open opportunities for the observation of rare and unexpected processes. Advances in accelerator technology must be accompanied by advances in detector technology as more complicated particle collisions are produced. Further advances in computing technology are necessary both to enable the processing
of complex data samples and to allow comparison between various theoretical models and experimental results.
Experimenters have often made great advances when they have been able to borrow a technology developed for other uses and modify it to allow advances in particle physics. Similarly, a technology has often been developed specifically to address the needs of the elementary-particle physics community but then has been adapted to meet the needs of society outside particle physics. Although it is often evident where the technical barriers in elementary-particle physics (EPP) reside, it is much more difficult to predict where the breakthroughs will be and how they will come about. Experience has shown, however, that innovative new technologies or innovative uses for existing technologies will find surprising applications beyond those originally conceived by the developers.
THE MACHINE FRONTIER
The development of particle accelerators has led to new tools for basic research, medicine, and industry. Synchrotron radiation, the bane of elementary-particle physicists in their quest for ever-higher energy electron accelerators, is now used in cutting-edge research in the materials sciences and in studies of biological systems. Accelerator-generated proton beams produce pulses of neutrons when they strike high-atomic-weight targets; these neutron sources play an important role in understanding the chemistry and physics of materials. Low-energy proton and pion beams, having served the needs of the elementary-particle physics community 40 years ago, are now used routinely in medical diagnostics and therapy. Cyclotrons, whose technology was first developed in the 1930s, now find medical application in the production of isotopes used for positron emission tomography (PET).
Industry employs particle beams for ion implantation in semiconductor devices, sterilization of materials, and x-ray lithography via synchrotron radiation. Using intense proton beams impinging on a target to produce neutrons can be a safer alternative to nuclear reactors. Proposed applications include the production of tritium, the destruction of plutonium and other high-level radioactive waste from nuclear weapons production and nuclear power plants, and even energy production by initiating fission in a subcritical reactor. National security is strengthened by current research into accelerator sources for explosives and contraband detection, neutron and proton radiography, and weapons effects simulations.
SYNCHROTRON RADIATION: USING X-RAY LIGHT TO SEE THE WORLD IN ATOMIC DETAIL
As described in Chapter 6, electrons, when forced to travel in circles, lose energy through the mechanism known as synchrotron radiation. For electron
energies of a few GeV (109 electron volts), this radiation is in the form of x rays. The need to replace the energy lost to radiation is a limiting factor in the design of high-energy circular electron machines. However, the x-ray radiation produced by electrons as they travel in their circular orbits can be used to probe the macroscopic structure of matter.
Circulating beams of electrons produce the most intense beams of x rays in the world. Early in the development of electron storage rings, facilities such as the Stanford Synchrotron Radiation Laboratory (SSRL) and the Cornell high-energy Synchrotron Source (CHESS) were built to take advantage of the synchrotron radiation produced as a by-product of storage ring operation for High-Energy physics research. Now, newly constructed dedicated facilities at Argonne, Brookhaven, and Lawrence Berkeley National Laboratory use high-current stored electron beams to provide synchrotron radiation for many researchers studying a tremendous variety of problems in different areas of science, including geology, biology, chemistry, physics, materials, and environmental sciences.
The high-energy of the x-ray photon gives it both its characteristic wavelength of about I angstrom (Å; 10−10 m) and the ability to probe the inside of dense matter. Since 1 to 2 Å is the typical distance between the atoms and molecules that compose our world, x rays have proven to be the most appropriate type of light for revealing the smallest details about our chemical and physical surroundings. Much of the work done at synchrotron facilities utilizes x rays to determine the structure and function of a material down to its finest details, usually by locating individual atoms. The small but intense x-ray beam is especially important for illuminating small specimens. Compared with a typical laboratory x-ray source, a millimeter-sized specimen might be exposed to from one to a hundred million times more photons at a synchrotron facility. As scientists study ever smaller specimens or smaller features of large specimens, the need for intense, collimated synchrotron radiation continues to grow.
The semiconductor industry, for example, has a developing need for synchrotron radiation. Because the sizes of wires and junctions on an integrated circuit are now smaller than the wavelength of visible light, standard microscopic methods are no longer sufficient for viewing or characterizing devices. Synchrotron radiation is used to study structures on silicon surfaces measuring one-millionth of a millimeter (Figure 8.1 ). From these studies, one gains understanding of a variety of processes that contribute to the production of a semiconductor device.
As a specific example, the use of x rays has helped refine several steps in the method of growing gallium nitride, a rare, wide-bandgap semiconducting material that produces blue light. At present, invention of the blue-light laser is a top priority in the international materials community because it will have an enormous technological impact. This impact stems from the fact that blue light has a shorter wavelength than the current generation of red-light-emitting diodes. Thus, a blue-light CD-ROM, for example, will be able to read and write smaller
bits of information and thereby lead to an increased storage capacity of at least a factor of 4 relative to current technology.
In a different field of study, biologists have fully realized the efficacy of using synchrotron x rays to study large molecules of physiologically important materials such as proteins and viruses. Because these huge molecules have many thousands of atoms, a technique called macromolecular crystallography makes use of digital detectors and computers to collect and analyze rapidly vast quantities of x-ray diffraction data. Two examples of this research are the determination of the structure of the mammalian rhinovirus HRV14, which causes the common cold, and the determination of the structure of HIV reverse transcriptase (RT) type 1 (Figure 8.2, in color well following p. 112).
These research projects represent a subset of a larger effort on the part of the biological and biochemical communities to use x rays on a routine basis for determining the structure of living materials. Revealing their structure is just the first step in the process of understanding how proteins and viruses function. The
most exciting and rewarding possibility then follows: By understanding the chemical structure and function of a virus, scientists hope to be able to design a drug that will inhibit the action of disease-causing agents. Although this field of so-called structure-based drug design is relatively new, it has already established itself as a nationwide (and worldwide) program, with a need for continuing access to synchrotron x-ray facilities.
One hundred years after the discovery of x rays, it is becoming evident that they can be more than just a tool to visualize atomic detail. The present trend toward ultraminiature design, exemplified by the rapid evolution of the integrated circuit, is creating the need to manufacture products with physical features smaller than the wavelength of visible light. A technique called x-ray lithography is just beginning to create micromechanical devices, machines with moving parts having dimensions as small or smaller than the width of a hair. It is easy to imagine that in the near future, x rays not only will play an increasingly important role in research and development but will have a significant impact on manufacturing as well.
SCIENCE AND INDUSTRY IN A PARTNERSHIP DOWN TO THE WIRE
Beginning in the late 1970s, Fermi National Accelerator Laboratory (FNAL) used thousands of spools of superconducting cable to build the magnets of the Tevatron, the world's first superconducting synchrotron. The use of superconducting magnet technology allowed a doubling of the magnetic field and a concurrent doubling of the energy. In the process of developing this technology, Fermilab brought experts in superconductivity together with physicists, engineers, materials scientists, and manufacturers of alloys, wire, and cable in a collaboration that helped boost the then-infant superconducting technology industry to a full-grown role in the billion-dollar world market created by magnetic resonance imaging.
Although ''technology transfer" suggests the orderly flow of information from the laboratory bench to the factory to the marketplace, like all real-life human endeavors, technology transfer is more complicated and less orderly. In this instance, Fermilab's advanced research in high-energy physics required large quantities of superconducting wire and cable, creating overnight a market for products that did not yet exist. It was this "demand pull," rather than a particular new discovery, that launched technological development and large-scale production up a steep learning curve. Meeting this demand created an industry with the capability to supply a commercial market, driven by the important new medical diagnostic tool called magnetic resonance imaging (MRI), which had not been foreseen at the start of Fermilab's research.
To build the Tevatron, Fermilab used 135,000 pounds of niobium-titanium based superconducting wire and cable between 1974 and 1983. At the project's
start, the annual world production of these materials was a few hundred pounds. After Fermilab created the collaboration to develop large-scale manufacturing techniques, capacity grew so that today the annual production is in excess of 300,000 pounds, about half of which finds commercial application, principally for MRI.
The development of superconducting wire showed that science and industry could find common ground where both could thrive. Successful companies in the collaboration had the motivation to innovate, experiment, and invest resources to supply the materials to build the world's highest-energy particle accelerator, and in doing so, they built a new industry. Elementary-particle physicists did not invent MRI, but they did push superconducting technology out of the nest so that when MRI came along, the industry was ready to fly. The future can never be foretold completely, but indications are that superconducting technology has just begun to soar. In the words of the late Robert Marsh of Teledyne Wah Chang, still the world's largest supplier of superconducting alloys, "Every program in superconductivity that there is today owes itself in some measure to the fact that Fermilab built the Tevatron and it worked."
THE DETECTOR FRONTIER
The particle detectors utilized by elementary-particle physicists have evolved in sophistication over the past 50 years, providing the ability to investigate phenomena at continually advancing energy and intensity frontiers. Requirements for more precise spatial resolution, higher rate capability, and the ability to function in very high radiation environments stimulate the particle physics community to develop new and improved techniques. The evolution of detector technologies in support of elementary-particle physics has also led to advances in other fields such as nuclear and atomic physics and has found fertile areas of application in industry and medicine.
Tracking detectors date back to the invention of the cloud chamber in 1911 and the bubble chamber in the 1950s. Since these early years of elementary particle detectors, the tracking chamber has evolved considerably. The multiwire proportional chamber (MWPC) is a good example of a device that was invented and perfected for physics research and has had great impact outside the field. Georges Charpak (1992 Nobel laureate in physics for his detector inventions and their impact on science) invented the MWPC in 1968. The versatility of proportional chambers has led to many applications in medicine, materials science, and biochemistry.
Driven by the need to measure the energy and position of electrons and photons with very high precision, elementary-particle physicists pursued the development of bismuth germanium oxide (BGO) in a partnership with industry dating back to the early 1980s. During the past 10 years, the need for even higher spatial resolution and the desire to construct physically smaller detectors
have led physicists to search for suitable alternative crystalline materials. A new development, lead tungstate crystals, is under way under the auspices of the Compact Muon Solenoid (CMS) experiment, which is under construction for the Large Hadron Collider project at CERN (the European Laboratory for Particle Physics). Industrial participation in this and other developments is not based solely on vendor expectation of an immediate financial payoff but also, and perhaps more importantly, on some form of secondary payoff. This could be from the potential for an extended market for the product or simply from the advantages of an enhanced reputation as a high-technology company.
The needs of several modern medical imaging techniques overlap the requirements of elementary-particle detectors described above in significant ways. A variety of techniques have been developed for nonsurgical imaging inside the body, such as computerized axial tomography (CAT), single photon emission computerized tomography (SPECT), and positron emission tomography. These and other medical diagnostic procedures depend on radiation detectors that have good spatial and energy resolution. Typically, short-lived radioactive isotopes are ingested by the patient in the form of drugs. The decay of these isotopes is then detected outside the body by detectors such as MWPCs. Detectors with sufficient spatial resolution can observe internal features of the body to a precision of several millimeters. Crystals, such as those described above, do an excellent job of measuring energy but cannot currently provide spatial resolution at the millimeter level, as required in medical applications, at an affordable cost.
An exciting new development in medical imaging is an outgrowth of the development of polarized gas targets for high-energy physics experiments. Based on a technique called "spin-exchanged optical pumping" targets of highly polarizable helium gas were developed in the 1990s for use in EPP experiments at the Stanford Linear Accelerator Center (SLAC) and at DESY in Hamburg. This technique has recently found application in a new MRI technique. In this case, the gas xenon is inhaled by a patient and, because of its enhanced polarizability relative to normal body tissue, an extremely high-resolution image of the gasfilled region can be obtained. Figure 8.3 shows a picture of a human lung, with a resolution far beyond that achievable by conventional MRI, produced using this technique. Other applications based on this new technology are being vigorously pursued in the medical community.
THE COMPUTING FRONTIER AND ELEMENTARY-PARTICLE PHYSICS
Elementary-particle physicists have made prompt and extensive use of the latest advances in computing for many decades and have contributed in important ways to the advancement of computing. Today, computing pervades the field with advances in detection, data acquisition, networking, and data analysis capabilities tied closely to the most advanced technical developments in the
computing field. Historically, elementary-particle physicists have created vast amounts of software for their research, with some having an impact on the world outside the field, for example, the invention of hypertext programming language and the World Wide Web.
The relationship between the elementary-particle physics community and the computing industry has evolved over the past 20 years. During the early years of the computer revolution, elementary-particle physicists were, by necessity, very creative in developing the computer applications and tools required to support their research. A prime example is the development of massively parallel processor "farms." To allow rapid analysis of events recorded in their experiments, physicists at particle physics labs started to build stripped-down (no monitor, no disk) computing engines. Vast arrays, or farms, of these computers were set to work on data reduction from experiments. Today, workstations and even
PCs are available in inexpensive configurations that are sufficiently powerful to serve as farms. This strategy, in one form or another, is being adopted (or rediscovered) anywhere that numerous similar computing tasks are required. Examples include statistical simulation (Monte Carlo) problems in other sciences, database searches in almost any field imaginable, and theoretical calculations as described in Chapter 9.
Experiments in high-energy physics are now conducted by worldwide collaborations, requiring wide-area coordination of data handling and rapid communications. These social structures have made rapid use of the evolution of widearea networks and effective new software standards. The need for effective networking facilities is particularly important to American scientists participating in experimental collaborations in Europe and Japan, but it clearly is an issue within the United States as well. It is, therefore, no surprise that physicists have been hooked up to the Internet and similar computer networks since these networks began. The general public, on the other hand, first began to appreciate the wonders of global computer communication in the mid- 1990s. The ensuing popular explosion of the Internet can probably be attributed primarily to the development of a comfortable, universal hypertext interface for exchanging text, graphics, and images. The World Wide Web was invented at CERN to enable the organization and exchange of information between particle physics collaborators at locations all over the world.
As the information revolution picked up steam over the past 10 years, scientists were able to rely to a greater extent on industry to develop the computer tools needed to support elementary-particle physics research rather than having to develop these tools themselves. However, industry still finds the EPP community a valuable resource for testing innovations and future directions. The
physics community provides an unquenchable demand for and knowledgeable evaluation of the technology. Computing technology will continue to advance rapidly, and elementary-particle physicists can be expected to apply the latest innovations to expand the reach of their discoveries.
TECHNOLOGIES FOR THE NEXT 20 YEARS
Continued advancement in the capabilities of the accelerators and detectors used in support of elementary-particle physics research is critically dependent on continuing advances in the underlying technologies. Although many of these advances are directly pursued in the laboratories and universities carrying out research in EPP, others occur independently as a result of research in other fields or in industry and are adopted by experimental particle physics. Likewise, developments in the technologies supporting experimental particle physics often find wider application in other areas of research or to the benefit of society at large. Although it is not possible to predict what technologies developed in support of experimental particle physics will impact the broader scientific research community and society in this country over the next 20 years, it is possible to indicate the technologies in which experimental particle physics is likely to invest considerable effort.
Superconductivity is the enabling technology for hadron accelerators and colliders. There is every reason to believe that the elementary-particle physics community will continue to pursue vigorously development in this field, often in close collaboration with industry. Recent rapid advances in the development of high-temperature superconductors give confidence that the development of a viable accelerator magnet, based on these superconductors, is likely in the first decade of the next century. Such a development could well parallel the first large-scale use of superconducting power distribution in this country and the enhanced availability of various superconducting-based medical devices. Although experimental particle physics will by no means be the sole driver for such a development, its participation will be significant.
Microwave power sources and accelerating structures represent the enabling technologies of electron colliders. Linear collider development can be expected to drive reductions in the fabrication and operating costs of such devices. Recent developments in periodic permanent magnet focusing klystrons could result in more affordable power sources and a corresponding increase in applications. Studies currently under way in the synchrotron radiation community are examining the possibility of utilizing the SLAC linac as a source of electrons for the world's first x-ray free-electron laser. Such a facility would provide significant new opportunities to the biomedical and materials research communities.
Detector development will continue to rely on increases in resolution and bandwidth. Pixel detectors will likely be required to support research at the very high luminosities contemplated at the Large Hadron Collider (LHC). Data trans-
mission rates approaching 1 GB/s will be required. Distributed computing and extensive use of microprocessors will form the basis of both triggering and data analysis systems. Enhanced pattern recognition techniques and software algorithms will be required to sort out signatures in detectors recording tens of simultaneous events with hundreds of tracks each. What applications, if any, these developments will find in other fields is impossible to predict, but based on history it is safe to say that many will find application in areas beyond experimental particle physics.