Generation Of Particle Accelerators Research Articles

Electron-activated carbon diffusion in niobium compounds for rf superconductivity

The higher energies planned for the next generation of particle accelerators and storage rings makes the use of superconducting hi-Q RF cavities highly desirable. Past efforts to produce reliable cavities for such projects have met with limited success. Among the barriers to achieving the maximum electric field gradient are oxide layer charging, single surface multipactoring and field emission. These are suface effects. At SLAC, a multi-technique surface analysis system has been constructed to examine possible sources of these problems and to suggest processes or surface coatings which will reduce or eliminate them. As one component of this analysis, we have investigated the time evolution of species on anodized Nb/sub 2/O/sub 5/-on-Nb surfaces as a function of electron bombardment. The surface concentration of C increases at an anomalously high rate under the exciting electron beam. Examination of the surface and gas phase indicate that the C source is in the underlying material. Estimates of the penetration depth of the beam are in agreement with the fact that there is a significant rate increase in the surface C buildup when the beam penetrates the anodized layer into the Nb bulk. Bulk analytical methods indicate, however, that the C concentration in the Nbmore » is very low. Grain boundary diffusion of C to the surface and/or an enhancement of the C diffusion coefficient due to localized beam pipe heating are examined as possible explanations of this effect.« less

The Hunting of the Quark

THE EXPERIMENTAL STATUS of quarks, hypothetical particles smaller than electron, has excited debate within physics community from 1977 on. The experimental developments that led up to this debate and course debate took up to mid-1979 reveal certain features of experimental method that have received less attention from historians and philosophers than they deserve. In particular quark debate reveals an intimate relationship between experimental practice and theoretical belief that has often been overlooked but has significant implications for our understanding of science. First, a thumbnail sketch of history of In early 1960s, with advent of a new generation of particle accelerators, list of elementary particles-strictly speaking strongly interacting particles such as protons and neutrons, collectively known as hadrons-was growing rapidly. More than two hundred distinct hadrons have now been identified. In 1961 Murray Gell-Mann and Yuval Ne'eman introduced a symmetry scheme, known as the eightfold way or SU(3), which injected order into proliferation of new hadrons by grouping them into multiplets, members of each multiplet having closely related properties. I The success of this scheme has been unchallenged ever since. In 1964 Gell-Mann and George Zweig independently pointed out that this multiplet structure could be understood if one postulated existence of a new layer of matter.2 They suggested that hadrons were not truly elementary particles, but were instead composites of just three subhadronic entities, which Gell-Mann christened quarks. Combinations of quarks were supposed to make up hadrons, just as combinations of hadrons make up nuclei, combinations of nuclei and electrons make up atoms, and so on. The economy of Gell-Mann and Zweig's proposal was striking, explaining an apparently indefinitely rising number of hadrons in terms of only three constituent One question that immediately arose was, and is, could isolated quarks be found? Experimentalists began to search for them in a variety of ways, all predicated upon unusual electric charges which had been postulated for quarks: either one third or two thirds of electric charge of electron (e). Since physicists had hitherto believed that all matter carried electric charge in

The Next Generation of Particle Accelerators

The Next Generation of Particle Accelerators

Probing the Depths of the Nucleus

The physics of objects smaller than an atom was once a single specialty. It was called nuclear physics because its main concern was the nature and structure of atomic nuclei. But in the last decade or so a separation has taken place between the physicists who studied subatomic matter at very high energies, the so-called particle physicists, and nuclear physicists proper. The particle physicists took as their domain the nature and behavior of individual particles, rather than collective entities like a nucleus. They have gone on to higher and higher energies and ever more startling discoveries in their search for the most fundamental constituents of matter. Lately the particle physicists have experienced a certain frustration. They have been having a hard time trying to mate theory and experiment, and many of the objects they have discovered are so ephemeral that some of them are beginning to wonder whether they have anything to do with the structure of stable matter. The structure of nuclei is fundamental to the structure of stable matter, and if it ever becomes well understood, the age-old dream of making the elements one desires instead of depending on what nature gives might be a step nearer. And among the nuclear structure physicists, there is a feeling of hope and an expectation that old frustrations are about to be relieved. The nuclear physicists are building a new generation of particle accelerators, which, they expect, will give them an entirely new dimension of information about nuclear structure. Heretofore their experiments have concerned the nucleus as a whole. Now they want to study the nucleus in more detail. They want to see how small regions of it look and how individual nuclear particles behave and interact with each other within the nucleus. So far they have not had the energy available to get such data. Technology is now allowing them to build the machines that will do it. And those are now being built at several sites in the U.S., as well as in the Soviet Union, Switzerland and Canada. The machines are commonly called meson factories because one of their primary functions is the production of copious beams of pi and mu mesons with which to probe nuclei. In some cases they also yield protons and negative hydrogen ions, hydrogen atoms with an extra electron each. Each of these particles interacts in a different way with the particles in the nucleus, and each gives a different perspective on nuclear structure. Putting the perspectives together, physicists hope, will give a comprehensive picture. The meson factories bring nuclear structure physics into an energy range, hundreds of millions of electron volts, where the physics has not usually been done. The energy is necessary to get the detailed information, but achieving the energy was not the major technological stumbling block in the construction of meson factories; intensity was the problem. The particle beams have to be very intense-contain a large number of particles-to make enough of the desired reactions happen to get meaningful data. Gradual improvements in beam handling techniques have made the management of very intense beams possible, but they still give designers problems with radioactivity. There are unique problems in the handling of intensely radioactive material in targets, says Dr. R. L. Burman of Los Alamos Scientific Laboratory, speaking of the now-building Los Alamos Meson Physics Facility. The solution, he says, is to design a so-called