Casting Light on Material Structures

SSRL is a national facility supported primarily by the Department of Energy for the utilization of synchrotron radiation for basic and applied research in the natural sciences and engineering. It is a user-oriented facility which welcomes proposals for experiments from all researchers. The synchrotron radiation is produced by the 3.5 GeV storage ring, SPEAR, located at the Stanford Linear Accelerator Center (SLAC). SPEAR is a fully dedicated synchrotron radiation facility which operates for user experiments 7 to 9 months per year. SSRL currently has 24 experimental stations on the SPEAR storage ring. There are 145 active proposals for experimental work from 81 institutions involving approximately 500 scientists. There is normally no charge for use of beam time by experimenters. This report summarizes the activity at SSRL for the period January 1, 1991 to December 31, 1991 for research. Facility development through March 1992 is included.

Some of the earliest X-ray Photoemission Spectroscopy (XPS) studies with high-energy X-rays took place in the 1970s as a way of demonstrating the feasibility of using synchrotron radiation from a multi-GeV storage ring to study materials. These developments took place at the SPEAR storage ring at the Stanford Linear Accelerator Center and showed that synchrotron radiation could be tamed but that the technologies of the time would not allow hard X-ray photoemission to become a tool for routine experiments. However, soft X-rays did fulfill that promise and dominated the XPS research for the next several decades with important contributions to the study of surfaces and interfaces. With the advent of second and particularly third generation synchrotron sources, XPS with high-energy X-rays has become a practical tool for the study of bulk properties and buried interfaces. The development of these techniques will be discussed as will the need for both hard and soft X-rays to provide complimentary details on bulk and surface properties of materials.

The availability of synchrotron radiation (SR) to the scientific community has literally revolutionized the way X-ray science is done in many disciplines, including low temperature geochemistry and environmental science. The key reason is that SR provides continuum vacuum ultraviolet (VUV) and X-ray radiation five to ten orders of magnitude brighter than that from standard sealed or rotating anode X-ray tubes (Winick 1987; Altarelli et al. 1998). Although SR was first observed indirectly by John Blewitt in 1945 (Blewitt 1946) and directly by Floyd Haber in 1946 at the General Electric 100-MeV Betatron in Schenectady, NY (see Elder et al. 1947; Baldwin 1975), it took 10 to 15 years before the first systematic applications of SR, which involved spectroscopic studies of the VUV absorption of selected elements (Tomboulian and Hartman 1956) using the 300-MeV synchrotron at Cornell University and of rare gases (Madden and Codling 1963) using the National Bureau of Standards SURF I synchrotron. As of September 2002, there are about 75 storage ring-based SR sources in operation, in construction, funded, or in advanced planning in 23 countries, with 10 fully dedicated SR storage ring facilities in the U.S.. A listing of these sources can be obtained at the following web site: http://www-ssrl.slac.stanford.edu/sr_sources.html . The first SR experiments relevant to low temperature geochemistry and environmental science, although not performed on earth or environmental materials, were X-ray absorption fine structure (XAFS) spectroscopy measurements on amorphous and crystalline germanium oxide conducted on the SPEAR storage ring at the Stanford Synchrotron Radiation Project in 1971 by Dale Sayers, Farrel Lytle, and Edward Stern (Sayers et al. 1971). Prior to the availability of SR in the hard X-ray energy range (> 5 keV), XAFS spectroscopy measurements were impractical because of the high X-ray flux required and the need for a continuously tunable …

Synchrotron radiation is now playing an increasingly important role in recent developments of new light sources. Synchrotron light emitted from a relativistic electron beam has a radiation pattern which makes it a unique source. The advantages with this type of radiation can be summarized as (a) continuous spectrum extending from the ir to the x-ray region, (b) strongly polarized, (c) highly collimated, (d) pulsed structure allowing time-resolution spectroscopy, and (e) high intensity making feasible the use of monochromators with narrow band pass. The Stanford Synchrotron Radiation Project has been in operation since May 1974 as a U.S. National Facility for uv and x-ray research in many disciplines using the radiation from the storage ring SPEAR at the Stanford Linear Accelerator Center. The radiation spectrum is characterized by the critical energy which varies as E3 (E=electron-beam energy) and is 11 keV for E=4 GeV. Useful flux is available out for approximately five times the critical energy. Five monochromators now share the beam run and cover the entire wavelength region from the visible to the hard x-ray region. Several experimental groups are involved in research programs including (a) uv and x-ray photoelectron spectroscopy, (b) optical reflectance and transmission studies, (c) soft x-ray absorption and extended x-ray absorption fine structure (EXAFS) studies, (d) low-angle scattering studies of certain biological materials, and (e) x-ray diffraction studies of protein crystals and other materials.

At DESY the Synchrotron Light Source PETRA III has been extended in the North and East section of the storage ring to accommodate ten additional beamlines. The PETRA ring was converted into a dedicated synchrotron light source from 2007 to 2009. Regular user operation started in summer 2010 with a very low emittance of 1 nm at a beam energy of 6 GeV and a total beam current of 100 mA. All photon beamlines were installed in one octant of the storage ring. Nine straight sections facilitated the installation of insertion devices for 14 beam lines. Due to the high demand for additional beamlines the lattice of the ring was redesigned to accommodate 10 additional beamlines in the future. In an one year long shut-down two new experimental halls were built. The recommissioning of PETRA III started in February 2015. We are reporting the current status of the synchrotron light source including the performance of the subsystems.

The Stanford Synchrotron Radiation Project (SSRP) is now in full operation as a national facility utilizing the intense ultraviolet and x-radiation from the storage ring SPEAR at the Stanford Linear Accelerator Center (SLAC). The experimenter-operated facility is designed to maximize access to, and utilization of the radiation by 5 or more simultaneous users, within the limits of parasitic operation on a high energy colliding beam storage ring. A novel experimenter-controlled personnel protection system permits independent access to each of 5 experimental areas. A vacuum monitoring and control system protects the storage ring vacuum from contamination, rising pressure, or catastrophic failure. The design and operation characteristics of these control systems and of the beam position monitoring and control system, vacuum system and thin beryllium windows are presented.

A fourth generation synchrotron at SLAC?

The x-ray absorption edge spectra of the Cu and Fe-centers in oxidized and reduced cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase: EC 1.9.3.1) have been obtained using synchrotron radiation from the SPEAR storage ring at the Stanford Linear Accelerator Center. In addition, oxidized and reduced plastocyanin as well as a number of model copper compounds in various oxidation states were also examined. A comparison of the absorption edge fine structure of cytochrome oxidase with those of the models indicates that one of the two coopers in the oxidized protein is in the +1 oxidation state. Upon reduction of the protein with dithionite, the second copper becomes Cu(I). The shift in the Fe K-edge of cytochrome oxidase upon reduction is small (about 2 e V or 3 times 10(-19 J) and is comparable to that previously observed for the reduction of the heme iron of cytochrome c.

An X-ray focusing monochromator for small-angle diffraction studies was designed for use with the synchrotron radiation from the storage ring, SPEAR, at the Stanford Linear Accelerator Center. It incorporates a 7 cm long silicon crystal, cut at 8°30′ to the (111) planes and bent to a logarithmic spiral curvature for horizontal focusing and monochromatization. A 120 cm long elliptically curved float-glass mirror is used for vertical focusing, and provides means of eliminating higher-order harmonics of smaller wavelength. With SPEAR operating at 3.7 GeV, 20 mA, the two elements produce a 0.5 × 0.5 mm focused beam with an intensity of 6 × 108 photons s−1. The diffraction pattern of frog sciatic nerve myelin obtained with this system was compared with that obtained with a 300 W conventional microfocus X-ray source and a toroidal camera. The new system shows a 190-fold gain in the integrated intensity on photographic film. Synchrotron radiation provides a broad X-ray spectrum. The monochromator is tunable to any wavelength between 0.5 and 3 A, with a total wavelength spread in the focused beam of about 0.01 at 1.74 A. The broad spectrum allows wavelength selection for anomalous-scattering experiments.

Photoionization and Velocity Map Imaging spectroscopy of atoms, molecules and clusters with Synchrotron and Free Electron Laser radiation at Elettra

The SPEAR storage ring was originally designed for electron-positron colliding beam experiments spanning an energy range of 1-4 GeV per beam. A racetrack lattice was chosen to create two long straight sections in the East and West ''pits''. A modified FODO lattice was chosen for the arcs, with the FODO cells separated by 3.1 m straight sections. Special matching-cell magnet configurations were required on either side of the East and West pits. Electrons and positrons were injected into the ring from the main SLAC linear accelerator (LINAC) at a maximum energy of 2.4 GeV, a level limited by the ratings of the transport line and SPEAR septum magnet. In 1972, SPEAR 1 was commissioned with an operating energy of 2.4 GeV, a level limited by the RF and magnet power supply systems. In 1974, these systems were upgraded for approximately 4 GeV operation. Soon after the SPEAR 2 upgrade became operational, the charmed {Psi}/J-particle was discovered at an energy of 1.5 GeV (per beam). SPEAR continued to operate at or near that energy for colliding beam experiments until the end of the high-energy physics program in the late 1980's. Two Nobel Prizes were awarded for work done at SPEAR: onemore » for the discovery of the {Psi}/J, and another for discovering the electron-like {tau} particle in 1975. The first synchrotron radiation experiments took place in 1973 as part of the Synchrotron Radiation Pilot Project. The first Stanford Synchrotron Radiation Project (SSRP) beam line was commissioned in 1974, operating parasitically with the high-energy physics program. As more beam lines were added, and the experimental program gathered momentum, SSRP evolved into a full-fledged laboratory. In 1977, under the aegis of NSF funding, SSRP became known as SSRL. Two years later, SPEAR operation was divided between the high-energy physics and the synchrotron radiation physics communities. SSRL operated in the 3-3.5 GeV energy range so as to produce a higher-energy photon spectrum, and at currents up to 100 mA (at 3 GeV), a level limited by photon-absorber power ratings. The straight sections in the modified FODO lattice proved fortuitous for the development of synchrotron radiation technology as the first wiggler and undulator insertion devices used for synchrotron radiation experiments were installed in SPEAR 2. In 1982, SSRL became a DOE-funded laboratory. SPEAR was operated by SLAC until the end of the 1980's, with SSRL officially an experimental user of the facility. At the end of the high-energy physics program, SPEAR became a fully dedicated synchrotron radiation source, SSRL became a department within SLAC, and SSRL took over responsibility for SPEAR operations. By the late 1980's, the availability of the SLAC LINAC for SPEAR injection had become limited by the SLAC Linear Collider (SLC) project, which required full-time LINAC operation. In 1991, the problem was resolved by the completion of a dedicated electron injector composed of a LINAC pre-injector and a synchrotron booster. While the booster was designed for 3 GeV operation, the transport line and SPEAR septum magnet ratings still limit the SPEAR injection energy to a maximum of 2.4 GeV, the routine injection energy is now 2.3 GeV. A new SPEAR lattice configuration, which lowered the emittance from 470 nm-rad to 160 nm-rad by altering ring-magnet strengths, was also implemented in 1991. Two previous attempts to reduce emittance, one in 1976 and one in 1985, were unsuccessful because the lattice configurations were not compatible with the two-kicker injection bump then in use. The problem was resolved during the 1991 lattice re-configuration by adding a third electron injection kicker. In 1994, the magnet strengths were altered again, without changing emittance, to reduce the strength of the strong-focusing quadrupole magnets in the East and West pits. Currently, SPEAR operations benefit from an ongoing accelerator improvement program that has increased accelerator reliability and beam quality. However, certain user experiments could profit from improvements in flux, focused flux density, and brightness. To reach the performance level expected of third-generation synchrotron light sources, SPEAR 3 would need to increase photon brightness by at least an order of magnitude. The SPEAR 3 upgrade project will achieve this increase by replacing (1) the existing magnet lattice, (2) the vacuum chamber and (3) the RF system. The new lattice will reduce beam emittance by an order of magnitude. As beam lines are upgraded, stored beam current will increase--first by a factor of two, and later by as much as fivefold. Although beam lines must be modified to accept higher power densities, the majority of the beam line infrastructure (including insertion devices) will be reused for SPEAR 3 to minimize the total downtime.« less

It is well known that when charged particles are accelerated they emit electromagnetic radiation. For example, when electrons traveling at nearly the speed of light are forced to move along a curved path by the magnetic fields of a storage ring, they emit photons into a narrow cone. Depending on the electron energy, this so-called synchrotron radiation can be extremely intense over a broad wavelength range extending all the way from the microwave to the x-ray spectral region. Because this radiation also tends to be vertically collimated and polarized, it has been used as a powerful research and development tool in chemistry, physics, material science, biology and medicine, and, in the last decade or so, has played a key role in the development of EUVL. A drawing of a typical synchrotron radiation source built around a storage ring is shown in Fig. 32.1. The storage ring, a many-sided doughnut-shaped tube, enables a current of electrons to circulate at essentially the speed of light in a closed orbit for many hours, all the while emitting synchrotron radiation. The electron beam decays slowly because of electron momentum loss during collisions with residual gas molecules, electron-electron collisions, and photoemission. If the momentum loss is too high, the electron is lost from within the storage-ring acceptance. To reduce electron-molecule collisions, the inside of a storage ring is maintained at extremely low pressure (in the range 10−9 to 10−10 Torr). Changing the electron beam's size can change the electron-electron collision rate, but the photoemission process is inherent in the quantum nature of photoemission. A system of magnetic lenses, bending magnets (dipoles), focusing magnets (quadrupoles), and steering magnets (sextupoles), strategically located around the ring, guides and focuses the electron beam. This so-called lattice determines the basic features of the circulating beam, such as its emittance and its transverse dimensions, and also determines the number, length, and location of any straight sections in the ring available for insertion magnets: devices that provide radiation with enhanced flux, brightness, and spectral range. The energy lost by the circulating electrons to synchrotron radiation is replenished by longitudinal electrical kicks imparted to the beam as it transverses one or more radio frequency (rf) cavities incorporated in the ring. The rf power in these cavities chops the circulating electron beam into bunches and hence determines the duration of the synchrotron radiation pulses; its frequency determines the minimum bunch spacing. The storage ring is filled with electrons from a source called an injector, e.g., a linear accelerator fed from an electron gun, a microtron, or a small booster ring fed by a linac or a microtron. Some of the synchrotron radiation from the bending magnets and most of the radiation from the insertion magnets (wigglers and undulators) escapes the storage ring through tangential ports, called beamlines, that allow the radiation to pass to experimental end stations located at convenient distances from the ring.

Utilization of a channel-cut double crystal X-ray spectrometer with vertical dispersion in synchrotron radiation experiment

The National Synchrotron Light Source II, completed in 2014, is a 3-GeV synchrotron radiation (SR) facility at Brookhaven National Laboratory and has been in steady operation since. With a design electron current of 500 mA and subnanometer radians horizontal emittance, this 792-m circumference storage ring is providing the highest flux and brightness x-ray beam for SR users. The majority of the storage ring vacuum chambers are made of extruded aluminium. Chamber sections are interconnected using low-impedance radiofrequency shielded bellows. SR from the bending magnets is intercepted by water-cooled compact photon absorbers resided in the storage ring chambers. This paper presents the design of the storage ring vacuum system, the fabrication of vacuum chambers and other hardware, the installation, the commissioning, and the continuing beam conditioning of the vacuum systems.

Synchrotron light sources have undergone three generations of development in the last two decades. The National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory has two "second generation" storage rings that currently provide the world's most intense sources of photons in the VUV and X-ray spectral ranges. There are almost 90 beam lines serving a community of 2600 scientists from 370 institutions. They are engaged in basic and applied research in physics, chemistry, biology, medicine, materials science and various technologies. When design of the NSLS began in 1977, emphasis was given to the stability of the concrete slab on which the storage rings and experimental beam lines were placed. Stability is the result of controlling: . vibration from sources internal and external to the building, . thermal effects of air and water temperature variations, . foundation settlement and contact between the slab and underlying subsoil. With the advent of new research where highly focused beams of x-rays must be placed on increasingly smaller targets located 35 meters or more from the source, and the development of x-ray lithography with resolutions approaching 0. 1 micron at chip exposure stations, even greater attention to stability is required in building designs.