Synchrotron Radiation Sources To Provide New Probes of Matter


Synchrotron Radiation Sources To Provide New Probes of Matter...

1 downloads 45 Views 632KB Size

SCIENCE

Synchrotron Radiation Sources To Provide New Probes of Matter Lawrence Berkeley's device will produce radiation in vacuum ultraviolet and soft x-ray regions, Argonne's in hard x-ray region Rudy M. Baum, C&EN San Francisco

Two new synchrotron radiation sources now in the early stages of being built promise to provide sci­ entists in numerous fields with pow­ erful new probes of matter. The Advanced Light Source (ALS) at Lawrence Berkeley Laboratory, Berkeley, Calif., and the Advanced Photon Source (APS) at Argonne National Laboratory, Argonne, 111., are billed as "third-generation" light

sources in that they are dedicated to the production of synchrotron radiation and designed for use of devices—known as "wigglers" and "undulators"—that vastly increase the brilliance of the synchrotron ra­ diation produced. The two machines are comple­ mentary. The ALS, with an electron beam energy of 1 to 2 Giga electron volts, will produce radiation in the vacuum ultraviolet and soft x-ray regions of the spectrum, and the APS, with a positron beam energy of 7 GeV, will produce radiation in the hard x-ray region of the spec­ trum. These two different types of radi­ ation will provide the ability to per­ form different types of experiments. The APS, for example, will produce light particularly well suited for

x-ray diffraction studies; radiation from the ALS, on the other hand, will allow, for example, the combi­ nation of x-ray microscopy and spec­ troscopy with unprecedented spa­ tial and energy resolution. "I think we are on the threshold of a real revolution in the quality of radiation sources available to re­ searchers/' says LBL director David A. Shirley of the ALS and APS ma­ chines. He likens the revolution he foresees to the one that occurred in the scientific use of optical radia­ tion ushered in by the development of lasers. "I think these sources are going to play the role in the 1990s that lasers played in the 1970s," Shirley says. Over the past 40 years, synchro­ tron radiation has evolved from a nuisance faced by high-energy phys-

Advanced Photon Source produces hard x-rays Electron gun

Electron linear accelerator

Positron target

Positron linear accelerator

Booster synchrotron

ΈΖΖΙΆX-rays

Storage ring Insertion devices

22

September 19, 1988 C&EN

icists to an important research tool utilized by scientists in a variety of fields. Whenever a charged particle is accelerated, it gives off electromagnetic radiation. At relativistic speeds, the radiation produced by such particles increases enormously in intensity and directionality because it is emitted over a very narrow angle. Synchrotron radiation spans the electromagnetic spectrum, but it is by far the most powerful source of radiation in the vacuum ultraviolet to hard x-ray regions of the spectrum. Thus research requiring these wavelengths of radiation—ranging from 1 to about 100 A—has come to rely heavily on synchrotron radiation facilities. Synchrotrons are circular accelerators that bring electron beams to high energy; storage rings maintain such a high-energy electron beam for a long period of time. Both use arrays of two types of magnets: focusing magnets to keep the beam traveling in a narrow path, and bending magnets to force the beam to arc around the circular chamber of the accelerator. Whenever the electron beam is bent by a bending magnet, synchrotron radiation is produced. The stronger the magnetic field of the bending magnet, the higher the energy of the most energetic photons produced. High-energy physicists want to produce the highest energy electrons possible. Synchrotron radiation, is an annoyance in such a pursuit in that it is an unavoidable drain on the electron energies that can be achieved. The first s y n c h r o t r o n light sources operated essentially as adjuncts to the high-energy physics being conducted at the accelerator. An example is the Stanford Synchrotron Radiation Laboratory (SSRL), a highly successful facility at which synchrotron radiation is produced by the 4-GeV storage ring, SPEAR, and the 15-GeV storage ring, PEP, operated by the Stanford Linear Accelerator Center in Palo Alto, Calif. In general, SPEAR is dedicated to the production of synchrotron radiation during 50% of its operating time, or about four months per year. The remainder of the time, synchrotron radiation may be used by re-

THE ROBERT A. WELCH FOUNDATION CONFERENCE ON CHEMICAL RESEARCH XXXII VALENCY October 31-November 2, 1988 The Westin Oaks Hotel, Houston, Texas PROGRAM 9:00 9:05

9:15

10:45

1:45

3:00

9:00

10:30 12:00 1:00

2:15

9:00

10:30

Monday, October 3 1 , 1988 JACK S. JOSEY, Welcoming of Guests W. O. BAKER, Introductory Remarks PRINCIPLES OF ELECTRON FUNCTION IN COMPOUNDS AND CRYSTALS P. W. ANDERSON Discussion to be led by JEREMY K. BURDETT CONCEPTS OF CHEMICAL BONDING: APPLICATION TO SYSTEMS OF MATERIALS AND BIOLOGICAL INTEREST WILLIAM ANDREW GODDARD III Discussion to be led by JOHN A. POPLE VALENCY IN MOLECULES AND CLUSTERS LOUIS E. BRUS Discussion to be led by WILLIAM P. SLICHTER VALENCY AND CHARGE DISTRIBUTION IN ALKALIDE AND ELECTRIDE SALTS JAMES L. DYE Discussion to be led by FRANCIS J. DISALVO, JR. Tuesday, November 1, 1988 VALENCY IN STRANGE DIMENSIONS DUDLEY HERSCHBACH Discussion to be led by HARRY G. DRICKAMER VALENCY CONSIDERATIONS IN NOVEL INORGANIC STRUCTURES ARTHUR W. SLEIGHT Discussion to be led by RICHARD E. SMALLEY Luncheon RICHARD B. BERNSTEIN, 1988 WELCH AWARDEE VALENCY AND ELECTRON TRANSFER IN BIOLOGICAL SYSTEMS: BIRDS DO IT, BEES DO IT GEORGE L. MCLENDON Discussion to be led by NORMAN SUTIN Wednesday, November 2, 1988 VALENCE FORMATION IN CHEMICAL DYNAMICS RICHARD B. BERNSTEIN VALENCY IN THE PERIODIC TABLE—CONCERNING THE LIMITS OF OXIDATION OF THE ELEMENTS NEIL BARTLETT Discussion to be led by KONRAD SEPPELT

ADVANCE REGISTRATION FORM . I will attend the conference. . I will attend the complimentary luncheon on Tuesday, November 1, 1988. . Please add my name to The Robert A. Welch Foundation mailing list. Dr. Mr. Mrs. Ms

{PLEASE PRINT OR TYPE)

(Last)

(First)

(Middle)

Position _ Organization. Address

Advance registrations will be acknowledged and accepted in order of their receipt, to within the capacity of the available space. Make your hotel reservations directly with The Westin Oaks Hotel, Telephone No. 1-800-228-3000 or 713-960-8100 x6990, prior to October 14, 1988. Please return by October 14 to:

T h e Robert A. W e l c h Foundation 4 6 0 5 Post O a k Place, Suite 2 0 0 Houston, T e x a s 7 7 0 2 7

September 19, 1988 C&EN

23

Science searchers in a "parasitic" mode during colliding beam runs for highenergy physics experiments. Operation on PEP is generally parasitic. Second-generation synchrotron radiation facilities, typified by the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory in Upton, N.Y., are dedicated to producing synchrotron radiation. At NSLS, two electron storage rings, one operating at 750 MeV for producing vacuum ultraviolet radiation and one operating at 2.5 GeV for producing soft x-rays, accommodate about 900 scientists working on more than 50 beam lines. The numerous beam lines at synchrotron radiation facilities are possible because the electron beam in a storage ring must be forced to bend by b e n d i n g m a g n e t s n u m e r o u s times in its transit of the ring. Although the storage ring and its support facilities are characteristic of the large-scale scientific endeavors physicists have grown accustomed

to, the experiments carried out on each beam line often resemble those in a typical university chemistry laboratory. "A big science machine for the little science community , / is what one Argonne scientist calls the APS. "Clearly, results can be obtained by a single investigator. Experiments will not require a team of 100 people the way high-energy physics experiments require such teams," he says. Like the NSLS, both the ALS and APS will be storage rings dedicated to producing synchrotron radiation. What sets them apart from previous synchrotron radiation facilities is their optimization for the use of wiggler and undulator insertion devices in a quest for the highest possible radiation brilliance or brightness, which are similar measures that take into consideration not only the flux of photons from a source but, in a sense, how "concentrated" those photons are.

Organic Intermediates through Ishihara Sangyo's Advanced Technology;

NEW PYRIDINE DERIVATIVES P-021 CF3

ciT^s\.ci

P-022

a

c,

fy'

C/-^\

Ν

2.3.5.6,4-TCTF

2,3.4,5.6-TCTF

P-024

P-025

P-023 CF3

Τ 4 Λif

F

F

f^

CF3

F

P-026 CF.,

f\ ISK

24

Ml

Ν

Ν

2.3.4,5,6-TFTF

2.6.4-ACTF

P-027

P-028

Ν

2,3,5,6.4-TFTF

^CF:,

Ν

ISHIHARA SANGYO has added the above new compounds to its product line. Although these new developmental compounds are only in the research stage, small samples can be prepared for research and development purposes. We wish to make the best use of our processes to assist in the synthesis oï your products.

CF.

rii

Ν

Ν

2,6.4-DATF

2.4-ATF

FsCv^MlMH,

U 3.5-ATF

ISHIHARA SANGYO KAISHA, LTD. 10-30, Fujimi 2-Chome, Chiyoda-ku, Tokyo, Japan, Telex:2324306 ISK J I S K ( E U R O P E ) S . A . 33 31 Rue Montoyer, Box 8 1040 Bruxelles Tel:(02)512-54-50-512-72-78 Telex:23362 ITDD Β I S H I H A R A C O R P O R A T I O N ( U . S . A . ) Transamerica Pyramid 42nd Floor 600 Montgomery Street, San Francisco, Ca. 941 1 1 Tel:415-421-8207 Telex:278010 ICUSA UR

September 19, 1988 C&EN

Technically, photon flux is the number of photons over a given en­ ergy bandwidth per second. Bright­ ness, expressed in photons per second-milliradian 2 , defines the an­ gular density of the photon flux. Brilliance, expressed in number of photons per second-mm 2 -milliradian 2 , is the photon intensity emitted in the unit phase-space volume of the radiation field. Spectroscopists and other scien­ tists who use radiation in their re­ search increasingly appreciate the importance of brightness, says LBL's Shirley. "You get a lot of photons out of a candle, but a laser beam occupies a very small area of phase space. Therefore you can bring the photons from a laser to bear on your sample. That is the essence of why the ALS and APS are going to be so readily usable," he says. Synchrotron radiation naturally possesses high brilliance because the emission is highly directional. In­ sertion devices at the APS and ALS will increase the brilliance of exist­ ing synchrotron radiation sources by four to five orders of magnitude, according to Yanglai Cho, APS as­ sociate division director, who is in charge of the facility's accelerator systems and design. Insertion de­ vices use arrays of permanent mag­ nets to bend the electrons along an undulating path. This causes light emitted by the electrons to be com­ pressed along the axis of the elec­ trons' motion. Radiation from undulators installed in the ALS and APS will have brilliance on the or­ der of 1018 to 1019 photons per second-mm 2 -milliradian 2 , Cho says. Wigglers and undulators differ in the amount each device deflects the electron beam from its path. Wig­ glers deflect the beam through an angle that is large compared with the natural emission angle of syn­ chrotron radiation. A wiggler pro­ duces a continuous spectrum that is similar to that produced by a bend­ ing magnet of similar field strength, but with a flux and brightness that exceed that produced by a single bending magnet by a factor propor­ tional to the number of magnetic poles of the wiggler. An undulator deflects the electron beam by a much smaller amount, comparable to that of the natural

Cho: four- to fivefold rise in brilliance emission angle. This produces radiation that emerges in a very narrow cone and, because of interference effects, sharply peaked at certain wavelengths and suppressed at others. Undulator radiation possesses significant coherence. An undulator is similar, in fact, to a free-electron laser. An undulator increases the brightness of radiation at specific wavelengths over that of a bending magnet of similar magnetic field strength by a factor proportional to the number of magnetic poles of the device squared. The wavelengths of light from an undulator can be tuned by mechanically opening or closing the gap between the undulator faces. Prototype wigglers and undulators have been developed and installed at existing synchrotron radiation facilities. However, those facilities do not have relatively long, straight sections incorporated into their storage rings to accommodate insertion devices of optimal size. Insertion devices impose constraints on the design of the storage ring dedicated to synchrotron radiation because they require such straight sections of beam. The 3500foot circumference of the APS, for example, is much greater than that required for a 7-GeV storage ring, Cho points out. This circumference

will accommodate 40 straight sections, 35 of which will be dedicated to insertion devices. The APS, which is scheduled for completion in 1995, will cost about $456 million to build. Operating costs likely will run around $50 million per year, depending on how many beam lines ultimately are built. Those beam lines—up to as many as 100, most of which will be financed by outside consortia of users—probably will add another $300 million to the final cost of the facility. One high-ranking scientist close to the APS project says he is confident of continued funding support for the APS. "My perception is that Congress and other key figures in Washington believe that this is a good project," he says. "It will have an impact both in the basic research community and in the industrial sector." Despite confidence that the APS will be built, however, in the current tight budget climate the project could experience delays due to decreases in annual budget appropriations. The ALS, which is scheduled for completion in 1992, will cost about $99 million to build. By the end of the current fiscal year, about $20 million already will have been spent on the project, according to Shirley,

Marx: distribution of beam line time

Beno: revolutionize x-ray diffraction and another $25 million has been included in the 1989 fiscal year budget. Funding for the ALS "is very secure at this point," Shirley says. As at the APS, most beam lines at the ALS will be built by the users. Up to 60 beam lines, 11 from insertion devices, will be built at the ALS. Three types of insertion device teams (IDTs) will be created at the ALS, w i t h the designation depending on whether the ALS or the IDT pays for the insertion device and beam line. Different IDT types will have different access to the beam line. The goal, according to ALS project director Jay Marx, is io achieve the most equitable and productive distribution of beam line time between various users. At one level, of course, it is as impossible to predict the impact of the ALS and APS on chemistry, physics, and biology as it would have been to predict in 1960 the impact lasers would have over the next 25 years on those same areas of science. Lasers have changed the face of science, but in 1960 many of the experiments lasers would make possible had simply not yet been thought of. Shirley, who in addition to his position at LBL is a physical chemistry professor at the University of California, Berkeley, points out that September 19, 1988 C&EN

25

Science the potential of synchrotron radiation often does not manifest itself completely until a scientist begins applying it in his or her research. Shirley began using synchrotron radiation at SSRL in the development of x-ray photoelectron spectroscopy in the mid-1970s. He says that the experience resulted in a dramatic change in the way he thought about his research in terms of the types of experiments he considered carrying out. On the other hand, Shirley is quick to note, the design specifications of the ALS make clear many of the directions researchers will pursue at the facility. Numerous user groups have conducted workshops over the past two years to delineate research priorities at the ALS. One such workshop on the chemical applications of undulator radiation concluded that the short pulse duration, high brightness, variable polarization capability, and the possibility of a vacuum ultraviolet free electron laser would make possible a number of important experiments. For example, a number of pumpprobe experiments involving the synchrotron radiation beam in conjunction with a laser likely will be possible at the ALS. Such experiments include high-resolution infrared spectroscopy of radical intermediates in molecular beams, the spectroscopy of high-lying Rydberg states, and probing reaction product state distributions. Another class of experiments possible at the ALS are fast-timing experiments. According to a summary of the workshop prepared by Tomas Baer, a physical chemistry professor at the University of North Carolina, the most ambitious timing experiment proposed for the ALS is a "two-color, pump-probe experiment in which a picosecond laser is synchronized to the synchrotron pulse repetition rate. By delaying one pulse with respect to the other, fast reaction rates can be measured directly by using one of the two sources to pump the molecule to an excited state and using the other pulse to probe the state as a function of time." Such experiments are carried out currently using two lasers, but the synchrotron radiation will provide very-high-energy pho26

September 19, 1988 C&EN

Westbrook: fast x-ray area detectors tons that are easily tuned, as well as a much higher repetition rate than a typical 10-Hz laser. The high energy of the photons will allow researchers in this area to study reactions otherwise inaccessible to them. There will be a number of biological applications of ALS synchrotron radiation, says project director Marx. Recently, the ability to focus soft x-rays with sufficient flux to do microscopy and holography was demonstrated at the NSLS. Much higher fluxes will be possible at the ALS, and microscopy and spectroscopy with very good spatial and energy resolution likely will be possible. Unlike an electron microscope, samples in an x-ray microscope do not need to be under a vacuum. Thus, it should be possible to study cells in an aqueous environment, which is not possible with an electron microscope. The x-ray microscope avoids a number of sample preparation procedures required for electron microscopy. These procedures quite possibly alter the structures of biological samples. Because there is a strong component of undulator radiation that is coherent, biologists also likely will be able to produce three-dimensional holograms of intact cells using ALS radiation, Marx notes. Additionally, the same sort of combination of microscopy and spec-

troscopy can be applied to inorganic materials, such as semiconductor devices, and surfaces. Similar considerations suggest broad new areas of research will open up when the APS comes online. "I think x-ray diffraction is going to be revolutionized by this m a c h i n e , " says Mark Beno, an Argonne chemist. The high-brilliance, highly focused beam produced by the APS will allow study of very small crystals, on the order of a micrometer in size, that "will open up areas that have not been amenable to analysis before." An example of this kind of research will be x-ray diffraction studies of zeolites. Determinations of protein structure by x-ray diffraction also likely will be dramatically affected by the synchrotron radiation produced at APS, according to Edwin M. Westbrook, APS coordinator for biophysical instrumentation, who is developing, among other research activities, the fast x-ray area detectors that will be needed for such studies. The x-ray beams produced by the APS will allow data collection at a much more rapid rate than in the past, on the order of a few seconds or less for entire data sets from protein crystals. This will enable the careful structural study of the many variants of a protein that can be produced by site-directed mutagenesis. The tightly focused x-ray beams also will permit crystallographers to use much smaller protein crystals than was possible in the past. A particularly intriguing possibility—time-resolved studies of protein crystal structure—illustrates how increasing the rate at which data can be collected opens entirely new areas of scientific investigation. According to Westbrook, the APS will produce x-ray beams bright enough to produce diffraction data sufficient for structural determinations in times on the order of a few microseconds. If carried out sequentially, this would allow research workers to "image as a function of time the evolution of a protein structure," Westbrook says. Such a system would enable the scientists to produce "movies" of phenomena such as substrate binding to an enzyme. D