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Aug. 7, 2015 Perspectives Physics / Astronomy

Brighter, shorter x-ray pulses to share

The RIKEN SPring-8 Center is making some of the most advanced x-ray sources accessible to researchers working on high-energy physics.

From a small group of pioneering researchers using the radiation wasted during high-energy physics experiments has grown an entire scientific field dedicated to constructing synchrotron radiation facilities. The demand for the most brilliant source of x-rays from researchers of every hue has been relentless. The RIKEN SPring-8 Center is addressing this need by making some of the most advanced x-ray sources accessible to them all.

Synchrotron radiation was quite a nuisance to high-energy physicists. They spent the last half of the last century investigating the basic building blocks of matter by first accelerating particles—the faster, the better—and then smashing them into other particles. Large donut-shaped facilities called synchrotrons made it possible to accelerate charged particles like electrons almost to the speed of light by making them undergo continuous revolutions around a ring of magnets.

But when charged particles are forced to speed up, slow down or change direction, they emit electromagnetic radiation or light. At slower speeds, this radiation appears in the form of weak, long, low-frequency waves moving in almost every direction. As a particle is accelerated to nearly the speed of light, it produces a spectacular beam of light whose wavelengths span a large portion of the spectrum, extending all the way up to the ultraviolet and x-ray regions.

For decades, this so-called ‘synchrotron’ radiation represented the single largest source of energy loss in high-energy physics experiments. Then a group of researchers in the 1960s started making use of the waste product parasitically, which led to an explosion in x-ray science.

Whipping x-rays

Image of SACLA Figure 1: Thanks to RIKEN’s in-vacuum undulator technology, the SPring-8 Angstrom Compact free electron LAser (SACLA) at RIKEN can emit a coherent x-ray laser beam with wavelengths about a tenth of a nanometer long. © 2015 RIKEN

The resolution of observations using light is limited by the wavelength. In the visible regime, for example, wavelengths are just under a micrometer long, and can therefore be used in microscopes to observe the micro-world. X-rays, with wavelengths more than a thousand times shorter, allow us to see the nano-world, from individual atoms to twisting DNA and carbon nanotubes. Prior to synchrotron radiation, however, the only x-ray sources were x-ray tubes, which involved slamming high-speed electrons against a metal target. First demonstrated by Wilhelm Rӧntgen in 1895, the x-rays produced using this method could not be tuned to a desired wavelength and fell far short of the brilliance of synchrotron radiation, which was developed subsequently.

By the 1980s, researchers, especially those in the fields of spectroscopy and crystallography, had lobbied for facilities solely designed to generate synchrotron radiation. In these second-generation synchrotrons, bunches of electrons confined in a central storage ring emit x-rays into beamlines. At the end of each beamline is a tiny sample waiting to be hit, and a scrutinizing researcher.

Synchrotrons saw major upgrades in the 1990s, with the construction of three mega-facilities: the European Synchrotron Radiation Facility in France, the Advanced Photon Source in the United States and, finally in 1997, SPring-8 in Japan. These third-generation synchrotrons were fitted with straight sections in which undulators were installed. The undulators consisted of two parallel rows of magnets having alternating polarities through which electrons undulate like tiny ripples to produce brilliant x-rays.

The brightest light of them all was generated by SPring-8. Established through collaboration between RIKEN and the former Japan Atomic Energy Research Institute, the storage ring at SPring-8 has the largest circumference (1,436 meters) and can maintain the highest electron energy (8 gigaelectronvolts) of all three facilities. This means that it can penetrate deep into heavy atoms like uranium and plutonium. But the crucial innovation is in its insertion devices, where the parallel rows of magnets are brought inside an ultrahigh vacuum chamber, closer to the zigzagging electron beam to produce even more brilliant radiation.

Soon after the completion of SPring-8 came another major breakthrough with the construction of the SPring-8 Angstrom Compact free electron LAser (SACLA) in 2011. The in-vacuum undulator technology was again adopted in SACLA, which has a linear structure (see image). The radiation is emitted as a single coherent beam of x-rays—a distinguishing characteristic of lasers—in short pulses so powerful that they destroy the sample. SACLA’s x-ray laser is a billion times more brilliant and ten thousand times shorter than the radiation emitted at SPring-8.

SACLA is one of only two free-electron lasers in the world capable of producing hard x-rays with wavelengths about a tenth of a billionth of a meter (a tenth of a nanometer) long—the other is the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in the United States. Thanks to RIKEN’s in-vacuum undulator technology, SACLA requires much less energy than the LCLS (8 versus 14 gigaelectronvolts) to produce a shorter wavelength x-ray laser (0.06 versus 0.1 nanometer). And the total length of the SACLA facility at 700 meters is considerably shorter than the two-kilometer facility in the United States. Indeed, SACLA is the first compact x-ray laser source1.

Protein pumps and photosynthesis

Image of photosystem II Figure 2: The three-dimensional structure of photosystem II was determined using an ultrashort laser pulse with a wavelength of less than 0.2 nanometers produced by SACLA. © 2014 Jian-Ren Shen, Okayama University

With these advances in the production of synchrotron radiation a realization has emerged among scientists of its potential for interdisciplinary applications. Large facilities like SPring-8 and SACLA have made it possible for researchers of various scientific backgrounds to access the light source. Close to 60 beamlines tap into SPring-8’s storage ring, and nearly half are publically accessible to researchers from any institution. The facility welcomes about 20,000 researchers each year, adding up to more than 180,000 users over the past two decades of operation. The results have been tremendous, spawning more than 10,000 papers in a wide range of fields, including life sciences, materials science, Earth science and nuclear physics.

One group of researchers from the University of Tokyo, for example, has crystallized the calcium pump proteins embedded in muscle cell membranes that induce muscle motion2. By analyzing the patterns of x-ray diffraction from their samples at SPring-8, the researchers were able to determine the three-dimensional structures of these protein pumps, at various stages of calcium attachment and non-attachment.

Tokyo Institute of Technology researchers have also collaborated with SPring-8 to recreate the structure of a mineral found 2,900 kilometers underground where pressures reach above 125 gigapascals and temperatures above 2,000 degrees Celsius3.

Coming full circle to high-energy physics, an international team of physicists discovered a subatomic particle consisting of five of the most fundamental elementary particles called quarks (in contrast, protons and neutrons are made of only three quarks).

Some of the research at SPring-8 has had direct industrial benefit, such as the measurement of silica nanoparticles found in rubber, which led to the commercialization of low-resistance automobile tires that improve fuel efficiency by approximately 6 per cent.

SACLA has enabled researchers to further refine their scope of investigation, from molecules arranged periodically in a crystal to whole, stand-alone molecules. And by producing intense laser pulses every one-sixtieth of a second, researchers can capture events as fleeting as changes in the bonds holding atoms together to create molecular movies.

Researchers at RIKEN and Okayama University have used SACLA to determine the first high-resolution crystal structure of an undamaged protein called photosystem II (see image)4 that generates the oxygen we breathe via the photosynthetic process. And SACLA successfully induced a nonlinear optical effect in which two lower-frequency x-ray photons are absorbed in a crystal of pure germanium and emitted as one higher-frequency photon5. More than 2,000 researchers have used SACLA since its two beamlines opened to the public in March 2012. By popular demand, RIKEN started operation of another beamline in April 2015.

Brighter synchrotron radiation

Though located within walking distance from each other, free-electron lasers and synchrotron facilities currently exist in leagues of their own. The next frontier in x-ray science will be to produce a light as coherent as SACLA but that has mild to moderate intensity so that it does not damage the target. This can be achieved within a decade by upgrading the original synchrotron design. RIKEN is taking the first steps in this direction with a scheduled upgrade of SPring-8 to be completed by the early 2020s.

SPring-8-II will rely on a revolutionary technology called the multi-bend achromat that can further concentrate the electrons in the storage ring to produce more coherent light. Current storage rings use double-bend achromats made of two magnets per unit. SPring-8-II will have five-bend achromats containing five magnets per unit to emit an x-ray beam that is 60 times more brilliant.

Brighter and more coherent sources of x-ray radiation will continue to be sought, but the real challenge remains satisfying the voracious appetite for the technology. By delivering faster results, SPring-8-II will be able to shorten user turnaround. And RIKEN plans to construct SACLA-2 that will be ten times shorter than SACLA, and could be reproduced in universities and research centers across Japan. The construction of a mid-sized synchrotron facility is also under discussion. These developments will allow researchers as far afield as the social sciences to reap the rewards of synchrotron radiation.


  • 1. Ishikawa, T., Aoyagi, H., Asaka, T., Asano, Y., Azumi, N., Bizen, T., Ego, H., Fukami, K., Fukui, T., Furukawa, Y. et al. A compact X-ray free-electron laser emitting in the sub-ångström region. Nature Photonics 6, 540–544 (2012). doi: 10.1038/NPHOTON.2012.141
  • 2. Toyoshima, C. & Mizutani, T. Crystal structure of the calcium pump with a bound ATP analogue. Nature 430, 529–535 (2004). doi: 10.1038/nature02680
  • 3. Iitaka, T., Hirose, K., Kawamura, K. & Murakami, M. The elasticity of the MgSiO3 post-perovskite phase in the Earth’s lowermost mantle. Nature 430, 442–445 (2004). doi: 10.1038/nature02702
  • 4. Suga, M., Akita, F., Hirata, K., Ueno, G., Murakami, H., Nakajima, Y., Shimizu, T., Yamashita, K., Yamamoto, M., Ago, H. & Shen, J.-R. Native structure of photosystem II at 1.95 Å resolution viewed by femtosecond X-ray pulses. Nature 517, 99–103 (2015). doi: 10.1038/nature13991
  • 5. Shwartz, S., Fuchs, M., Hastings, J. B., Inubushi, Y., Ishikawa, T., Katayama, T., Reis, D. A., Sato, T., Tono, K., Yabashi, M. et al. X-ray second harmonic generation. Physical Review Letters 112, 163901 (2014). doi: 10.1103/PhysRevLett.112.163901

About the Researcher

Tetsuya Ishikawa

Image of Ishikawa

Tetsuya Ishikawa has been director of the RIKEN SPring-8 Center since 2006. He joined RIKEN in 1993 and became chief scientist at the Coherent X-Ray Optics Laboratory in 1998. In 2012, Ishikawa received a Medal of Honor with Purple Ribbon from the Government of Japan for his enormous contribution to x-ray optics.