Of X-rays and Synchrotrons
In 1895 German physicist Wilhelm Röntgen was in his laboratory at the University of Munich researching cathode rays – the phenomena where an electrical current passing through an evacuated glass tube causes the end of the tube to glow (or fluoresce). Röntgen covered the entire evacuated tube with black cardboard and turned off the lights to check whether any light was escaping from the tube. Whilst the tube seemed to contain the light from the tube he noticed that a screen painted with barium platinocyanide over a metre away on his lab bench was shimmering. Intrigued, he placed additional cardboard between the vacuum tube and the screen. Then he tried a book, and a wooden shelf board. Still the screen glimmered. Röntgen had discovered X-rays, for which he went on to win the inaugural Nobel Prize for Physics in 1901.
Röntgen’s discovery attracted the attention of medical researchers, given the ability of the new “rays” to penetrate beneath the skin, but other physicists were more interested in trying to establish the nature of the mysterious rays. Maxwell’s theory of electromagnetism, documented in 1873, described the wave-like nature of light. In 1912 Max von Laue realised that, if X-rays were part of the electromagnetic spectrum with a shorter wavelength than visible light, they would interact with crystal structures to produce an interference pattern characteristic of the crystal properties. This led him to produce the first X-ray diffraction pattern from a crystal, for which he was awarded a Nobel Prize in 1914.
Shortly after von Laue’s data was published, it caught the attention of William Lawrence Bragg, then working in Cambridge. After discussions with his father, William Henry Bragg, he set about re-interpreting the X-ray pictures published by von Laue’s group. He realised that a focussing effect seen by the German group would arise if the X-rays were reflected by planes of atoms in the crystal. This enabled him to reformulate von Laue’s conditions for diffraction into what became known as Bragg’s Law, which gives a direct relationship between the crystal structure and its diffraction pattern. This made it possible to use X-ray diffraction to determine crystal structures, and the Braggs used their new technique to ‘solve’ the structure of a number of crystals, including sodium chloride, Calcite and Pyrites: X-ray Crystallography was born.
Diffraction was not the only application of the newly discovered X-rays. Scottish physicist Charles Barkla discovered that the various elements emit X-rays that are characteristic of that element, and Henry Gwyn Jeffreys Moseley performed a systematic study of X-ray spectra, determining the mathematical relation between the radiation wavelength and the atomic numbers of the emitting elements. He pointed out the existence of holes in the periodic table for elements that were discovered much later. Barkla won the 1917 Nobel Prize “for his discovery of the characteristic Röntgen radiation of the elements”. Tragically Moseley enlisted in the British Army and was killed in the Battle of Gallipoli in 1915.
By 1911 Ernest Rutherford had established himself as a pioneering physicist. He famously developed the planetary model of the atom, a small nucleus surrounded by orbiting electrons, based on an experiment where alpha particles were fired at a sheet of gold foil and the resulting trajectories analysed. Rutherford realised that accelerated particles provided a useful tool for investigating the structure of matter. In 1928 addressing the Royal Society he issued a call to arms, “I have long hoped for a source of positive particles more energetic than those emitted from natural radioactive substances.” Particle accelerators were the answer.
Physicists around the world were looking into ways to accelerate particles. Ernest Lawrence at Berkeley University was one of them. He came across a thesis by the Norwegian scientist Rolf Widerøe. Lawrence couldn’t understand the language, but the maths and the diagrams were enough to spark an idea in his head of a circular accelerator. In 1930 Lawrence took on a graduate student, Milton Stanley Livingston and assigned him the task of building the machine. By the end of the year, and a huge amount of work from Livingston, the first cyclotron had been built. Livingston later commented dourly, “Lawrence was my teacher when I built the first cyclotron – he got a Nobel prize for it – I got a PhD.”
Cyclotrons however had fixed magnetic fields. Because the bending of a charged particle is inversely proportional to its momentum, cyclotrons were limited to fairly low energies before they became unaffordably large. By collecting the particles into bunches and synchronising a rise in magnetic-field strength with the increasing energy of the charged particles, the particles could be accelerated to higher energies while being constrained to a fixed circular path. This concept became known as the synchrotron.
At this stage particle accelerators were designed for the purpose of particle collisions of the kind Rutherford desired. However, even before the first machine was built there was a potential problem. Einstein’s theory of special relativity applied to Maxwell’s equations predicted that charged particles travelling along a circular path at relativistic speeds would generate a very intense, narrow cone of light, parallel to the direction of motion of the charged particles. In 1947 a team of scientists at General Electric in the US switched on the world’s first purpose-built synchrotron, and simultaneously made the first observation of artificial synchrotron light.
For the General Electric machine this light was predominantly in the visible region of the electromagnetic spectrum. But with larger, higher energy machines came higher energy radiation, into the X-ray region. In 1956, two American scientists, Diran Tomboulian and Paul Hartman, were granted use of the 320 MeV synchrotron at Cornell University. In addition to confirming the spectral and angular distribution of the light they saw, they carried out the first X-ray spectroscopy study using synchrotron light.
Over the years the number of experiments increased, all using machines built for high energy particle physics. This changed in 1980 when the UK built the world’s first synchrotron dedicated to producing synchrotron light for experiments. For scientists carrying out spectroscopy experiments, the brightness of the light reaching the sample determined the resolving power of the results. For crystallographers, especially those looking at small crystals with large unit cells, high brightness was important to resolve closely spaced diffraction spots. The development of insertion devices provided new opportunities. Insertion devices are arrays of magnets that create a very bright and tuneable beam, with intensity peaks with a wavelength that can be varied by adjusting the field strength.
Now there are more than 50 synchrotron light sources around the world, carrying out a huge range of experiments with applications in engineering, biology, materials science, cultural heritage, chemistry, environmental science and many more. Several Nobel prizes have been awarded for research that depended on synchrotron light, most recently Prof Venki Ramakrishnan, joint winner of the 2009 Nobel Prize in Chemistry for his work on the structure of the ribosome. Synchrotrons play a vital role for the global scientific infrastructure, by increasing international collaborations and being truly multidisciplinary. Synchrotrons are here to stay.