11June 2022 ESRFnews
radiation should cause the orbit of an electron in a betatron to shrink. He confirmed his numbers by experiment a result that has also been considered the first observation of synchrotron radiation.
What happened next? For a time, not much. Synchrotron radiation was characterised by teams at both GE Labs and the Lebedev Institute in Moscow in the Soviet Union. Later, in the mid-1950s, a group at Cornell University in New York state, US, were able to use it to perform a few experiments, measuring the low- energy transmission of beryllium and aluminium foils. It was not until 1961, however, that a proper experimental programme got underway, at the US National Bureau of Standards in Maryland. As with other experimental programmes of the decade, NBS s use of synchrotron radiation was parasitic: the main application of the accelerator was still collisions of the electrons themselves, for nuclear physics. Modern synchrotron X-ray science, employing a dedicated second generation electron storage-ring, began with the Synchrotron Radiation Source at Daresbury Laboratory in the UK, in 1981.
Where does the ESRF come in? In its original incarnation, beginning operations in 1992, the ESRF was the first third generation synchrotron light source, far brighter than its predecessors due to the inclusion of devices in its storage ring known as wigglers and undulators. As scientists from more and more fields of research became interested health, advanced materials, the environment, agriculture, cultural heritage so the demand for synchrotron radiation grew: today, there more than 50 synchrotron light sources across the world. Thanks to its EBS upgrade two years ago, the ESRF is still leading the pack today, as the first high-energy fourth generation synchrotron light source. The light that was first glimpsed 75 years ago is showing no signs of fading.
estimated the spectrum of radiation emitted by orbiting electrons). Haber is said to have been given credit for the discovery within GE Labs. He eye-balled synchrotron radiation ahead of all those scientists who might have seen it sooner, Pollack once wrote. Outside the labs, however, Haber s name remains almost entirely unknown.
Did other people contribute to the discovery? There were certainly many steps along the way, as attendees reflected at a symposium for the anniversary of the event organised by the community website lightsources.org. One of these was the construction of the first betatron by Donald Kerst at the University of Illinois Urbana- Champaign in the US in 1940. A more primitive type of circular electron-accelerator, the betatron can generate X-rays by driving its electrons into a fixed metal target. Then in 1945, a year before Haber s chance encounter, John Blewett, a Canadian-American physicist working at GE Labs, predicted the degree to which the loss of energy by
A bright spot of optical synchrotron radiation emanates from the GE Labs synchrotron in 1947.
What actually happened? In late 1946, Herbert Pollack, a physicist at the General Electric (GE) Research Laboratory in New York state, US, directed the construction of a circular accelerator to test a new method of driving electrons towards relativistic energies. Previous accelerators had struggled to do this because according to the general theory of relativity a particle s mass greatly increases as it enters this regime, and therefore the electrons fall out of sync with the fluctuating voltages designed to accelerate them. Proposed independently by the Soviet physicist Vladimir Veksler and the US physicist Edwin McMillan, the synchrotron principle involved decreasing the frequency of the voltage fluctuations as the electron energy increased. Pollack s was one of the first synchrotrons to be built, and fortunately for science, at least his team had not taken the precaution of installing radiation shielding. During its early use, on 24 April 1947, a technician, Floyd Haber, peered into the accelerator tube with a mirror to check for sparking and instead saw a bright arc of light.
Did he know what he had seen? Not according to Robert Langmuir, an electrical engineer on Pollack s team. The light was first seen by the technician I don t remember his name, he recollected in a letter to Pollack several decades after the event. He thought it was sparking. I, and almost immediately thereafter you, recognised it to be what I called at the time, Schwinger radiation (another name for synchrotron radiation, in reference to the eminent theoretical physicist Julian Schwinger at Harvard University in Massachusetts, US, who had
75 years ago, scientists fi rst observed synchrotron light. Materials science was never the same again.
The light that started it all
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