Synchrotron radiation

Nearly 100 years ago, theorists postulated the mechanism for the creation of synchrotron radiation. However, a manmade, controllable source of such radiation was not found until the middle of the wentieth century when accelerators for charged particles first appeared. High-energy electron accelerators emerged as viable synchrotron radiation sources because, as electrons approach the speed of light, the synchrotron radiation increasingly is emitted in a narrow, forward-directed cone.

Thus, the radiation is concentrated in a small solid angle and can be readily used by researchers. Early synchrotron light sources used photons that were created as the undesirable by-product of electron accelerators operated for high energy physics research. This parasitic use of synchrotron radiation showed such high promise that in the 1980’s accelerators were built expressly for the purpose of generating synchrotron radiation.

These accelerators, called “second-generation” synchrotron radiation light sources, typically consisted
of a number of curved sections, in which the synchrotron radiation was generated, connected by straight sections; together, the curved and straight sections formed a closed, approximately circular orbit.

After testing of GE's 100-MeV betatron commenced in 1944, Blewett suggested a search for the radiation losses, which he expected from the work of Ivanenko and Pomeranchuk to be significant at this energy. However, two factors prevented success: whereas, according to Schwinger's calculations, the radiation spectrum for the 100-MeV betatron should peak in the near- infrared/visible range, the search took place in the radio and microwave regions at the orbital frequency (and low harmonics) and the tube in which the electrons circulated was opaque. Although quantitative measurements reported in 1946 of the electron-orbit radius as it shrunk with energy were in accord with predicted losses, there was also another proposed explanation with the result that, while Blewett remained convinced the losses were due to synchrotron radiation, his colleagues were not.

Advances on another accelerator front led to the 1947 visual observation of synchrotron radiation at GE. The mass of particles in a cyclotron grows as the energy increases into the relativistic range. The heavier particles then arrive too late at the electrodes for a radio-frequency (RF) voltage of fixed frequency to accelerate them, thereby limiting the maximum particle energy. To deal with this problem, in 1945 McMillan in the U. S. and Veksler in the Soviet Union independently proposed decreasing the frequency of the RF voltage as the energy increases to keep the voltage and the particle in synch. This was a specific application of their phase-stability principle for RF accelerators, which explains how particles that are too fast get less acceleration and slow down relative to their companions while particles that are too slow get more and speed up, thereby resulting in a stable bunch of particles that are accelerated together.

Under Madden and Codling, measurements began at the new NBS facility (Synchrotron Ultraviolet Radiation Facility or SURF) to determine the potential of synchrotron radiation for standards and as a source for spectroscopy in the ultraviolet (the wavelength for peak radiated power per unit wavelength was 335 Å). Absorption spectra of noble gases revealed a large number of previously unobserved resonances due to inner-shell and two-electron excitations, including doubly excited helium, which remains today a prime test bed for studying electron- electron correlations. These findings further stimulated the growing interest in synchrotron radiation. Establishment of SURF began the first generation of synchrotron-radiation facilities, sometimes also called parasitic facilities because the accelerators were built and usually operated primarily for high-energy or nuclear physics. However, the NBS synchrotron had outlived it usefulness for nuclear physics and was no longer used for this purpose.

If SURF headed the first generation, it was not by much, as activity was also blossoming in both Europe and Asia. At the Frascati laboratory near Rome, researchers began measuring absorption in thin metal films using a 1.15-GeV synchrotron. In 1962, scientists in Tokyo formed the INS- SOR (Institute for Nuclear Studies-Synchrotron Orbital Radiation) group and by 1965 were making measurements of soft x-ray absorption spectra of solids using light from a 750-MeV synchrotron. The trend toward higher energy and shorter wavelengths took a big leap with the use of the 6-GeV Deutsches Elektronen-Synchrotron (DESY) in Hamburg, which began operating for both high-energy physics and synchrotron radiation in 1964. With synchrotron radiation available at wavelengths in the x-ray region down to 0.1 Å, experimenters at DESY were able to carefully check the spectral distribution against Schwinger's theory, as well as begin absorption measurements of metals and alkali halides and of photoemission in aluminum.