“Synchrotron radiation” sounds like nuclear physics, but the everyday picture is simpler: push electrons to almost the speed of light, then make them turn. Turning costs the electrons energy, and that energy leaves as light. The light spans infrared, ultraviolet and hard X-rays, and it is several orders of magnitude brighter than a laboratory X-ray tube.

So a synchrotron is not a bigger X-ray machine. It is a whole facility — accelerator, storage ring and experimental stations chained together. Later posts deal with soft X-ray absorption spectroscopy (SXAS); this one is about the light source itself.

Aerial view of the European Synchrotron Radiation Facility (ESRF) in Grenoble. The storage ring sits inside the circular building; beamlines radiate outward. Image: Wikimedia Commons.

What the machine looks like

A modern third-generation source has three stages:

  1. Linear accelerator (linac) — the first kick.
  2. Booster — raises the energy to hundreds of MeV or a few GeV.
  3. Storage ring — electrons circulate in a vacuum pipe for hours; around the ring sit bending magnets, focusing quadrupoles and insertion devices.

The most important insertion devices are undulators and wigglers: rows of magnets with alternating polarity that force the electrons onto a snaking path. The more regular the snake, the brighter and narrower the emitted spectrum — and the easier it is to tune the photon energy onto the absorption edge you care about.

Schematic of ESRF: electrons enter the storage ring from the linac and booster; beamlines carry the light to individual end stations. Image: Wikimedia Commons.

The ring itself is not where experiments happen. Light is extracted at tangent points into beamlines: slits, a monochromator and mirrors shape the beam before it lands on the sample in an end station.

Magnet lattice inside the Australian Synchrotron storage ring. The electrons travel in the vacuum pipe threaded through these magnets. Image: Wikimedia Commons.

Why it beats a lab X-ray tube

Materials science, chemistry, biology and earth science use synchrotrons not because the energy is higher, but because of the quality of the light:

  • Brilliance — enormous photon flux per unit area, solid angle and bandwidth, so weak signals survive.
  • Continuous tunability — swap a crystal or grating and the beam sits on the carbon K-edge, the oxygen K-edge or a transition-metal L-edge.
  • Collimation and polarisation — linear and circular polarisation are both available; the latter matters for magnetism and orbital symmetry.
  • Time structure — electrons travel in bunches, so the light is naturally pulsed.

Soft X-rays (roughly 50 eV to 2 keV) add one practical constraint: air absorbs them almost completely. Soft X-ray end stations therefore run under ultra-high vacuum, and samples go in through load locks. That, more than the physics, is why SXAS has a reputation for being hard.

What comes next

The synchrotron only supplies the light. Once you have a tunable soft X-ray beam, one of the most common things to do with it is X-ray absorption spectroscopy (XAS): scan the photon energy and watch how the sample “drinks” light near an absorption edge. The soft X-ray branch of that family is what people call SXAS — the subject of the next post.