Logo Properties of Synchrotron Radiation
Course Material Index  Section Index  Previous Page  Next Page 

Properties of Synchrotron Radiation

As already explained, the electron bunches emit radiation as they are radially accelerated by the dipole magnets. This radiation is contained within a fan-like region (see below):

Now this fan-like distribution of radiation (coloured mauve on the diagram) is quite different from that of the radio-transmitter which obviously needs to transmit radio waves in all directions. The reason for this is that with a synchrotron the electrons are travelling very close to the speed of light (e.g. at 2 GeV their velocity is around 0.9999 of the speed of light and the electron mass increases to 4000 times its rest mass). It is a consequence of relativity that at such (relativistic) speeds the spatial bounds of the synchrotron radiation are contracted to a narrow fan in the forward direction of the electron bunches, typically 0.3 mrad (1 mrad ≈ 0.06 degrees) in the vertical plane. However since the electron bunches are circulating horizontally there is a continuum of such fans spread out over an arc (see below) so that the final fan is wider horizontally (typically degrees) than vertically as illustrated above.
This then represents one of the great advantages of synchrotron radiation: that it is condensed into a small angular fan, thus imparting much greater intensity and collimation than can be obtained from conventional laboratory sources.

But what is this radiation? As already remarked it extends from the radio-frequency to the X-ray regions of the electromagnetic spectrum although we will be principally interested in the X-ray region. Such radiation is often referred to as white (since it contains all wavelengths); its performance is measured by a power spectrum which is a plot of radiation intensity versus wavelength, λ; a typical version is shown below:

This characteristic shape is common to all dipole radiation. It shows a maximum typically at about λ = 10 Å and spreads out either side, slowly on the high wavelength side (termed soft radiation) all the way to the radio-wave region, and rather abruptly on the short wavelength side (termed hard radiation since it penetrates matter more easily). The intensity appears to be measured in strange units, and this therefore requires some explanation. All forms of electromagnetic radiation show both wave-like and particle-like properties, and so sometimes we may prefer to describe the radiation as a stream of particles, rather than as a propagative wave. We refer to these particles as photons (which have discrete amounts of energy E given by hc/λ - yet another fundamental principle of physics). A common goal is to maximise the number of X-ray photons of a desired wavelength hitting a sample, and so the intensity of a synchrotron radiation beam has been traditionally expressed in units of photons per second per 0.1% bandwidth per mrad2; this is so that comparisons with other sources can be made, accounting for the time of collection (per second), that the white source contains all wavelengths (per % bandwidth), and that the radiation is spread out over a fan (per mrad2). It is customary now also to refer to brilliance (photons -1 0.1% bandwidth-1 mrad-2 mm-2) whereby the effective size of the electron bunches (per mm-2) is also taken into consideration. Whatever the units, one should realise that we are invariably talking in terms of numbers like 1012 photons per second and this can represent an enormous transmission of energy; a point of some concern with fragile specimens.

The radiation also has some other remarkable properties: It is horizontally polarised in the plane of the electron orbit and circularly polarised above and below the orbit. As the schematic below illustrates, whereas with laboratory sources the X-ray electric vector vibrates in all directions perpendicular to the propagation of the X-ray; with the synchrotron this vibration is horizontally polarised. This has advantages which can be put to use in both synchrotron diffraction and spectroscopy.

Synchrotron Laboratory
Finally, the X-ray beam is not actually continuous in time but fires in extremely short bursts. This is simply a property of the electron bunches; the X-rays are produced only as the bunches pass through the dipole magnet. Depending on the number, size and speed of the bunches, each flash typically lasts for 100 ps (100 × 10-12 second) and is repeated typically every 1-300 ns (1 ns = 10-9 second). This time structure to the radiation can also be put to use in certain specialised experiments.

We should finish this part by summarising the five main features of synchrotron radiation:

All these features except the last are regularly exploited in synchrotron powder diffraction.

Course Material Index  Section Index  Previous Page  Next Page 
© Copyright 1997-2006.  Birkbeck College, University of London.
 
  		Author(s): Paul Barnes
Simon Jacques
Martin Vickers