Comment on ‘The Brightest X-Ray Machine in the World probes Molecules’

I recently retweeted a link to an Science Daily piece on the ‘Brightest X-ray Machine in the World’, which refers to the Linac Coherent Light Source (LCLS) at Stanford. Calling it the ‘Brightest X-ray Machine’ seems to need some clarification, which I will attempt to do here.

First, we must remember that X-rays, whether we think of them as electromagnetic waves or photons, have energies that depend on their frequencies (or wavelengths). ‘Hard’ X-rays have wavelengths in the range 0.1-0.01 nm, which correspond to energies of about 12 to 120 keV (energies and wavelengths or frequencies being related by the Planck equation). It is the wavelength (or frequency) that determines the energy of the X-ray photon. It is easy to confuse this with the ‘brightness’ of the beam, which refers to the number of X-ray photons emitted, and which can be measured in units of power per unit area (i.e. the number of photons of a particular energy striking a given surface area). In the article, a beam intensity of 10 to the power of 18 Watts per square centimetre is quoted for the LCLS machine.

It is the energies of the X-rays that determine what they can be used for in an experiment. In order to ‘strip’ electrons off an atom, energies that are greater than or equal to the binding energies of the electrons are needed. The advantage of having a brighter (more intense) X-ray machine like the LCLS is that more photons are available, so for the above case, more electrons can be removed, making their detection easier, and meaningful results more possible to obtain.

This can, of course, be directly related to the photoelectric effect experiment, where light is shone onto a metal surface, and electrons observed to be emitted from the surface provided the light frequency is high enough. Increasing the light intensity can liberate more electrons, but only if the frequency is high enough.

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