MadSci Network: Physics

Re: How to describe x-ray in metal?Wave or particle?

Date: Tue Oct 17 08:16:10 2006
Posted By: Steve Nelson, research physicist
Area of science: Physics
ID: 1161059694.Ph

As photons approach the violet end of the spectrum, their wavelengths shrink. As that wavelength shrinks with increasing energy, the characteristic size of atoms and molecules begins to approach a few percent of the size of the wavelength itself. At these wavelengths, other scattering processes begin to dominate the scattering. This is part of why the sky is blue (it's actually blue and violet at the same time, your eye just can't see it), because Rayleigh scattering dominates at these wavelengths in air. Around the blue-ultraviolet wavelengths, photon energies begin to exceed the energy for separating an electron from the metal (an energy known as the work function). At this energy, the photoelectric effect as was first correctly interpreted by Einstein begins to take over. Photons transition from interacting with the collective group of mobile electons in the metal as their wavelengths get shorter and single-atom scattering processes like the photoelectric effect becomes more powerful. As photon energies increase through the UV and into the x-ray spectrum, their wavelengths approach the size of atoms themselves and Rayliegh scattering becomes less and less important. For even higher energy photons (gamma rays) of a few million electron Volts (MeV) of energy, Compton scattering dominates for a while, then the wavelength becomes too small and it becomes less likely that the gamma ray will scatter from an individual electron. That is when pair production (where a gamma ray creates an electron-positron pair) takes over as the dominant scattering process. So you see, the scattering of a photon will be dominated by a process which can be determined largely by looking at its characteristic wavelength. Your rule of the scattering of a wave from a metal only applies for wavelengths much greater than the spacing between the atoms, because the wave must induce a current over a macroscopic distance. If it's too small to create a current because it can only hit one atom, you have to consider the processes differently. This is the length scale where quantum mechanics takes over in a more obvious way for classical electrodynamics, but a further discussion of that is best left for a course in the subject.

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