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The high energy electron can
also cause an electron close to the nucleus in a
metal atom to be knocked out from its place. This
vacancy is filled by an electron further out from the
nucleus. The well defined difference in binding
energy, characteristic of the material, is emitted as
a monoenergetic photon. When detected this X-ray
photon gives rise to a characteristic X-ray line in
the energy spectrum. C. Barkla observed these
lines in 1908-09 and was given the 1917 Nobel Prize
for this discovery. He also made the first
experiments suggesting that the X-rays are
electromagnetic waves.
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The Nobel Prize
in Physics 1917 - Charles Glover Barkla
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