2.5.1. Electromagnetic Spectrum

 

German physicist Wilhelm Conrad Röntgen discovered X-rays on November 8, 1895, at the University of Wurzburg in Germany. X-rays are still called Röntgenstrahlen in Europe. Röntgen used electrons to bombard inert gas in tubes, and discovered that nearby photographic plates had been exposed by some sort of unknown ("X") radiation. He demonstrated that X-rays travel in straight lines and are very penetrative, traversing all materials to varying degrees. He received the first Nobel Prize in Physics in 1901 for his discovery, donating the prize money (then about $40,000) to the University of Wurzburg.

Wilhelm Conrad Röntgen (1845-1923).

X-rays are electromagnetic radiation.  All X-rays represent a very energetic portion of the electromagnetic spectrum (Figure 2.5.1a) and have short wavelengths of about 0.1 to 100 angstroms (Å). They are bounded by ultraviolet light at long wavelengths and gamma rays at short wavelengths. X-rays in the range from 50 to 100 Å are termed soft X-rays because they have lower energies and are easily absorbed.

Figure 2.5.1a. Electromagnetic spectrum

The range of interest for X-ray analysis is approximately from 0.1 Å to 100 Å. Although, angstroms are used throughout these notes, they are not an accepted SI unit. Wavelengths should be expressed in nanometers (nm), which are 10^-9 meters (1 Å = 10 ^-10m; 1 nm = 10 Å ), but most texts and articles on microprobe analysis retain the use of the angstrom. Another commonly used unit is the micron, which more correctly should be termed micrometer (µm); a micrometer is 10⁴ Å.

The relationship between the wavelength of electromagnetic radiation and its corpuscular energy (E) is derived as follows. For all electromagnetic radiation:

For all wavelengths,

Thus,

Substituting values:

The conversion to angstroms and electron volts (1 eV = 1.6021 x 10-19 Joule) yields the Duane-Hunt equation:

Note the inverse relationship. Short wavelengths correspond to high energies and long wavelengths to low energies. Energies for the range of X-ray wavelengths are 124 keV (0.1 Å) to 124 eV (100 Å). The magnitudes of X-ray energies suggested to early workers that X-rays are produced from within an atom.  Those produced from a material consist of two distinct superimposed components: continuum (or white) radiation, which has a continuous distribution of intensities over all wavelengths, and characteristic radiation, which occurs as a peak of variable intensity at discrete wavelengths.

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