Generation of Characteristic X-Rays

Darrell Henry, Louisiana State University

Energy shells associated with electron excitation

Energy shells

Characteristic X-rays are generated when an energetic beam of electrons interacts with the inner shell electrons by inelastic scattering with enough energy to excite inner shell electrons to outer shell orbitals, leaving inner-shell vacancies. As outer-shell electrons fall to the various inner shell orbitals, characteristic amounts of energy are generated that are a function of the target element and the type of orbital decay. Much of the energy is emitted from the atom or some may be internally absorbed and knock out another outer shell electron (Auger electron) with additional energy emissions.

These phenomena are useful because:

There is a relationship between energy (E) and the wavelength (λ) given by the expression: λ = 12.398/E (where λ is in � and E is in KeV). Detectors of X-ray emission use wavelength as a reference for measurement of a specific element.
The energy, and its associated wavelength, yielded from a specific shell, is the characteristic X-ray for a given element. These characteristic X-rays are used for:
- general identification of a material (EDS)
- quantitative or semi-quantitative analysis (EDS, WDS, others)
- generation of X-rays of a specific wavelength or small range of wavelengths as a source of crystal structure studies (XRD ) or quantitative studies (XRF, others).

Two treats for the price of one: the relationships that describe how Xrays are generated are used in two important ways:

In an Xray instrument, Xrays of a fixed wavelength are generated in an Xray tube by exciting a "target" made of a known element by a stream of electrons. Usually, Xray tubes (used by geologists) have either a Cu or Mo metal strip, which will produce an Xray beam of fixed wavelength (filters are needed to screen out K_β radiation so that only K_α radiation is emitted. The production of monochromatic Xrays is important for both single crystal and powder diffraction Xray techniques.
When analyzing unknown minerals using the electron microprobe, a focused electron beam is used to excite all elements in a sample, thus producing a spectrum of characteristic Xrays. By analyzing the wavelengths of these secondary Xrays, it is possible to obtain a qualitative analysis by identifying the elements present in a sample using energy dispersive spectrometry (EDS) or quantitative analyses are routinely acquired by comparing the Xray yield (count rate) to standards (and applying other correction factors) using wavelength dispersive spectrometry (WDS)