X-RAY FLUORESCENCE BASICS (XRF)
X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by bombarding it with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis of rocks and minerals.

The work is usually done in a laboratory along with other petrographic assessments. Used mostly to log cores, it can also be used on individual rock samples. Handheld and portable core logging versions are available for use at the wellsite.


Because XRF can give quantitative values for the elements in a rock, it is often used to help evaluate the results from elemental capture spectroscopy (ECS) logs. The elemental composition from XRF in the lab or ECS in the wellbore can be inverted to a mineral composition using a least squares inversion algorithm.



XRF Energy spectrum of a material showing energy peaks for specific elements in the sample. The relative amplitudes indicate the relative concentration of each element in the material. A non-negative least squares inversion can transform element concentrations into mineral weight percent.

 

The range of elements that can be observed varies with the design of the instrument. A typical handheld can only recognize the elements between Magnesium and Lithium, for example. A full scale lab model can handle all the way to Uranium.

X-RAY FLUORESCENCE EXAMPLE


Example of an XRF log taken on a core (core photo at left). (image: Woods Hole Oceanographic)


HOW X-RAY FLUORESCENCE WORKS
Source Wikipedia

When materials are exposed to short-wavelength X-rays or to gamma rays, ionization of their component atoms may take place. Ionization consists of the ejection of one or more electrons from the atom, and may occur if the atom is exposed to radiation with an energy greater than its ionization potential. X-rays and gamma rays can be energetic enough to expel tightly held electrons from the inner orbitals of the atom. The removal of an electron in this way makes the electronic structure of the atom unstable, and electrons in higher orbitals "fall" into the lower orbital to fill the hole left behind. In falling, energy is released in the form of a photon, the energy of which is equal to the energy difference of the two orbitals involved. Thus, the material emits radiation, which has energy characteristic of the atoms present. The term fluorescence is applied to phenomena in which the absorption of radiation of a specific energy results in the re-emission of radiation of a different energy (generally lower).

Each element has electronic orbitals of characteristic energy. Following removal of an inner electron by an energetic photon provided by a primary radiation source, an electron from an outer shell drops into its place. There are a limited number of ways in which this can happen. The main transitions are given names: an L→K transition is traditionally called Kα, an M→K transition is called Kβ, an M→L transition is called Lα, and so on. Each of these transitions yields a fluorescent photon with a characteristic energy equal to the difference in energy of the initial and final orbital. The wavelength of this fluorescent radiation can be calculated from Planck's Law:
      1: Lambda = h * c / E

Where:
  h =  Planck's constant
  c = speed of light in a vacuum
  E = energy difference
  Lambda =  wavelength of emitted photon

The fluorescent radiation can be analyzed either by sorting the energies of the photons (energy-dispersive analysis or EDXRF) or by separating the wavelengths of the radiation (wavelength-dispersive analysis or WDXRF). The latter technique is no longer in widespread use as the advances in computer and detector technology favour the energy dispersive method.

Once sorted, the intensity of each characteristic radiation is directly related to the amount of each element in the material.
 

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