X-Ray Spectroscopy

Origin of X-rays

The X-ray region of the electromagnetic spectrum consists of wavelengths in the region of about 0.1 to 100Ao. For analytical purposes, the range of 0.7 to 2.0Ao is the most valuable region.

X-rays are produced when high-velocity electrons strike a metal target. The process of producing X-rays may be visualized in terms of Bohr’s theory of atomic structure.
A range of X-ray techniques and methods are in use but we shall categorize all methods into three main categories. These are:

• X-ray absorption method
• X-ray fluorescence method
• X-ray diffraction method

X-Ray Fluorescence

The spectrometer is used routinely for nondestructive analysis of rock, minerals, sediments, and fluids. It works on the wavelength dispersive principle used for work analysis for larger fractions of geological material.

The instrument consists of the X-ray source, sample analyzing crystal, and detector.

In this method, X-rays are produced by the fluorescent sample. By measuring the wavelength and intensity of the produced X-rays, qualitative and quantitative analysis can be performed. X-ray fluorescence method is non-destructive. Moreover, it requires very little sample preparation before the analysis is to be carried out.

The X-ray fluorescence method depends upon the principle that is common to several other instruments as SEM. XRD that is involving interaction between electron beams and x-rays to the sample. The analysis of trace elements is possible because of the behavior of the atom to interact with X-ray radiation. The sample is illuminated by intense x-ray radiation that is the incident beam. The energy is absorbed and is sufficient to dislodge an electron from the inner orbitals and the gap is filled by electrons of the outer orbital releasing energy due to the difference between the binding energy of the inner orbital compared to the outer one. The radiation emitted has less energy, then the primary incident radiation and is recorded by a scintillation counter which is the most common form of detector which is used in x-rays spectroscopy.

Scintillation

Scintillation generates photons in response to the incident radiation. A sensitive photomultiplier that will convert this photon into an electronic signal and process it. The energy released from the emission of the radiation is characteristics to that of the atom. The wavelength-dispersive spectrometer is used for the separation of complex x-rays spectrum into characteristics wavelength of elements when the atom of different elements are present in the sample. The intensity of the energy measured by the detector is directly proportional to the abundance of that element in the sample.

Applications of X-Ray Fluorescence Spectroscopy

X-ray fluorescence is a method of elemental analysis used for qualitative analysis as well for quantitative analysis.

• This technique is used in agriculture for the determination of trace elements in plants and food, detection of insecticides in fruit and leaves; determination of phosphorus in fertilizers etc.

• It is applied for direct determination of sulfur in protein, chloride in blood serum, strontium in blood serum and bone tissue etc.

• It is used for the examination of ores, tailings, concentrates and drilled cores; determination of silica in flowing slurries of ores, determination of lead in lead-tin alloys, etc.

• Other applications includes the determination of additives in motor oil by determining barium, zinc, phosphorus, calcium and chloride, and the determination of lead or sulfur in gasoline.

• It is used in rubber industry for the determination of vulcanizing elements.

X-Ray Diffraction

Diffraction methods are based on the dispersion of X-rays by crystals. By means of these methods, we can identify the crystal structures of numerous solid compounds. These methods are extremely important as compared to X-ray absorption and X-ray fluorescence methods.

X-ray diffraction is done to achieve:

• Measure the average spacing between the layers or the atoms.

• Orientation of the single crystal.

• Crystal structure of unknown sample.

• Shape, size, and stress of crystalline region.

• The atomic plane of the crystal causes the incident beam to interfere with one another they leak with the crystal. Diffraction occurs only when the brag law is satisfied with the condition for constructive interference from the planes with spacing ‘d’.

• X-ray from the source is bombarded onto the sample packed tightly in the sample cell. The diffracted beams are recorded at 2Φ that using a detector or photographic plate. The peak position, width intensity help in the determination of structure and stress on the sample.

Bragg Law

nλ = 2dsinΦ

English physicists Sir W.H. Bragg and his son Sir W.L. Bragg developed a relationship in 1913 to explain why the cleavage faces of crystals appear to reflect X-ray beams at certain angles of incidence. The variable ‘d’ is the distance between atomic layers in a crystal, and the variable lambda is the wavelength of the incident X-ray beam; n is an integer. This observation is an example of X-ray wave interference commonly known as X-ray diffraction (XRD), and was direct evidence for the periodic atomic structure of crystals postulated for several centuries.

The Braggs were awarded the Nobel Prize in physics in 1915 for their work in determining crystal structures beginning with NaCl, ZnS and diamond.

Applications of X-Ray Diffraction

• Structure of Crystals:

The analytical applications of X-ray diffraction are numerous. The method is non-destructive and gives information about the molecular structure of the sample. Perhaps its most vital use is to measure the size of crystal planes.

• Polymer Characterization:

Powder method can be used to establish the degree of crystallinity of the polymers.

• Annealing of metals:

Well annealed metals are in ordered crystal form and give sharp diffraction lines. If the metal is subjected to drilling, hammering, or bending, it becomes “worked,” or “fatigued,” that is, its crystals become broken and the x-ray pattern becomes more diffused.

X-Ray Absorption

X-ray absorption spectroscopy is a well-established technique used for the characterization of semiconductor solid, liquid amorphous crystalline bulk, or nano sterile. It is the energy-dependent on the fine structure of the absorption spectrum of a particular element initial intensity Io all incident on a sample and the extent of absorption depends upon the energy and sample thickness so the sample thickness becomes the path length. According to Bragg’s law, I1 can be calculated with a formula.

A= -log Ix/ Io

For X-ray energies that can be absorbed by the photons, the photoelectron gets excited to unoccupied the state of the absorbing atom. This leads to an increase in absorption coefficient at a particular X-ray energy corresponding to the energy difference between the core level and unoccupied state for higher X-ray energies the photoelectron prompted to a free or continuum state, the wave thus created propagates outward and scattered at the neighboring atoms.

Applications of X-Ray Absorption Methods

The various applications are as follow:

• Qualitative Analysis:

This is based upon the simple fact that there is a large difference in the mass absorption coefficient of one element from another.

• Quantitative Analysis:

Amount of X-rays absorbed is directly proportional to the concentration of the element present. This characteristic is widely used to detect broken bones, impurities, segregations, etc.