X- Ray Spectroscopy

X rays are electromagnetic radiation. X-rays are very energetic portion of the electromagnetic spectrum 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.

The relationship between the wavelength of electromagnetic radiation and its energy (E) is as follows.

For all electromagnetic radiation: E = h ν

where: h is the Planck constant (6.62 x 10-34 J.s); ν is the frequency expressed in Hertz.

Properties of X-Rays

• Invisible;
• Propagate with velocity of light
• Unaffected by electrical and magnetic fields;
• Differentially absorbed in passing through matter of varying composition, density and thickness;
• Reflected, diffracted, refracted and polarized
• Capable of ionising gases;
• Capable of affecting electrical properties of solids and liquids
• Capable of blackening a photographic plate
• Able to liberate photoelectron.
• Emitted in a continuous spectrum
• Emitted also with a line spectrum characteristic of the chemical element;

Generation of X-rays:

• Bombarding a metal target with a beam of high energy electrons.
• Exposing a substance to a primary beam of X-rays to generate a secondary beam of Xrays of lower energy.
• Using a radioactive source emitting X-rays during its decay process.

Characteristic X-Rays
X-Rays emitted from heavy elements when their electrons make transitions from a higher to a lower atomic energy level.

X Rays are

• unique for each element as energy emitted is proportional to the binding energy of the element.
• Can be used to identify the particular element from which they are emitted.
  
  Based on the properties of X-rays, a number of analytical tools have been developed over the years. These include:
• X-ray emission spectroscopy
• Augur emission spectroscopy
• X-ray fluorescence spectroscopy
• Electron spectroscopy
• X-ray Absorption
• X-ray Diffraction

In this post we are going to take up X-ray fluorescence spectroscopy in detail. X-ray fluorescence (XRF) spectrometry is an elemental analysis technique with broad application in science and industry. XRF is based on the principle that individual atoms, when excited by an external energy source, emit X-ray photons of a characteristic energy or wavelength. By counting the number of photons of each energy emitted from a sample, the elements present may be identified and quantitated. Emission of characteristic secondary X rays from a material that has been excited by bombarding with high energy X-Rays.

In X-Ray Fluorescence, all the elements in the sample are excited by the absorption of the primary beam which emit their own characteristic fluorescence. Hence, XRF is a powerful tool for the qualitative and quantitative estimation of all but the lightest elements having atomic number greater than oxygen i.e. >8.

Types of XRF spectroscopy

There are two types of XRF spectroscopy

• Energy Dispersive X-Ray Fluorescence spectroscopy (ED-XRF) – It Uses a solid state detector and distinguishes each peak according to its energy.
• Wavelength Dispersive X-Ray Fluorescence spectroscopy (WD-XRF) – It Uses the scanning crystal as the dispersive element and distinguishes each peak according to its wavelength.

Components of XRF

Important instrument components of XRF includes

Source
A transducer/detector
A signal processor
A read out

Energy Dispersive X-Ray Fluorescence spectroscopy (ED-XRF)

In Energy Dispersive X-Ray Fluorescence spectrometry (ED-XRF), the identification of characteristic lines is performed using detectors that directly measure the energy of the photons. In the simplest case an electron is ejected from an atom of the detector material by photoabsorption. The loss of energy of this just created primary electron results in a shower of electron-ion pairs in the case of a proportional counter, optical excitations in the case of scintillation counter, or showers of electron-hole pairs in a semiconductor detector. The resulting detector signal is proportional to the energy of the incident photon, in contrast to wavelength dispersion in which the Bragg reflecting properties of a crystal are used to disperse X-rays at different reflection angles according to their wavelengths. Although energy dispersive detectors generally exhibit poorer energy resolution than wavelength dispersive analyzers, they are capable of detecting simultaneously a wide range of energies.

EDXRF has a polychromatic source (X-ray source or radioactive material), a sample holder, a semiconductor device and various electronic components for energy discrimination.

Radiation Source

1. Coolidge’s (X-ray) tube: All modern X-ray tubes owe their existence to Coolidge’s hot-cathode X-ray tube. It consists essentially of a vacuum sealed glass tube containing a tungsten filament for the production for electrons, an anode and a beryllium window. From variety of modifications, two geometries have emerged as the most suitable for all practical purposes: the end-window tube and the side window tube, both having their own merits and limitations. The main differences between tube types are in the polarity of the anode and cathode and the arrangement of the exit window.conditions.
   
   All X-ray tubes work on the same principle: accelerating electrons in an electrical field and decelerated them in a suitable anode material. The region of the electron beam in which this takes place must be evacuated in order to prevent collisions with gas molecules.
   
   Detectors
   X-ray detectors X-ray detectors are usually operated as photon counters. The individual pulses of the charge are produced as quanta of radiation which are absorbed by transducer and counted. In ED-XRF, the detector allows the determination of the energy of the photon when it is detected. Detectors historically have been based on silicon semiconductors, in the form of lithium-drifted silicon crystals, or high-purity silicon wafers.
   
   
   We will cover up WD- XRF in our next post, as we feel too much information in one go can be overwhelming for a lot of students. Take a deep breath, revise what you have learnt from this post, then move to the next post covering WD-XRF.