Questions and answers about x-ray methods

X-ray methods questions and answers.

Chemistry is the branch of Science which deals with the structure of matter and composition of substances, their transformation, analysis, synthesis and manufacturing. To facilitate the study of Chemistry, it has been divided into various branches viz; Physical, Inorganic, Organic, Analytical etc. The knowledge of the basic concepts is absolutely essential for better understanding of the subject matter. To augment the ability of the students to understand the subject.

Questions and answers about x-ray methods
The article in your hand Questions and answers about x-ray methods primarily meant for graduate and postgraduate students of Chemistry with the intention that they could show better performance during various competitive examinations and interviews held for Civil and Military Services. At the same time it is equally useful for those who desire to be well informed in various disciplines of chemistry.

Q. How will you define X-rays?

The interaction of radiation with matter results in elastic collisions in which potential energy of target species is raised. A common type is one in which electron is excited to higher energy level. When electron comes back to its ground state, energy is released as electromagnetic radiation, which may be visible light (λ = 400-800nm), ultraviolet (λ =10-400nm) оr X-rays (λ =0,005-10nm).

So in simple words X-ray is that type of electron spectroscopy in which emitted radiations fall in λ=0.005-10nm.

Q. What is the basic principle of X-rays?

When a beam of high energy electrons or X-ray photons impinges on a target material (usually metal), a sufficient energy knock out a planetary electron of target atom completely. Then another electron falls into vacancy from higher orbital (outer orbital) to emit X-ray radiation. with a wave length dependent on the energy levels, and hence the characteristic of the element.

Q. What is Bremsstrahlung?

When electrons from some source collide with electrons of target material these are slow down by multiple interaction with the electrons of the target. The energy lost is converted into a continuum of X-radiation, called Bremsstrahlung, meaning breaking radiation.

Q. Differentiate between Kα and Kβ?

X-rays due to transitions from the L to the K shell are called Kα X-rays. Kα1and Kα2 corresponding to electrons originating in different sublevels of the L-shell. X-rays due to transition from the M to K-shell are called Kβ, and so on.

Q. How for analytical purposes X-rays are obtained?

For analytical purposes, X-rays are obtained in three ways.
  1. Bombardment of a metal target with a beam of high energy electrons.
  2. By exposure of a substance to a primary beam of X-rays inorder to generate a secondary beam of fluorescent rays, and 
  3. By a radioactive source whose decay process results in X-ray emission.

Q. What analytical information is provided by X-rays?

For analytical purposes x-rays can be utilized in several different ways For example,

  1. Absorption of X-rays gives information about the absorbing material, just as in other spectral regions.
  2. The diffraction of X-rays permits analysis of crystalline materials with a high degree of accuracy and specificity.
  3. Wavelength measurement identifies the element in the excited state samples.
  4.  Measurement of radiant power at given wavelength can be quantitative indicator of the composition of the sample.

Q. Which instrument can be used for X-rays measurement and what are its components?

The instrument used for X-ray studies is the X-ray spectrometer absorptiometer. It consists of a high intensity X-ray tube, a sample chamber, a goniometer with collimator and analyzer crystal and a detector.
In both emission and absorption, the X-rays must be transmitted from the X-ray tube target to the counting volume of the detector.

Q. What detectors are being used in X-rays absorption and emission analysis?

There are three types of detectors X-rays absorption and emission analysts. These are:
  1. Photographic detectors
  2. Gas ionization detector
  3. Scintillation detector

Q. Make a comparison between X-ray absorption and X-ray emission?

  1. For general elemental analysis the X-ray emission is versatile than X-absorption. Emission has advantages of spectral line specificity and sensitivity of one or two orders of magnitude greater than absorption.
  2.  X-ray emission is applicable to a wide range of elements without changing any instrumental parameter other than Bragg angle.
  3.  X-ray emission methods are applicable to all elements having atomic number 11 to 92, with some practical and theoretical limitations, in the case of low atomic number elements.
  4. A unique advantage of the absorption edge method over emission is the elimination of matrix effect which is inherent in emission analysis.
  5. Concentration can be calculated from absorption edge measurement without recourse to the use of standards.
  6. The lower limit of detection by emission method is a function to peak to back ground ratio and the number of photons mounted.

Q. What do you know about X-ray diffraction?

In this analytical technique X-rays are employed in investigating the interior of a crystal to contain the special arrangement of the structural units of a substance in crystalline state.

Q. Why crystals diffract X-rays?

It might be possible to diffract X-rays by means of crystals, because
The crystals act as a three dimensional natural grating for X-rays
X-rays act as a part of electromagnetic radiation.
X-rays are actually the radiation of very small wavelength probably of the order of 10-8 cm.

Q. Define Bragg law?

X-ray diffraction is explained best by Bragg’s law, which states” As the in a crystal are orderly arranged, a beam of X-rays is reflected from plane of atoms similar to the reflection of light wave from a plane mirror.”

                                                  nλ=2d sinθ   
n is an integer 1,2,3 etc, known as order of reflection.
d is inter planar distance of crystal/
is wavelength.

Q. What are the most important requirements of diffraction?

The most important requirements of diffraction are:
  1. The spacing between the layers of atoms must be roughly the same as the wavelength of radiation.
  2. The scattering centre must be specially distributed in a highly regular way.

Q. What are advantage and disadvantage of X-ray methods?

Advantages of X-ray methods
  1. X-ray methods are non-destructive in nature.
  2. X-ray analysis can be applied to amorphous or crystalline samples in any physical state of sub-division.
  3. There is no consideration of electrical and mechanical properties of samples.

Disadvantages of X-ray methods

  1. The accuracy of method depends upon the surface preparation, reliability of standards, stability of the X-ray tube output and number of X-ray photons counted.
  2. At one percent concentration level the line intensity and line to background ratio is sufficiently high.
  3.  Instrumental and sample variables affect the analysis. Three variables can, however, be reduced to less than using standardized techniques and modern instruments.

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