X-Ray Diffraction Method
X-Rays are electromagnetic radiations having a range of wavelength 0.1-100Aº, however, their analytical range is 0.7-2.0Aº. X-ray radiations are produced when a high-velocity electron impinges or collides with the target. The following process illustrates the process of production of x-ray radiations:
Fast-moving electrons collide with the atoms of the target
↓
Loss of electron from the inner shell of the atoms that results in vacant orbital
↓
The electron from the penultimate outer shell falls into the inner shell
↓
Emission of X-rays
The energy of x-rays equals the difference in energies of the two shells. For example, if electrons from K shell is lost and electron from shell L falls into K shell then,
E = Eₗ – Eₖ
Where,
E = Energy of x rays
Eₗ = Energy of L shell
Eₖ = Energy of K shell
The other factors associated with x-rays are the frequency, wavelength, and atomic number of the metal atoms. The relationships are as follows:
𝛎 = (Eₗ – Eₖ)/h
Where,
𝛎 = frequency of x rays
h = Plank’s constant
𝛌min = hc/Ve
Where,
𝛌min = minimum wavelength of x rays
c = speed of light
V = voltage applied
e = electronic charge
c/𝛌 = a(Z-𝛔)²
Where,
a = a constant
Z = atomic number of the atom
𝛔 = a constant that depends upon a series of lines
Interaction of X-rays With Matter
When the x-rays come in contact with the matter, they result in three characteristic processes: absorption, diffraction, and fluorescence.
- X-Ray Absorption
X-rays like other radiations also show the properties of absorption. When the electrons of atoms constituting the matter interact with them, they absorb the radiations and get excited which results in the emission of secondary radiations, characteristic of those atoms.
The process follows Beer’s law. The extent of absorption by a given element depends on the number of atoms of that element in the path of x-rays but is independent on the physical or chemical state of that element.
- X-Ray Fluorescence
When a sample is irradiated with an x-ray beam, the sample sometimes emits other x-ray beams, called x-ray fluorescence. The frequency of those rays gives quantitative analysis whereas their wavelength enables qualitative analysis.
- X-Ray Diffraction
Diffraction is the property of x-rays exhibited only by crystalline substances. When the x-rays are incident on a substance, the electrons of atoms of the substance become small oscillators. The oscillators oscillate at the same frequency as that of incident rays.
The electrons arranged in a crystal lattice emit electromagnetic radiation in all directions at the same frequency as that of incident rays. Constructive interference of the scattered radiations causes diffraction and that process follows Bragg’s Law, which states:
n𝛌 = 2d sin𝚹
Where,
n = integer
𝛌 = wavelength of the x-rays
d = spacing between crystal planes
𝚹 = angle of incidence
X-ray diffraction is a common technique that determines a sample’s composition or crystalline structure. For larger crystals such as macromolecules and inorganic compounds, it can be used to determine the structure of atoms within the sample, whereas, for small-sized crystals, it can determine sample composition, crystallinity, and phase purity.
Types of X-Ray Diffraction Methods
The following x-ray diffraction methods are used to study the structure of the crystal:
- The Laue Method
- Bragg’s Spectrometer Method
- Rotating Crystal Method
- Powder Crystal Method
Instrumentation of X-Ray Diffraction
The XRD instrument is composed of:
- X-ray source
- Collimator
- Monochromator
- Detector
- Recorder
1. X-ray source
The source of the rays is a circuit composed of an anode, a cathode, and a battery. The electric current from the battery heats the cathode which is made up of a tungsten filament, that emits thermionic electrons. The emitted electrons hit the anode which has the target metal, transferring the energy to the surface of the target.
The target material selected should have an atomic number greater than that of the elements being examined in the sample.
The energy of the rays emitted by the target material should be greater than that required to excite the elements being irradiated.
2. Collimator
The target material produces the x-rays which are always randomly directed. They are converted into a narrow beam of rays by a collimator. The collimator consists of two metal plates separated by a small gap. It absorbs all the rays except the narrow beam that passes through the gap.
3. Monochromators
There is a wide range of wavelengths possessed by the x-rays but only a short range is useful for the analysis. Monochromators help to pass the rays only with desirable wavelengths. There are usually two types of monochromators used in XRD instruments:
- Filters
Filters act as a window of material that absorbs undesirable radiations but allows the radiation of the required wavelength to pass. This method makes use of the significant difference in the mass absorption coefficient on the other side of an absorption edge. For example, a zirconium filter is used for molybdenum radiation.
- Crystal Monochromator
A crystal monochromator is positioned in the path of the x-ray beam so that the angle reflecting planes satisfies Bragg’s equation for the required wavelength. The beam is split up by the crystalline material into the component wavelengths in the same way as a prism splits up white light into rainbow colors. This is called analyzing crystals. They can be flat crystal or curved crystal monochromators of lithium fluoride, sodium chloride, or quartz.
4. Detectors
The detectors are placed after the monochromators which detect the signals and transfer them to the recorder. The detectors used in XRD can be based on the following methods:
- Photographic Method
In order to record the position and intensity of an x-ray beam, a plane cylindrical film is prepared. The film is developed after being exposed to x-rays. The blackening of the developed film is always expressed in terms of density units D.
D = log Io / I
Where,
Io = Incident intensities
I = Transmitted intensities
D = Total energy that causes the blackening of the film.
- Counter Method
There are 5 types of detectors used in XRD which are based on the counter method. They are:
- Geiger-Muller Tube Counter
This detector consists of a tube, two electrodes, and a counter. The tube is filled with argon gas. The cathode used is a grounded metallic casing and an anode is a rod in the middle of the detector. When the x-ray particles enter the tube, they ionize the argon gas, producing a large number of Ar⁺/e⁻ ion pairs. Ar⁺ moves towards the cathode while e⁻ move to the anode and this movement of charges produces an electric potential which is measured by the counter.
- Scintillation Detector
This detector consists of a scintillator made up of sodium iodide and a photomultiplier tube. The radiations fall on the scintillator which converts the rays into visible radiations. When these visible radiations fall on the photocathode, the photons present in visible radiations get multiplied due to reflection on the dynodes present in the photomultiplier tube. The anode collects these photons that show signals on the readout.
- Semiconductor Detector
This detector is divided into three regions namely- a p-type region made up of silicon, an n-type region up of silicon doped with lithium and an intrinsic region made up of silicon doped with lithium ions. X-rays enter the intrinsic region and excite electrons to a conduction band. This results in a resistance decrease in the region which allows the flow of current through the device. The detection is based on the increase in conductivity when struck by radiation.
- Proportional Counter
This detector is similar to the Geiger-Muller tube detector. The tube is filled with heavy gases such as xenon or krypton as they are easily ionized. It is operated at a lower voltage than the Geiger-Muller detector. The output pulse is dependent upon the intensity of x-rays falling on a proportional counter.
- Solid-State Semiconductor Detector
In this detector, the electrons produced by x-ray beams are promoted into the conduction and the current which flows is directly proportional to the incident x-ray energy.
5. Recorder
The signals detected by the detectors are then amplified and recorded by a computer system in the form of peaks. The intensity of the peaks is related to the number of molecules in that phase or with that spacing. The greater the intensity of the peak, the greater the number of crystals or molecules with that distinct spacing. The width of the peaks is inversely proportional to the crystal size.
Application of XRD in Forensic Analysis
XRD is an analytical instrumental method used in forensic science.
- It is applicable in forensic toxicology where inorganic metallic poisons can be detected from the sample. It is also useful in elucidating the structure of the drugs seized from a place.
- The XRD is helpful in the examination of jewelry made up of gold, silver, or any other metal, seized in cases of robbery or burglary.
- Examination of paint is also possible from the XRD methods.
- In forensic medicine, the XRD is helpful in the determination of any bone disease or deformity in the deceased. X-ray of dental records plays a vital role in the identification process.
- The crystalline structure of the suspected material found at the crime scene can be done by using the XRD method.
Conclusion
XRD is an instrumental method widely used in forensic analysis. It is a helpful technique with multidisciplinary applications. It is one of the robust techniques that gives satisfactory results.
Also Have a Look at:
- How Does Atomic Absorption Spectroscopy Work?
- Comparison Microscope: Principle, Features & Applications
- Neutron Activation Analysis
