In forensic ballistics, instruments have been used successfully to identify bullets, firearms, and guns involved in crimes. Some of them are as follows:
Neutron Activation Analysis
Neutron activation is a general term for irradiating the material with neutrons to create radionuclide. Neutron Activation Analysis was discovered by Hevesy and Levi in the 1936 year. They discovered that samples containing certain rare earth elements became highly radioactive after exposure to a source of neutrons. From this observation; they quickly recognized the potential of employing nuclear reactions on samples followed by measurement of the induced radioactivity to facilitate both qualitative and quantitative identification of many elements. NAA is an extremely sensitive technique useful for performing both qualitative and quantitative multi-element analysis of major/minor and trace elements in the sample from almost every conceivable field of activity scientific of technical interest. For many components and applications, NAA provides sensitivities on the order of parts per billion (ppb) or better, which are superior to those obtained by other methods.
Methodology of Neutron Activation Analysis
The essentials required to carry out an analysis of a sample are:
- A source of neutrons and their moderation
- Instrumentation suitable for detecting gamma rays
- Knowledge of the reactions that take place when neutrons interact with target nuclei in detail.
It’s important to note that:
i. Prompt Gamma Rays are emitted, and their study is defined as PG NAA, with measurements taken during irradiation.
ii. The measurements in Delayed Gamma-ray Neutron Activation Analysis (DG NAA) follow radioactive decay.
Out of these two, the latter operational method is more common. As a result, when NAA is mentioned, it is generally assumed that the delayed gamma-ray is being measured. About 70% of elements have properties suitable for measurements by NAA.
► Natural sodium i.e. 23 Na is taken as the target. Capturing of one neutron will be converted into 24 Na (Radioactive Sodium).
► The conventional approach is introducing the sample to be studied, as well as several relevant standards, into a nuclear reactor’s neutron field.
► The sample is then analyzed using an appropriate gamma-ray detector system. The 24Na decays to stable 24Mg with the release of Gamma rays having energies of 1368.53 & 2754.9 KeV.
► The gamma-ray energies are converted to an electrical signal when striking on a suitable detector. Then, it is processed as a count in an energy spectrum.
► The accumulation of gamma counts at particular energy generates a curve, the area of which is proportional to the radioactivity of the characteristic radionuclide.
► Comparing against standards allows determination as well as an abundance of a particular element.
► Most of the instrument used in Forensic Science makes use of the changes that occur in the energy levels of electrons of the atom under different conditions such as excitation, bombardment, vibrational stretching, bending, etc.
► As we know, chemical differences between elements are a function of the number of protons in the nucleus of their atoms.
► The nuclei of atoms of a given element always contain the same number of protons. However, the number of neutrons in an atom of the element may vary.
► We also know that the nucleus of an atom is a tightly packed structure, unlike the electrons which can be easily dislodged or changed in various ways.
► Atoms of a single element containing a different number of neutrons are called isotopes of that element.
► Every element has several isotopes and for the most part, they are stable. However, when the neutron imbalance becomes too great, the nucleus becomes unstable and the element becomes radioactive.
► Radioactive elements decompose into smaller atoms to become stable. In the process of decomposition, they emit energy in the form of alpha particles, beta particles (electrons), and gamma rays. During decomposition, each element emits gamma rays at wavelengths uniquely characteristic of the element.
Applications of NAA
► It has wide applications in Agriculture, Anthropology, Biology, Chemistry, Geology, and Medicine.
► it used to know the Source of clays and pottery
► Sourcing and composition of igneous rocks, sediments, and basalts
► It is used to identify contaminants in salts, pure crystals, and metals
► Composition and contaminants in metals, thin film deposits plastics
► Trace elements in oil and lipids
► Toxins and trace elements in hair, skin, and nail samples
► Toxins in Fish and agricultural products
► It is used in detection of trace levels of naturally occurring radioactive material such as thorium and uranium
► Precious Metal Assay
► It is used in high fidelity measurements of precious and rare earth metals in geological samples
► Detection of trace levels various toxic metals such as mercury, uranium, and thorium
► It is used in the analysis of bullets, paints, glass, metals Gunshot residues (GSR Particles).
X-Ray Fluorescence and Wave Length Dispersive Analysis
Wavelength Dispersive Analysis Many substances show fluorescence under Ultraviolet (UV) light. It means such substances have the power to receive radiant energy of a certain wavelength and to convert this energy into a higher wavelength within the visible spectrum consisting of different colors. Thus fluorescence is a phenomenon where a low wavelength of Ultraviolet light (invisible to eyes) gets converted into a higher wavelength in the visible spectrum and one can observe the same being visible to eyes.
For example, an invisible small stain of semen when exposed to ultraviolet radiation from a UV lamp gives a whitish blue Colour glow. Similarly, we can carry examination for many other common articles of physical evidence namely comparison of paper, currency notes, secret writing, sealing waxes, glass, threads, anthracene powder, and urine stains.
The essential difference between UV fluorescence and X-ray fluorescence is that in UV fluorescence the incident UV of lower wavelength exhibits fluorescence in the visible spectrum which can be seen. In the case of X-ray fluorescence, the incidents X-ray as well as fluorescent radiation both are invisible to the human eyes as both the radiation incident and fluorescence lie in the invisible spectrum, but the basic fact remains the same meaning thereby that in the incident X-ray radiation will be of lower wavelength and fluorescence radiation of higher wavelength.
Principle of X-ray Fluorescence
X-ray interacts with matter in many ways like absorption, scattering, and diffraction, etc. Absorption of X-ray is used for determining the elemental composition of a sample and the method is known as XRF or X-ray fluorescence. The sample to be analyzed for elemental composition is placed in the path of the X-ray beam which will get absorbed by the sample. The absorbing atoms get excited and emit an X-ray of the characteristic wavelength of elements present in the absorbing material and this process is known as fluorescence.
As the wavelength of the fluorescence is characteristic of excited atoms of particular elements, the measure of this wavelength would enable one to identify the fluorescing element. The greater amount of elements will yield greater intensity and a small amount will show less intensity for the sample, the reason being that intensity of fluorescence depends on how much of that element is present.
According to Bragg’s equation:
λ = 2 d Sin θ
d is the space between the crystal layers of the analyzing crystal and hence known.
Therefore, the known value of d and measuring θ can be calculated.
The element is identified once the wavelength has been calculated and a chart of fluorescence wavelength of all known elements consulted.
(i) Source for producing X-ray
(ii) Suitable filtering of primary X-ray
(iii) Production of Fluorescent radiation characteristic of element present when the sample is hit by the beam.
(iv) Passing of fluorescent radiation, through a collimator
(v) Separation of different wavelengths by the analyzing crystal
(vi) The intensity of each wavelength is determined and recorded using a synchronized detector that rotates in an arc around the analyzer to cover different angles for different elements present in the sample.
- Characterization of soils
- Trace elements in plants
- Detection of insecticides (poison) on leaves, fruits, and vegetables Analyzing of ores and alloys
- To distinguish between good quality and poor quality of rubber based on presence of Sulphur
- Analyzing of ceramics
- Determination of additives in motor oil
- Examination of Antiques
- This is a non-destructive technique and thus the forensic exhibit remains unaltered.
- Sample preparation, in general, is not required
- It is a very sensitive method of analysis
- The detection limit can be 10 ppm, far superior to normal chemical methods. Amount of sample required is very small which makes it more suitable for Forensic work.
Flame-Less Absorption Spectrometry
This absorption spectrometry is an extremely sensitive and convenient technique for the evaluation of GSR particles. It is so sensitive that it can detect elements in microgram and picogram ranges. It works on a simple principle that, element absorbs radiation of the same wavelength as it emits when excited. The suspected element (in GSR) is taken in an atomizer (a graphite tube or a Tantalum strip) and is excited by heating. The radiations from a discharge tube with are suspected metal electrodes are passed through the atomizer. The loss of intensity in radiation gives the quantity of the metal in question. The technique is very useful and popular for the examination of GSR particles.
(a) It is cost-effective.
(b) Time taken for analysis is reasonably less.
(c) It can detect lead also.
(a) One element can be detected at one time.
(b) Method is destructive.
Electron microscopes are used for getting enlarged images of small objects or a small portion of large objects using electron beams. The lenses used in these microscopes are electrostatic and/ or electromagnetic lenses. Since glass lenses do not affect the electron beam, they are not used. Electron Microscope has high magnification. In certain cases the magnification can be achieved up to 2, 00,000 X. It is extensively used in the field of Biology and other Material Sciences. In Scanning Electronic Microscope (SEM), the three-dimensional external shape of an object with greater depth of the field can be obtained. Unlike the Light Microscope, the Electron Microscope uses electrons as the former one uses Photons (light) for visualization. Compared to the optical microscope, electron microscopes are a very recent invention. The image is made visible by the use of a fluorescent screen that has the coating of PbS (lead sulfide). The basic design of an electromagnetic lens is a Solenoid (a coil of wire around the outside of a tube) through which electric current is passed thereby inducing an electromagnetic field and the beam is controlled by adjusting current as electrons are very sensitive to the magnetic field.
There are two types of electron microscopes
1. TEM- Transmission Electron Microscope
2. SEM- Scanning Electron Microscope
In the TEM, an electron beam is passed through an extremely thin section of the specimen, and a 2D cross-section of the specimen is obtained.
SEM, in contrast, visualizes the surface structure of the specimen, providing a 3D impression.
Electron Microscope Usage
A wide variety of samples encountered in crime investigation can be examined by scanning electron microscope which includes.
(2) Fibers including paper fibers
(3) GSR, Gunshot residues
(4) Biological specimens
(5) Metallic fragments especially broken parts of bullets.
The important or main parts of the electron microscope are as follows:
- Electron Gun:
It serves as electron source and supplies electrons.
- Condenser Lens:
It controls the illumination of the specimen by the beam of electron and may be a magnetic field. To provide intense illumination double condenser or two lenses may be used.
- Objective Lens:
An objective lens inserted within the apertures for limiting it.
- Projection lens:
This is the lens that has been replaced by an eyepiece in a light microscope.
- Fluorescent Screen:
The fluorescent screen shows the final image with the direct impact of the electron beam on it. The material used for coating the screen is lead sulfide.
Advantages of SEM analysis
Analysis of GSR particle by SEM is beyond doubt the best method because of several reasons:
- It does not destroy the sample as it is non-destructive.
- The result of the analysis of GSR particles can be verified if required at any time.
- It can provide documentary proof of analysis.
- The material required for analysis is quite small
- SEM has a very high magnification, better resolving power, and greater depth of focus.
SEM- EDX Method [Scanning Electron Microscope with Energy Dispersive]
X-ray Analysis Primary electrons having higher energy (accelerated by the high voltage of about 15 kV to 50 kV) are made to fall on the particles under analysis. The X-rays emitted are then detected by an X-ray detector. This X-ray detector measures the energy in each photon of X-ray radiation and these quanta of energy can be stored electronically. A histogram of several quanta versus the energy can be drawn. From this histogram, elemental analysis of particles under examination can be determined.